EPA-600/2-77-233
November 1977
Environmental Protection Technology Series
FIELD MEASUREMENT OF DENITRIFICATION
Robert S. Kerr Environmental Research Laboratory
Office of Research and Development
U.S. Environmental Protection Agency
Ada, Oklahoma 74820
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RESEARCH REPORTING SERIES
Research reports of the Office of Research and Development, U.S. Environmental
Protection Agency, have been grouped into nine series. These nine broad cate-
gories were established to facilitate further development and application of en-
vironmental technology. Elimination of traditional grouping was consciously
planned to foster technology transfer and a maximum interface in related fields.
The nine series are:
1. Environmental Health Effects Research
2. Environmental Protection Technology
3. Ecological Research
4. Environmental Monitoring
5. Socioeconomic Environmental Studies
6. Scientific and Technical Assessment Reports (STAR)
7. Interagency Energy-Environment Research and Development
8. "Special" Reports
9. Miscellaneous Reports
This report has been assigned to the ENVIRONMENTAL PROTECTION TECH-
NOLOGY series. This series describes research performed to develop and dem-
onstrate instrumentation, equipment, and methodology to repair or prevent en-
vironmental degradation from point and non-point sources of pollution. This work
provides the new or improved technology required for the control and treatment
of pollution sources to meet environmental quality standards.
This document is available to the public through the National Technical Informa-
tion Service, Springfield, Virginia 22161.
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EPA-600/2-77-233
November 1977
FIELD MEASUREMENT OF DENITRIFICATION
by
Dennis E. Rolston
Francis E. Broadbent
Land, Air and Water Resources
University of California
Davis, California 95616
Grant No. R804259
Project Officer
Arthur G. Hornsby
Source Management Branch
Robert S. Kerr Environmental Research Laboratory
Ada, Oklahoma 74820
ROBERT S. KERR ENVIRONMENTAL RESEARCH LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
ADA, OKLAHOMA 74820
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DISCLAIMER
This report has been reviewed by the Robert S. Kerr Environmental
Research Laboratory, U.S. Environmental Protection Agency, and approved for
publication. Approval does not signify that the contents necessarily reflect
the views and policies of the U.S. Environmental Protection Agency, nor does
mention of trade, names or commercial products constitute endorsement or
recommendation for use.
ii
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FOREWORD
The Environmental Protection Agency was established to coordinate
administration of the major Federal programs designed to protect the quality
of our environment.
An important part of the Agency's effort involves the search for infor-
mation about environmental problems, management techniques and new technolo-
gies through which optimum use of the Nation's land and water resources can
be assured and the threat pollution poses to the welfare of the American
people can be minimized.
EPA's Office of Research and Development conducts this search through a
nationwide network of research facilities.
As one of these facilities, the Robert S. Kerr Environmental Research
Laboratory is responsible for the management of programs to: (a) investigate
the nature, transport, fate and management of pollutants in groundwater;
(b) develop and demonstrate methods for treating wastewaters with soil and
other natural systems; (c) develop and demonstrate pollution control technolo-
gies for irrigation return flows; (d) develop and demonstrate pollution
control technologies for animal production wastes; (e) develop and demonstrate
technologies to prevent, control or abate pollution from the petroleum refin-
ing and petrochemical industries; and (f) develop and demonstrate technolo-
gies to manage pollution resulting from combinations of industrial wastewaters
or industrial/municipal wastewaters.
This report contributes to the knowledge essential if the EPA is to meet
the requirements of environmental laws that it establish and enforce pollu-
tion control standards which are reasonable, cost effective and provide
adequate protection for the American public.
William C. Galegar
Director
Robert S. Kerr Environmental
Research Laboratory
iii
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ABSTRACT
The amount of NO- in irrigation return flow waters is dependent upon
each of the components of the N cycle in soils. One of those components for
which absolute amounts and rates are not well known is denitrification.
Volatile denitrification products, primarily NJD and N», are evolved whenever
anoxic sites develop within the soil and when sufficient carbon is available.
Absolute amounts and rates of denitrification from a Yolo loam field profile
at Davis, California, were studied in relation to the influence of soil-water
content, organic carbon source, and soil temperature. Field plots were inten-
sely instrumented with soil atmosphere samplers, soil solution samplers, and
tensiometers. Soil-water pressure heads (h) in the upper 15 cm of soil were
maintained constant at -15 and -70 cm of water and at -8 and -50 cm of water
for the soil temperature treatments (5 cm depth) of 23 and 8°C, respectively.
Plots cropped with ryegrass, uncropped plots, and plots to which manure was
mixed in the top 10 cm of soil were used to establish three different carbon
levels. Fertilizer was applied at the rate of 300 kg N ha~ as KNO, enriched
with 20 and 40% 15N for the h = -15 or -8 and h = -70 or -50 cm treatments,
respectively. The flux of volatile gases at the soil surface was measured
from the accumulation of N«0 and •'•%- beneath an air-tight cover placed over
the soil surface for 1 or 2 hours per day and from measured soil gaseous dif-
fusion coefficients and concentration gradients. Denitrification from gas
fluxes occurred for the high-temperature experiment in order of decreasing
magnitude in manure (h = -15 cm), manure (h = -70 cm), cropped (h = -15 cm),
cropped (h = -70 cm), uncropped (h = -15 cm), and uncropped (h = -70 cm)
plots. Approximately 70% of the fertilizer N was denitrified for the manure
(h = -15 cm) treatment. Approximately 1% of the added fertilizer was deni-
trified in the uncropped (h = -70 cm) treatment. Denitrification from gas
fluxes for the low-temperature experiment occurred in the same order as that
of the high-temperature experiment except that the rate and absolute magni-
tude were much smaller. Approximately 11% of the fertilizer N was denitri-
fied for the manure (h = -8 cm), and no measureable denitrification occurred
in the uncropped (h = -50 cm) treatment. The amount of N_ produced was much
greater than N»0. The N-0 flux at the soil surface varied between 5 and 26%
of total denitrification.
Denitrification measured from gas fluxes compared reasonably with that
determined by difference of all other components of the N cycle for the
wettest water treatments. For the drier treatments, denitrification measured
directly was smaller than that determined by difference due to the inability
to measure very small fluxes of N« over relatively long time periods,
inability to continuously monitor gas fluxes, and uncertainty in measuring
denitrification by difference. Determining denitrification by difference
was highly uncertain due primarily to variability of the water flux and NO.,
concentrations within the soil.
iv
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Since nearly all denitrification occurred in the upper 30-60 cm of soil,
the amount of N07 leaching below the root zone was strongly dependent upon
the soil-water treatment and the amount of carbon for denitrification. Nit-
rate was completely leached from the 180 cm profile after approximately 7
months in the h = -15 and -8 cm treatments, whereas nearly all NO^ remained
in the upper 60 cm of soil for the h = -70 and -50 cm treatments.
The proportion of volatile products and rates of denitrification as
influenced by field-soil environmental conditions can only be ascertained by
measuring the flux of the gases produced. A thorough, quantitative evaluation
of the rate and magnitude of denitrification, leaching, and plant uptake
provides a means for making management decisions to control irrigation return
flow water quality.
This report was submitted in fulfillment of Grant No. R-804259 by
University of California, Davis, under the sponsorship of the U.S. Environ-
mental Protection Agency. This report covers a period from December 1, 1975,
to July 31, 1977, and work was completed as of August 8, 1977.
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CONTENTS
Foreword ill
Abstract iv
Figures viii
Tables xi
Abbreviations and Symbols xiii
Acknowledgments xv
1. Introduction 1
2. Conclusions 3
3. Recommendations 6
4. Experimental Procedures 8
Field installation 8
Experimental procedures - field 13
Analytical techniques 16
5. Results and Discussion 18
High-temperature experiment 18
Plot characteristics 18
Soil gases 22
Gas fluxes 28
Plant uptake 37
Leaching 38
Residual soil N 40
Mass balance 43
Low-temperature experiment 45
Plot characteristics 45
Soil gases 51
Gas fluxes 56
Plant uptake 63
Leaching 65
Residual soil N 68
Mass balance 69
References 72
Publications 74
vii
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FIGURES
Number Page
1 Schematic of plot layout and treatments 8
2 Bulk density as a function of soil depth for experimental area. . . 10
3 Organic carbon as a function of soil depth for the uncropped and
manure plots of the summer and winter experiments 11
4 Soil-water content as a function of the soil-water pressure
head for surface soil within plots 12
5 Soil-water content as a function of soil-water pressure head for
several soil depths 13
6 Mean soil temperature of two probes at the 5-cm soil depth of
six plots as a function of time after fertilizer was applied
to the plots of the summer experiment. The arrows give the
fertilizer application for each plot with the symbols given
in Table 2 18
7 Mean soil-water pressure head of three tensiometers in each
plot at the 5- and 15-cm soil depth as a function of time
after fertilizer application to the two water treatments of
the summer experiment. The arrows give the fertilizer
application for each plot with the symbols given in Table 2 ... 19
8 Mean soil-water pressure head as a function of soil depth for
the three plots of each water treatment of the summer
experiment 20
9 Mean soil-water content as a function of soil depth for the
three plots of each water treatment of the summer
experiment 21
10 Mean soil-air content as a function of soil depth for the
three plots of each water treatment of the summer
experiment .....22
11 Oxygen concentration as a function of soil depth in the plots
of the summer experiment. Symbols are given in Table 2 23
12 Nitrous oxide concentration with depth on one day in the
h = -15 cm treatment of the summer experiment 25
viii
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Number Page
13 Nitrous oxide concentration with depth on one day in the
h = -70 cm treatment of the summer experiment 26
14 Concentration of ^ with depth on one day in the h = -15 cm
treatment of the summer experiment 27
15 Concentration of N£ with depth on one day in the h = -70 cm
treatment of the summer experiment 28
16 Flux of N20 calculated from measurements beneath cover with
time for the manure and cropped plots of the summer
experiment 29
17 Flux of N£O calculated from measurements beneath cover with
time for the uncropped plots of the summer experiment 30
18 Flux of N£ calculated from measurements beneath cover with
time for the manure and cropped plots of the summer experiment. . 31
19 Flux of N2 calculated from measurements beneath cover with
time for the uncropped plots of the summer experiment 32
20 Plant uptake of fertilizer N with time of the summer experiment . . 38
21 Concentration of fertilizer derived NO, in the soil solution as
a function of soil depth for the h = -15 cm treatments of the
summer experiment 39
22 Soil extractable N from fertilizer as a function of soil depth,
5.5 months after fertilizer application, of the summer
experiment 42
23 Soil digestible N from fertilizer as a function of soil depth,
5.5 months after fertilizer application of the summer
experiment 43
24 Mean soil temperature at the 5-cm soil depth as a function of
time after fertilizer was applied to the plots of the winter
experiment. The arrows give the fertilizer application for
each plot with the symbols given in Table 2 46
25 Mean soil-water pressure head at the 5- and 15-cm soil depth as
a function of time after fertilizer application to the two
water treatments of the winter experiment. The arrows give
the fertilizer application for each plot 47
26 Mean soil-water pressure head as a function of soil depth for
the two water treatments of the winter experiment 48
ix
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Number Page
27 Mean soil-water content as a function of soil depth for the
two water treatments of the winter experiment .......... 49
28 Mean soil-air content as a function of soil depth for the two
water treatments of the winter experiment ............ 50
29 Oxygen concentration as a function of soil depth in the plots
of the winter experiment. The symbols are given in Table 2 ... 51
30 Nitrous oxide concentration with depth on one day in the
h = -8 cm treatment of the winter experiment .......... 53
31 Nitrous oxide concentration with depth on one day in the
h = -50 cm treatment of the winter experiment .......... 54
32 Concentration of -"N^ with depth on one day in the h = -8 cm
treatment of the winter experiment ........... ....55
33 Concentration of N? with depth on one day in the h = -50 cm
treatment of the winter experiment ............... 56
34 Flux of N£0 calculated from measurements beneath a cover with
time for the manure and cropped plots of the winter
experiment ........................... 57
35 Flux of N£0 calculated from measurements beneath a cover with
time for the uncropped plots of the winter experiment ...... 58
36 Flux of N2 calculated from measurements beneath a cover with
time for the manure and cropped plots of the winter experiment. . 59
37 Plant uptake of fertilizer N with time during the winter experiment 64
38 Concentration of fertilizer derived NO- in the soil solution as
a function of soil depth for the h = -8 cm treatments of the
winter experiment ........................ 65
39 Soil extractable N from fertilizer as a function of soil depth,
4 months after fertilizer application in the winter experiment. . 68
40 Soil digestible N from fertilizer as a function of soil depth,
4 months after fertilizer application in the winter experiment. . 69
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TABLES
Number Page
1 Texture With Soil Depth 10
2 Fertilizer Application and % Enrichment of Each Treatment .... 14
3 Concentration (ppmv) of N20 Within Profiles Before Application
of Fertilizer of the Summer Experiment. Ambient Concentration
of N20 in Air is Approximately 0.30 ppm 24
4 Denitrification as N20, N2, and Total for the Summer Experiment
as Determined From Measurements Beneath a Cover Placed Over
the Soil 33
5 Ratios of N2 to N20 Produced from Denitrification in the Plots
of the Summer Experiment from Measurements Beneath a Cover
Placed Over the Soil 34
6 Soil Gaseous Diffusion Coefficients Determined by the C02 Flux
Method for the Summer Experiment 35
7 Soil Gaseous Diffusion Coefficients Determined by the Transient-
State Method for the Summer Experiment 36
8 Percentage Recovery of Fertilizer N Leached in the h = -15 cm
Plots of the Summer Experiment Calculated by Various Ways of
Determining the Soil-Water Velocity, vs. The Mean Recovery
of all Depths for Each Plot is Given in Parentheses 41
9 Mass Balance of Fertilizer N for the Summer Experiment 44
10 Concentration of N20 (ppmv) Within Profiles Before Application
of Fertilizer of the Winter Experiment. Ambient Concentration
of N20 in Air is Approximately 0.30 ppmv 52
11 Denitrification as N20, N-, and Total for the Winter Experiment
. as Determined from Measurements Beneath a Cover Placed Over
the Soil 60
12 Ratios of N2 to N20 Produced from Denitrification in the Plots
of the Winter Experiment from Measurements Beneath a Cover
Placed Over the Soil 61
13 Soil-Gas Diffusion Coefficients for the Winter Experiment 62
xi
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Number Page
14 Percentage Recovery of Fertilizer N Leached in the h = -8 cm
Plots of the Winter Experiment Calculated by Various Ways
of Determining the Soil-Water Velocity. The Mean Recovery
of all Depths for Each Plot is Given in Parentheses 67
15 Mass Balance of Fertilizer N for the Winter Experiment. The
Numbers in Parentheses Were Obtained by Calculating Leaching
from Concentration Versus Soil Depth. The Minus Sign
Indicates Greater Recovery Than Fertilizer Applied 70
xii
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LIST OF ABBREVIATIONS AND SYMBOLS
ABBREVIATIONS
cm
m
ml
°C
kg ha
mg
ppm
ppmv
cm^
g
Ug
-1
— centimeter
— meter
— milliliter
— degrees Centigrade
— kilograms (10^ grams) per hectare
— milligram (10 grams)
— parts per million on a weight basis
— parts per million on a volume basis
— cubic centimeters
— gram
— micro grams (10~ grams)
SYMBOLS
N
NH+
NO-
NO"
NH3
N20
N2
15N
C02
02
f
DP
c
x
h
Nitrogen
Ammonium
Nitrite
Nitrate
Ammonia
Nitrous Oxide gas
Nitrogen gas
Nitrogen isotope of mass 15
Carbon dioxide gas
Oxygen gas
gas flux (mg cm day~l)
soil gaseous diffusion coefficient (cnr day"-'-)
— o
gas concentration (mg cm )
distance (cm)
soil-water pressure head (cm)
xiii
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SYMBOLS (continued)
3 —3
6 — soil-water content (cm cm )
3 —3
e — soil-air content (cm cm )
vs — soil-water velocity (cm day )
xiv
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ACKNOWLEDGMENTS
The authors gratefully acknowledge additional financial support for this
research from the National Science Foundation - RANN (Grant No. G134733X),
the Kearney Foundation of Soil Science, and the University of California
Agricultural Experiment Station. The laboratories and mass spectrometer
facilities of the Department of Land, Air and Water Resources provided strong
support for this research.
The authors gratefully acknowledge the able assistance, ingenuity, and
dedication of the three technicians primarily responsible for the establish-
ment, operation, and analytical procedures of this research, David L. Hoffman,
David A. Goldhamer, and Dianne W. Toy. Several students including
Paul Brooks, Nora H. Monette, and Brian D. Brown assisted in analyses, and
their help is greatly appreciated.. The assistance of Tyler Nakashima and
James Burgess, in mass spectrometer maintenance is greatly appreciated. The
use of the gas chromatograph under the control of Professor C. C. Delwiche
was indispensable and is gratefully acknowledged.
xv
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SECTION 1
INTRODUCTION
The amount of N07 reaching the ground water of irrigated lands is
dependent upon each or the components of the N cycle in soils. The amount
of fertilizer N applied and crop uptake of N are easily measured parameters
of the total balance. The other components of the N balance, however, are
not as easily measured or determined. The leaching component of the N balance
has been a subject of much study and is the primary concern in terms of NOo
in ground water. The natural spatial variability of the leaching component
has been demonstrated to be large (Biggar and Nielsen, 1976). The other
components such as residual soil N, denitrification, and NH^ volatilization
losses are also not easily measured. The residual soil N is difficult to
measure due to the large organic N pool of most soils and complicated by the
natural spatial variability of that pool (Rolston, 1977). The transient
processes of immobilization and mineralization within the large organic N
pool also contribute to the complexity of the measurement. Volatilization
loss of fertilizer as NHo occurs to varying degrees in calcareous soils.
This component can generally be minimized by incorporating NH^ fertilizer
below the soil surface. Another loss of N from the soil system for which
absolute amounts and rates are not well known is denitrification. Volatile
denitrification products, primarily ^0 and N£, are evolved whenever anoxic
sites develop within the soil and when sufficient carbon as supplied by soil
organic matter, plant materials, and manure is available.
Simulation models of the N balance in soil systems attempt to predict
the amount and concentration of NOT in irrigation return flow water as a
function of irrigation and cropping practices (Mehran and Tanji, 1974;
Donigian and Crawford, 1976; Shaffer et al., 1976; Tanji and Gupta, 1977; and
van Veen, 1977). In general, the denitrification component of the various
mathematical models has not had adequate input data especially for the rates
of denitrification. Total denitrification of applied fertilizer is used
quite frequently such as 10-15% of the fertilizer N applied (Fried et al.,
1976).
Very few experiments have evaluated the absolute amounts and rates of
denitrification in the field. Rolston et al. (1976) demonstrated that the
volatile gases from denitrification could be measured in a field profile.
Total denitrification from gas fluxes compared reasonably with denitrifica-
tion determined by difference for a small, intensely-instrumented, field plot.
Total denitrification was determined by integrating with time the flux of the
gaseous denitrification products as determined from measured soil gaseous
diffusion coefficients and concentration gradients. However, these studies
only evaluated the amount of denitrification under one cropping or carbon
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input system and one soil-water content near saturation.
The objectives of the research reported here were:
a. To measure denitrification in a field soil directly from the fluxes
of N£ and ^0 at the soil surface.
b. To compare denitrification obtained from N« and ^0 gas fluxes with
denitrification obtained by difference.
c. To measure the amount of denitrification from an applied inorganic
fertilizer source as affected by an actively growing crop or manure
amendment, soil-water content, and temperature of a field soil.
o
The research was conducted on small 1-nr field plots because of the large .. _
cost of NOg fertilizer tagged with high enrichments of the stable isotope TJ.
The use of •*--% tagged NOo is presently the only positive means of measuring
N£ gas resulting from denitrification in the field. The experiments were
conducted at a high and low soil temperature, at two water contents near
saturation, and at three levels of carbon input (cropped, uncropped, and
manure).
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SECTION 2
CONCLUSIONS
The results of this research demonstrate that denitrification can occur
at very large rates with the largest rates occurring soon after application
of N07 fertilizer to soil maintained constantly wet. The rate of denitrifi-
cation generally decreases rapidly after the initially large rate due to a
decrease in NOT concentration and to the movement of the N03 pulse out of the
zone of high carbon content and low oxygen concentrations. For soils in
which the profile development is such that large bulk densities occur near
the surface, denitrification is limited to the top 30 to 60 cm of the soil
profile inasmuch as that is the zone where water content is greatest, soil-
air content is smallest, and biological activity is greatest due to generally
high carbon values. Thus, denitrification would be greatest in the surface
zone. This is not an uncommon situation for many alluvial, irrigated soils
of the West.
Denitrification occurs over a very narrow range of soil-water content on
Yolo loam soil. Very little denitrification occurred in plots at which the
soil-water pressure head was maintained at -70 cm of water. This soil-water
pressure head corresponds to a soil-air content of 9-10%. It would be expec-
ted that for alluvial, loam soils such as Yolo, that very little denitrifica-
tion would occur if soil-water pressures were maintained smaller than
approximately -100 cm of water.
The presence of the crop root system has a large influence on denitrifi-
cation. For the Yolo loam soil, very little denitrification occurred even at
the water content nearest water saturation if a crop was not growing on the
soil. The influence of the root system in providing carbon and consuming Q£
results in anoxic development and provides the carbon necessary for denitri-
fication. As expected, the addition of manure to the soil greatly increased
denitrification over that of the cropped plots. It would be expected that
the addition of other organic materials such as crop residues along with the
NOg fertilizer would also have the effect of substantially increasing deni-
trification. With manure added to the soil and water contents maintained
very close to saturation, approximately 73% of the fertilizer was lost by
denitrification at 23°C. Only 14% of the fertilizer was lost in the cropped,
wet treatment of the summer experiment. Only 3% of the NO^ was lost for the
wet treatment of the summer- experiment without a crop growing on the soil.
At a soil temperature of between 8 and 10°C, very little denitrification
occurred. This was primarily due to limited microbial activity as reflected
in relatively constant Q£ concentrations with soil depth. The largest deni-
trification loss at the low soil temperature occurred in the wet, manure
treatment with 11% of the fertilizer denitrified. The uncropped plots lost
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very little NO^ at the low soil temperatures.
The predominant gas evolved was ^ with total amounts being at least 6
times greater than that for ^0, and in some cases 20 times more N2 than N20
was produced under very anoxic conditions. The N2 to ^0 ratios were general-
ly small (1 to 5) early in the denitrification process and then became much
larger as denitrification proceeded.
Measurement of denitrification from the gaseous components was slightly
smaller than that measured by difference. This failure to account for the
total amount of N apparently denitrified was most likely due to the inability
to continuously monitor gas fluxes,to very small rates of denitrification
over an extended time period after the initially large rates had decreased,
and to errors in the difference method. The sensitivity of the instrumenta-
tion was not sufficient to measure denitrification rates less than approxi-
mately 1 kg N ha"-'- day"*- over many days or weeks. Although denitrification
measured from gas fluxes was less than that determined by difference due
primarily to the lack of sensitivity in measuring small denitrification rates,
measuring N£ and ^0 is the only means that the rates and position of denitri-
fication in the profile can be ascertained. The sensitivity for measuring
small gas fluxes can be increased by increasing the "N enrichment of the
added fertilizer and by increasing the time that covers are left in place at
the risk of changing the normal soil condition.
Calculations of the denitrification gas flux from the soil gaseous dif-
fusion coefficient and gas concentration gradients measured within the soil
compared reasonably with measured fluxes of N£ and N£0 as determined by
taking gas samples from beneath the cover placed over the soil surface for
1- or 2-hour time intervals on each sampling day. This statement is valid
only if the diffusion coefficient is measured immediately before and after
an experiment is conducted, since the values of the diffusion coefficient
were very much dependent upon surface soil conditions. Since No concentra-
tions within the soil were greater than those under a cover, increased sensi-
tivity should be possible if concentration gradients and diffusion coeffi-
cients can be accurately measured.
All NO^ had been leached from the profile of the wet treatments by 7
months after application of the fertilizer. In the dry plots, most of the
NO^ was still in the upper 60 cm of the soil profile after 5 months. There-
fore, the amount of water applied and the soil-water velocity has an enormous
effect on the amount of N0~ leaching and the position of the NO, within the
soil profile.
Determination of the leaching component was highly uncertain due to
errors in determining the soil-water flux and variability in Ng^ concentra-
tions. For the low-temperature experiment, it was impossible to obtain a
mass balance inasmuch as N leaching was greater than N applied as fertilizer.
This discrepancy was attributed primarily to an overestimation of the soil-
water flux resulting from spatial variability, flux changes with time, and
anion exclusion or immobile water. Errors are also possible in measuring the
residual soil N. Thus, the determination of denitrification by difference is
highly uncertain.
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Both cropped plots of the wet and dry treatments took up fertilizer N
at approximately the same rate until the NO^ was either denitrified or leached
from the upper part of the profile. The rate of uptake for the wet treatments
decreased to near zero after the NO^ had been leached from the upper 60 to 90
cm of the profile, whereas uptake continued in the dry treatments where the
NOo was still in the upper portion of the profile and being taken up by the
grass.
It would be expected that considerable fertilizer N would be immobilized
as live and dead roots in the cropped plots. However, there was also consid-
erable immobilization of fertilizer N in the manure and uncropped plots.
This immobilization most likely occurred as microbial biomass and the roots
of a few weeds and moss which were difficult to keep from the plots.
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SECTION 3
RECOMMENDATIONS
These results demonstrate that the addition of manure with the fertilizer
greatly increased denitrification of the fertilizer NO^. Thus, it is recom-
mended that manure not be applied with fertilizer. Manure applications should
be timed such that the NO^ from fertilizer does not occur at the same position
within the profile as does the carbon from manure. It also becomes important
that water management be controlled even more carefully when manure is applied
than with inorganic fertilizers in order to prevent anoxic development and
denitrification. There is most likely some loss of N which is mineralized
from the manure added to soil due to anoxic development during periods of
high water content. Thus, the N in manure may not be as effective as an equal
amount of N from inorganic fertilizers due to the inclusion of considerable
carbon in the manure to increase denitrification.
Irrigation management practices directed at minimizing N loss from deni-
trification should be designed with the idea of maintaining soil-water
pressure or water content within a range that anoxic conditions will most
likely not develop for that part of the profile with the largest amount of
carbon. For loam soil similar to Yolo, very little denitrification should
occur if soil-water pressures are maintained smaller than -100 cm (-10 centi-
bars) or tensions greater than 10 centibars. This limit of tension would be
larger for a clay soil and smaller for a sandy soil. The degree of soil
aggregation would also influence these limits.
Another management recommendation which may be made is that if water
contents or pressures cannot be maintained outside the range where denitrifi-
cation will occur, it may be possible to time fertilizer application so that
NOo will not occur in the zone along with the carbon at a time when water
contents will be close to saturation. It may be possible, for instance, to
apply NHj fertilizer immediately before a large irrigation for wetting a
profile at the beginning of the irrigation season. The next irrigation would
be timed and managed such that the NOg would be moved out of the zone where
most of the carbon occurs, yet remain within the zone where roots could obtain
the N. This management approach would minimize the potential for denitrifi-
cation in soils similar to the profile described in this report.
t
Field research should be conducted to determine such factors as the rate
that microbial populations increase after irrigating an initially dry soil.
Along with the population increase, the irrigation frequency and water con-
tents maintained between irrigations should have an enormous influence on
denitrification during normal irrigation cycles. Management decisions to
maximize fertilizer use efficiency and minimize NO^ leaching to ground water
-------
can be made only after the dynamics of microbial population growth,
leaching, and denitrification rates in field sites are ascertained.
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SECTION 4
EXPERIMENTAL PROCEDURES
FIELD INSTALLATION
Twelve, 1-m2 field plots were established on Yolo loam soil (Typic
Xerorthents) at Davis, California. The Yolo loam soil is a deep, well-
drained, alluvial soil in the Sacramento Valley. The soil is a prominent
agricultural soil of the area and is similar to other soils of extensive
acreage. A schematic diagram of the experimental location and the treatment
layout is given in Figure 1. Each of the 1-m2 plots was established with a
MANURE
UNCROPPED
CROPPED
DRY TREATMENT
HIGH
TEMPERATURE
EXPERIMENT
LOW
TEMPERATURE
EXPERIMENT
UNCROPPED
CROPPED
WET TREATMENT
Figure 1. Schematic of plot layout and treatments.
-------
60-cm deep redwood barrier around the outside edges of each undisturbed block
of soil. The redwood barriers were installed by digging a trench around the
l-m2 area, slipping the redwood over the undisturbed block of soil, and back-
filling the trench on the outside of the redwood. The space between the
redwood barrier and the soil on the inside was sealed by pouring melted
paraffin into the small crack between the soil and the wood. Each of the
twelve plots was instrumented with tensiometers, soil solution samplers, soil
atmosphere samplers, and thermocouples. Five soil atmosphere samplers were
installed at the 2-, 5-, 10-, and 15-cm depths; three soil atmosphere samplers
were installed at the 20-, 30-, 60-, 90-, and 120-cm soil depths. Triplicate
samplers, designed to function as tensiometers or solution extractors, were
installed at the 60-, 90-, 120-, 150-, and 180-cm depths of the soil profile.
Triplicate tensiometers were also installed at the 5- and 15-cm depths. Du-
plicate thermocouples were installed at the 5-cm depth. Soil solution sam-
plers consisted of porous cups glued to polyvinyl chloride tubing. All of
the deeper solution samplers and gas samplers were installed on an angle begin-
ning on the outside of the 1-m^ plot with the cup or the tip of the gas sam-
pler ending up in the center of the l-m^ area of the plot. Soil atmosphere
samplers consisted of 0.1 cm inside diameter nylon tubing glued into a 5-cm
long perforated acrylic plastic tube. For the deeper soil depths the small
diameter nylon tubing was placed inside a 1.3-cm diameter polyvinyl chloride
tube and the nylon tubing glued into a milled plastic tip. For all samplers,
the volume of the sampling tubes was very small. Soil solution samples were
obtained by evacuating bottles connected to samplers. Soil atmosphere sam-
ples were obtained by withdrawing 1-ml increments of gas with plastic, dispos-
able syringes. All gas samples were analyzed within a few hours after
sampling.
Six plots were constantly maintained at a soil-water pressure head (h)
very close to water saturation and six plots were maintained drier than near
saturation but wetter than field capacity. The soil was maintained at a
constant water content or pressure with a spray irrigation system which
consisted of spray nozzles on a traveling boom. The system was activated by
a timer so that plots could be irrigated on as frequent an interval as neces-
sary to maintain constant soil-water content conditions.
In order to establish different carbon treatments within each of the two
water regimes, two plots were cropped with perennial ryegrass (Lolium Perenne)
for approximately 4 months prior to the experiment. Two plots remained un-
cropped, and manure at a rate of 3.4 x 10 kg ha was mixed in the top 10cm
of soil of four plots approximately two weeks before fertilizer was applied.
The two water content treatments and the three carbon treatments were
also conducted at two times of the year in order to evaluate the effect of
soil temperature on denitrification. One experiment was conducted in July
when the average soil temperature at the 5-cm depth was approximately 23°C
during the experimental period. Another experiment was conducted in January
when the average soil temperature at the 5-cm depth was approximately 8°C
during the experimental period. The entire experimental area was covered
with plastic sheeting in order to prevent rain from changing the constant
soil-water content conditions while experiments were being conducted.
-------
The particle size analysis as a function of soil depth for the Yolo loam
soil is given in Table 1 and the average bulk density at the field site is
given as a function of depth in Figure 2. The bulk density is greatest near
TABLE 1. TEXTURE WITH SOIL DEPTH
Depth
0
15
30
60
90
120
150
- 15
- 30
- 60
- 90
- 120
- 150
- 180
Sand (%)
41
40
42
38
38
32
25
Silt (%)
37
37
38
42
42
46
51
Clay (%)
22
23
20
20
22
24
Loam
Loam
Loam
Loam
Loam
Silt Loam
Silt Loam
BULK DENSITY (g cm"3)
1.20 . 1.30 L.40
o WINTER
SUMMER
1.50
Figure 2. Bulk density as a function of soil depth for experimental area.
10
-------
the soil surface with a minimum at the 120-cm depth. Bulk density was deter-
mined on 7.6 cm long, 7.6 cm diameter undisturbed soil cores.
The percentage of organic carbon as a function of soil depth for the
uncropped and manure plots of the summer and winter experiments is given by
Figure 3. Organic carbon was determined on soil samples taken after experi-
O
UJ
Q
o
c/)
0
20
40
60
80
100
120
ORGANIC CARBON (%)
0 0.2 0.4 0.6 0.8 1.0 1.2
.6 1.8
° Uncropped
— • Manure - Summer
* Manure- Winter
Figure 3. Organic carbon as a function of soil depth for the uncropped and
manure plots of the summer and x/inter experiments.
ments were conducted, so several months had elapsed after manure application.
The soil for organic carbon determination was ground to pass a 2mm sieve or
finer. A subsample was then thoroughly ground with a pica mill to pass a 60
mesh sieve. Approximately 0.2 grams of the soil sample were placed in a
crucible to which a small amount of iron and tin accelerator was added. The
sample was covered with a single hole lid and placed into an induction furna:e.
The (X>2 produced was collected in a Nesbit tower containing ascarite. The
tower was weighed before and after the burn to determine the amount of carbon
dioxide trapped. There was no difference in the % carbon between a soil
11
-------
sample that had been extracted with KC1 and a sample that had not been
extracted.
Figures 4 and 5 gives soil-water characteristic curves (soil-water con-
tent versus soil-water pressure) at several depths of the experimental site.
The data points are the means of at least three 7.6-cm diameter, 7.6-cm long
undisturbed soil cores. Figure 4 gives curves from surface cores obtained
within the l-m^ plots of the wet (South) and dry (North) water treatments
after the denitrification experiments were completed. Figure 5 gives curves
for cores obtained at various depths outside the 1-m2 plots.
0.48
•? 0.46
E
o
o
O
O
0.42
O.4O
0.38
^ 0.36
.J
O
w 0.34
0.32
SURFACE CORES
INSIDE SOUTH SUMMER
INSIDE NORTH SUMMER
INSIDE
NORTH WINTER
I
I
I
I
0 -20 -40 -60 -80 -100 -120 -140
SOIL-WATER PRESSURE HEAD h (cm H20)
Figure 4. Soil-water content as a function of the soil-water pressure head
for surface soil within plots.
12
-------
0.30
0 -20 -40 -60 -80 -100 -120 -140 -160 -180 -200
SOIL-WATER PRESSURE HEAD h (cm H20)
Figure 5. Soil-water content as a. function of soil-water pressure head for
several soil depths.
EXPERIMENTAL PROCEDURES - FIELD
After constant, steady-state water content conditions were achieved in
the plots, N03 solution equivalent to 300 kg N ha was uniformly applied to
the plots by applying approximately 3 ml of solution to each of 400 points at
the soil surface. The 15N enrichment of the added fertilizer was 20 and 40%
excess 1% for the wet and dry treatments, respectively. The actual amounts
and enrichments applied to each plot are given in Table 2.
Within 6 hours after applying the fertilizer solution, an airtight cover
was placed over the plots. The cover consisted of a thick sheet of acrylic
plastic with rubber tubing on the lower edge to make an airtight seal with
the top of the redwood border. Samples of the atmosphere beneath the cover
were taken after one or two hours with the lid in place and analyzed for 15N2
and 15N20. Soil atmosphere samples from within the soil profile were also
taken soon after applying the fertilizer. Soil atmosphere samples were taken
13
-------
TABLE 2. FERTILIZER APPLICATION AND 15N ENRICHMENT OF EACH TREATMENT
High-Temperature
Experiment
Cropped, h
Manure, h
Uncropped,
Cropped, h
Manure, h
Uncropped ,
= -15 cm
= -15 cm
h = -15 cm
= -70 cm
= -70 cm
h = -70 cm
Plot
#
IS
2S
3S
IN
2N
3N
% 15N Excess
19.4
19.3
19.2
38.3
39.2
38.4
Fertilizer
Rate (kg N ha"1)
296.0
300.6
304.9
297.4
298.4
292.8
Low-Temperature
Experiment
Cropped, h
Manure, h =
Uncropped ,
Cropped, h
Manure, h =
Uncropped,
= -8 cm
-8 cm
h = -8 cm
= -50 cm
-50 cm
h = -50 cm
4S
5S
6S
4N
5N
6N
19.8
19.8
19.4
39.9
40.0
39.6
299.9
294.3
294.0
296.5
297.6
294.2
in 1-ml aliquots and N20, C02, 02, and N2 analyzed by gas chromotography in
the laboratory. Another 0.5 to 1 ml of gas was taken to determine 15N2 with
a mass spectrometer. All soil samples to be analyzed by mass spectrometry
were pulled through ascarite, dehydrite (magnesium perchlorate), and a com-
mercial 02 scrubber. Gas samples from the profile and samples from beneath
the cover were taken daily for a few days after the fertilizer was applied
and then at less frequent intervals until ^^N2 and %20 could no longer be
detected above background.
Soil solution samples were taken at weekly intervals. Thg grass of the
cropped plots was cut periodically, and the total clippings dried for analysis.
After the NO^ had been completely leached through the 180-cm depth of the wet
treatments, both the wet and dry plots were allowed to dry and soil samples
taken in 15-cm increments down to 120 cm. The samples consisted of eight
separate holes taken with a Veihmeyer tube within the 1-m2 plot. On some
plots, each of these samples was run individually. On others, the eight
14
-------
samples from each depth were combined into two samples at each depth.
Soil gas-diffusion coefficients were determined for the plots by the CC>2
flux method. This method measures the CC>2 trapped in a NaOH solution from an
air stream flowing over the top edge of the plot. The C02 concentration gra-
dients within the soil were measured several times during the CC^-trapping
process. The C02 concentration gradient and CC>2 flux were used in calculat-
ing the diffusion coefficient of CC^ from
f = -Dp(3C/3x) (1)
where f is the gas flux (mg cm day "•'•) , D is the soil gas-diffusion
coefficient (cm2 day"-'-) , C is the gas concentration (mg cm~3) t and x is dis-
tance (cm) . The diffusion coefficient for ^0 is about equal to that for C02.
The diffusion coefficient for No is calculated by multiplying the diffusion
coefficient for CC>2 by 1.25, which is the ratio of the diffusion coefficient
of N2 to the diffusion coefficient of C02 in air at 0°C. The basic assump-
tion in the CC^ flux method is that there is no production or consumption of
CC>2 within the zone over which the gradient is measured or beneath the cover
and that steady-state conditions exist throughout the sampling period. Thus,
the method is not applicable to the cropped plots.
Soil gas-diffusion coefficients were also determined for the plots by a
transient-state method. The one-dimensional, transient-state equation for
diffusion of a non-reactive gas in soil is
?C (2)
3te 9x2
o
where C is concentration of the gas with reference to the gas phase (cm gas
cm" 3 soil air) , t is time (hours) , D is the soil gaseous diffusion coeffi-
cient (CTB~ hour~^) , e is the soil-air content (car air cm~3 soil) , and x is
soil depth (cm) . Values of D and e are assumed to be constant with respect
to x and t. Equation (2) was solved analytically using a time-dependent
upper boundary condition. Such a condition could be easily achieved experi-
mentally by feeding gas at a constant rate into an enclosed chamber above the
soil with ports for the mixed gas to leave the chamber. If N2 is the gas of
interest, the N~ concentration within a chamber above the soil could be
decreased with time by feeding gas of zero N2 concentration into the chamber
with the mixed gas leaving the chamber through exit ports. The N£ concentra-
tion at the soil surface would decrease as some function of time. For the
general case where the concentration at the soil surface varies with time by
a smooth function, the surface condition may be described by a step function
given by
15
-------
C(0,t) = SQ t0 < t
S
S2 t2 < fc - t3
Sk
Sn tn < * (3)
where Sfc is the surface concentration within any time interval k. The solu-
tion of Eq. (2) using Eq. (3) and
C = S0 x > 0 t = 0 (4a)
is
L
C(x,t) = SQ + Z (Sk - 5^}) erfc {x/ [2/Dm(t-tk) ] } (5)
k=l
where L = {k:t > tk>, that is, k is the largest integer such that tfc is less
than t, Dm = Dp/e, and k is a dummy variable. The only assumptions inherent
in the use of Eq. (5) for determining the diffusion coefficient are that Dm
is constant with respect to x and t and that diffusion is not allowed to
proceed so long that concentrations markedly change at the bottom of a field
box.
The general procedure for determining the diffusion coefficient D was to
fit Eq. (5) to concentration profiles measured in the field at several times,
using values of S^ and t obtained from interpolation of measured values at
the surface and at t = 0, and various values of Dm. The least absolute dif-
ference was used to evaluate the best fit of the predicted to the measured
profiles. The use of the absolute difference gave a more clearly defined
minimum (best fit) than did least squares.
ANALYTICAL TECHNIQUES
Oxygen, N£, CO^, and N-0 were analyzed by gas chromotography with a
thermal conductivity detector. The concentration of N-0 in the gas samples
was determined by chromatography with a helium ionization detector according
to the procedure described by Delwiche and Rolston (1976).
16
-------
The isotopic composition of N in gas samples was determined on samples
scrubbed for 02> CC^, and l^O by direct injection into the mass spectrometer.
An independent experiment was conducted in which manure was added to soil in
order to determine if gaseous compounds were being evolved from the manure
and soil mixture which would interfere with the -*--%2 analyses. These experi-
ments showed that no such evolution occurred to any significant degree.
Soil samples were analyzed for extractable and digestible nitrogen and
soil solution samples were analyzed for NET, NO^, and N07. The soil sample
was extracted with 1.0 N KC1 and the solution analyzed by MgO-Devarda alloy
reduction technique. The extraction procedure removed solution NHT, NO^,
NO^, and exchangeable NH^. The NHj~ and NO^ concentrations in all soil and
soil solution samples were negligible. The Kjeldahl method was used to
determine the total digestible N in the soil and plant samples. Two-g samples
of soil were digested with 36N H2SO^ and salts (KoSO^, CuSO,, and selenium)
for approximately 17 hours to convert the N to NHT. The same procedure was
used for the plant digests except that 0.25 g of plant material was used, and
digestion time was 6 hours. The N in the digests was determined by titration
of the NH+ liberated by distillation of the digest with 40% NaOH. Detailed
procedures for determination of N in soil, plant, and soil solution samples
are given by Bremner (1965).
17
-------
SECTION 5
RESULTS AND DISCUSSION
HIGH-TEMPERATURE EXPERIMENT
Plot Characteristics
Temperature at the 5-cm soil depth as a function of time after the ferti-
lizer was applied to the plots of the summer experiment is given by Figure 6.
0
10 20
30 40 50 60 70 80
TIME (days)
Figure 6. Mean soil temperature of two probes at the 5-cm soil depth of six
plots as a function of time after fertilizer was applied to the
plots of the summer experiment. The arrows give the fertilizer
application for each plot with the symbols given in Table 2.
18
-------
The arrows give the time that fertilizer was applied to particular plots.
The average soil temperature during most of the denitrification activity was
approximately 23°C. There was a slight increase in soil temperature begin-
ning about two weeks after the fertilizer was applied to the first plot. The
increase in soil temperature occurred at the time fertilizer was applied to
the cropped and uncropped plots of the dry treatment.
The increase in soil temperature at two weeks after fertilizer was
applied to the first plot coincided with a decrease in the soil-water pressure
of the dry plots. The soil-water pressure head, h, for the 5- and 15-cm
depths as a function of time after fertilizer application for the summer ex-
periment is given by Figure 7. For the h = -15 cm and h = -70 cm soil-water
-160
SUMMER
• 5cm depth
o 15cm
30
TIME (days)
Figure 7. Mean soil-water pressure head of three tensiometers in each plot
at the 5- and 15-cm soil depth as a function of time after ferti-
lizer application to the two water treatments of the summer experi-
ment. The arrows give the fertilizer application for each plot
with the symbols given in Table 2.
pressure treatment, the soil-water pressure head for the h = -15 cm treatment
was relatively constant throughout the denitrificafeion period, whereas the
19
-------
soil-water pressure for the h = -70 cm treatment varied due to changing tem-
perature and radiation conditions. Soil-water pressure values were main-
tained by reading tensiometers daily and making changes in number of passes of
the irrigation system in order to correct any increases or decreases in the
soil-water pressure head from the desired values. It was sometimes difficult
to maintain constant water content in the h = -70 cm plots during rapid
changes in air temperature. Values of the soil-water pressure head as a
function of depth are given by Figure 8 for the h = -15 and -70 cm treatments.
SOIL-WATER PRESSURE HEAD h(cmH20)
-40 -80 -120 -160 -200
SUMMER _
Figure 8. Mean soil-water pressure head as a function of soil depth for the
three plots of each water treatment of the summer experiment.
The soil-water pressure head values are average numbers of the three plots of
each soil-water treatment. In a similar manner the soil-water content, 0, as
a function of depth for the two water treatments is given in Figure 9. It is
obvious from Figures 8 and 9 that soil-water content, and soil-water pressure
both decrease with depth. This is due primarily to the fact that the soil
surface tended to seal with time resulting in lower infiltration rates in the
surface than in the remainder of the profile. This characteristic of decreas-
ing water contents and pressures with depth results in the possibility of
20
-------
SOIL-WATER CONTENT 6 (cm3 cm'3)
0.30
0.33
0.36
0.39
0.42
0.45
Figure 9. Mean soil-water content as a function of soil depth for the three
plots of each water treatment of the summer experiment.
limited denitrification occurring in the lower part of the profile. From soil-
water content and soil bulk density, the soil-air content, e, as a function
of depth can be calculated and is plotted in Figure 10 for the two soil-water
treatments. These values of e are average values for the three plots of each
water treatment. The'h = -15 cm and h = -70 cm treatments result in only
very small differences between the soil-air content in the surface soil.
However, as will be shown later, this small difference is sufficient to make
a very large difference in the denitrification rate and magnitude.
21
-------
SOIL-AIR CONTENT e(cm3cm~3)
0.08
0.16
0.24
Figure 10. Mean soil-air content as a function of soil depth for the three
plots of each water treatment of the summer experiment.
Soil Gases
The amount of 02 in the soil and the diffusion rate of 02 within the
soil profile is one of the very important factors determining whether denitri-
fication will occur or not. Figure 11 gives 02 concentrations within five
plots of the summer experiments as a function of soil depth. Although mea-
surements were not made on the uncropped, h = -15 cm treatment, the 02 concen-
tration would be expected to be no smaller than 10%. The oxygen concentra-
tions were measured during the time of the denitrification experiments.
For the three plots at h = -70 cm, 02 values decreased to no less than
approximately 16% and that occurred in the plot to which manure had been added.
For the plots maintained at a soil-water pressure head of -15 cm, 02 values
became as small as 9% at the 20-cm depth of the cropped plot and as low as
approximately 3% at the 10-cm depth in the plot to which manure had been added.
In all cases, the smallest concentration of 02 occurred at the 10-to 20-cm
22
-------
02 CONCENTRATION (%)
10
20
- 30
^j
I
^~~ 40
0-
5 50
CO
60
70
80
90
10
15
20
25
SUMMER
Figure 11. Oxygen concentration as a function of soil depth in the plots of
the summer experiment. Symbols are given in Table 2.
depth in the profile with higher concentrations toward the soil surface and
also higher concentrations deeper with depth in the profile. This phenomenon
has also been observed by Rolston et al. (1976) where G£ levels were smallest
near the soil surface and increased with depth due to smaller soil-water
pressures, smaller water contents, and more diffusion of 62 from the deeper
depths of the soil profile. Since these samples were determined by extract-
ing a volume of gas with a syringe, the concentrations reported here are most
likely those concentrations in the large, continuous pores of the soil profile.
Thus, the G£ concentrations only give a qualitative estimate of the anoxic
conditions within the profiles.
Table 3 gives initial concentration of N£0 within the six plots of the
summer experiment before application of NO^ fertilizer. This gives an idea
of the degree of reducing conditions within the profiles and the denitrifica-
tion of native nitrogen. These data indicate that the ^0 concentration
before fertilizer application was dependent upon the reducing conditions of
23
-------
TABLE 3. CONCENTRATION (PPMV) of NO WITHIN PROFILES BEFORE APPLICATION OF
FERTILIZER OF THE SUMMER EXPERIMENT. AMBIENT CONCENTRATION OF N~C
IN AIR IS APPROXIMATELY 0.30 PPM
Soil
Depth (cm)
2
5
10
15
20
30
60
90
Cropped
0.08
0.09
0.09
0.19
0.37
0.29
1.25
h = -15 cm
Manure
4.0
2.6
6.8
32.4
26.3
38.4
9.2
Uncropped
2.6
3.1
3.3
3.3
3.0
3.5
Cropped
0.81
0.46
0.23
0.78
0.96
1.2
h = -70 cm
Manure
32
39
40
37
41
30
16
Uncropped
1.33
0.71
1.19
1.06
1.52
4.31
the profile and the amount of soil nitrogen available. The concentrations in
the cropped plots were initially near ambient. This was most likely due to
the grass using nitrogen which was slowly mineralized from soil organic matter
and to the reducing conditions within the root zone caused by root and micro-
bial respiration with the overall result of reduction of any N20 to N2.
Almost all samples from the h = -15 cm, cropped plot were below ambient indi-
cating very little production of N20 or reduction of N20 to N2. The manure
plots initially had N20 concentrations ranging from 2 to 41 ppm. Apparently
there was some denitrification of nitrogen mineralized from the manure. The
h = -70 cm manure treatment had generally higher N20 concentrations than did
that of the h = -15 cm, manure treatment. Nitrous oxide concentrations in
the uncropped plots ranged from . 7 to 4 ppm. Apparently there was some miner-
alization of soil organic matter in the uncropped plots resulting in a small
amount of N which could be denitrified forming N20. However, conditions were
apparently not anoxic enough that the N20 was further reduced to N2.
Representative N20 concentration profiles from denitrification of the
added fertilizer as a function of soil depth for two days after fertilizer
application for the h = -15 cm treatments are given by Figure 12. The concen-
tration profiles demonstrate the enormous differences in N20 production among
the cropped, manure, and uncropped plots. It appears that the greatest
denitrification in the manure plot was occurring in the upper 1$ cm of soil,
whereas denitrification in the cropped plot appeared to be occurring slightly
deeper. Similar data for the plots of the h = -70 cm treatment for two days
after application of fertilizer are shown by Figure 13. There was much less
production of N20 in the h = -70 cm treatments than in the h = -15 cm treat-
ments. Since N20 concentrations did not become great enough to measure 15N
without concentrating the N20 sample, N20 concentrations greater than initial
values were considered to be derived from the fertilizer.
24
-------
N.O CONCENTRATION (ppm v)
I
I-
o.
0
10
20
30
200
400
600
800
O
to 40
50
60
SUMMER
Day I
h = -!5cm
CROPPED
MANURE
UNCROPPED
Figure 12. Nitrous oxide concentration with depth on one day in the h
cm treatment of the summer experiment.
= -15
25
-------
0
I
10
20
x
t-
30
o
co 40
50
60
N20 CONCENTRATION (ppm v)
100 200
300
SUMMER
Day 2
h = - 70 cm
• CROPPED
o MANURE
A UNCROPPED
Figure 13. Nitrous oxide concentration with depth on one day in the h = -70
cm treatment of the summer experiment.
t
Similar representative N£ profiles derived from the fertilizer for two
days after application of the fertilizer are given in Figures 14 and 15 for
the h = -15 and h = -70 cm treatments, respectively. The concentration
profiles for No gas follow the same pattern as do those of the ^0, with
considerable differences in N2 production among the cropped, manure, and
uncropped plots.
26
-------
ISL FROM FERTILIZER (mg N liter"1 soil air)
SUMMER
Day 2
h = -15 cm
• CROPPED
o MANURE
A UNCROPPED
Figure 14. Concentration of 15N2 with depth on one day in the h
treatment of the summer experiment.
= -15 cm
27
-------
X
I-
Q_
UJ
O
O
If)
N2 FROM FERTILIZER (mg N liter'1 soil air)
,0 0.2 0.4 0.6
10
20
* 30
40
50
60
0.8
SUMMER
Day 2
h =- 70 cm
• CROPPED
o MANURE
AUNCROPPED
Figure 15. Concentration of N2
depth on one day in the h = -70 cm
treatment of the summer experiment.
Gas Fluxes
Flux of ^0 derived from the fertilizer as a function of time for the
manure and cropped plots is given by Figure 16. These fluxes were determined
from the accumulation of N£0 beneath a cover placed over the soil surface for
1- or 2-hour intervals. The time interval that the covers remained on the
plots was calculated from the air volume of the cover above the soil, the
enrichment of the fertilizer, the minimum detectable limit of 1%2 by
spectrometer, and the criterion that an N£ flux of at least 1.0 kg N
was measurable. The numbers below each of the soil-water pressure head treat-
ments are total N£0 produced as determined from the area bene%th each of the
flux versus time curves. The flux calculated from the concentration increase
beneath the cover was corrected for the decrease in the concentration gradient
with time as the gases accumulated. The decrease in the concentration gra-
dient with time would result in an underestimation of the diffusive flux as
measured fromconcentration changes beneath the cover. A simple correction
for this underestimation of the flux can be made based upon the steady-state
28
-------
o»
cr
LU
8.0
6.0
>\
-g 4.0
2.0
£ o
n 0.8
0.6
0.4
CJ
0.2
"I—I—\
.4 kg N ha'
MANURE
CROPPED
= -!5cm
_4.3 kg N ha"
1.8 kg N ha"
l I l I I I l I I I I I I
0
5 10
TIME (days)
15
Figure 16. Flux of NnO calculated from measurements beneath cover with time
for the manure and cropped plots of the summer experiment.
diffusion equation,
f = -Dp(dC/dx)
(6)
where f is the gaseous flux, D is the soil gaseous diffusion coefficient, C
is gas concentration, and x is soil depth. If it is assumed that the concen-
tration at the soil surface is equal to the concentration beneath the cover
and that the concentration at the shallowest sampling depth (2 cm) does not
change with time, the following boundary conditions apply:
x = 0 cm
x = 2 cm
C = C2
and (7]
Dp = -(VL)(Atr1to[(C2-C0)/C2]
The solution to Equations (6) and (7) rearranged to solve for D is
(7a)
(7b)
(8)
29
-------
where V is the volume of the chamber placed over the soil surface, L is the
depth of soil for which measurements are taken (2 cm), A is the cross-
sectional area of the soil covered with the chamber, t is the time after
covering the soil at which the concentration beneath the lid (Co) was measured,
and An is the natural logarithm. The value of Dp was determined from Equation
(8) for the 0 to 2-cm depth interval from the change in concentration (Co) of
N20 over a 1- or 2-hour time period and the measured concentration at the 2-cm
depth (C2). The calculated Dp and the measured concentration gradient (dC/dx)
at t = 0 (assumed to be linear for 0 to 2 cm) were used to calculate the
corrected flux from Equation (6). This corrected flux was the best estimate
of the gaseous flux of N20 if the cover had not been placed over the soil
surface. Measurements of concentration at the 2-cm depth both before and
after the cover had been in place for 2 hours demonstrated that Condition (7)
was valid for a 2-hour period. The greatest correction in the calculated flux
was for the cropped (h = -15 cm) treatment. Corrections for all other plots
were less than 5% of the total denitrification.
Similar data for the flux of N20 from the fertilizer as a function of
time for the uncropped plots are given by Figure 17. The scale of the
0.48
CSJ
TIME (days)
Figure 17. Flux of N20 calculated from measurements, beneath cover with time
for the uncropped plots of the summer experiment.
30
-------
ordinate is considerably smaller than that for the cropped and manure plots.
Both Figures 16 and 17 show that ^0 could not be detected beneath the lid
after approximately 20 days.
Similar data for the flux of N£ derived from the fertilizer as a function
of time for the manure and cropped plots are given by Figure 18. These fluxes
10 20
TIME (DAYS)
30
Figure 18. Flux of N2 calculated from measurements beneath cover with time
for the manure and cropped plots of the summer experiment.
were also determined from the accumulation of N£ beneath a cover placed over
the soil surface and corrected for the decrease in the concentration gradient
as N£ accumulated. The numbers below each of the soil-water pressure head
treatments are total No produced.
The increase in the N2 flux at 1.4 days of the h = -15 cm, manure treat-
ment (Figure 18) was from a sample taken in the afternoon of a fairly hot day.
Thus, the fluxes may be slightly underestimated for a few days after applica-
tion due to increased denitrification as surface soil temperature increased
in the afternoon. The increase in the N£ flux for the h = -15 cm, cropped
31
-------
treatment at approximately 3 days after the fertilizer was applied coincided
with the increase in temperature (Figure 6). The N2 flux from the uncropped,
h = -70 cm treatment would also be expected to increase due to the temperature
increase. There appeared to be a slight increase in flux although the total
flux was small and any increase would have a small effect. Data for the flux
of N£ from the fertilizer as a function of time for the uncropped plots are
given by Figure 19. The scale of Figure 19 is considerably different than
O>
- 3
tr
UJ
N
^ 2
tr
LJ
O
DC
U_
CM
I I
UNCROPPED
hs-15cm
5.7 kg N ha
-i
h = -70cm
3.6 kg N ha'1
5 10
TIME (DAYS)
Figure 19. Flux of N£ calculated from measurements beneath cover with time
for the uncropped plots of the summer experiment.
that of Figure 18. The very high initial N£ denitrification rate and rapid
decrease in the rate are similar to the behavior of the ^0 flux. In all
cases, the amount of N£ produced was much greater than N20. f
The data given in Table 4 are the total amount of denitrification con-
sisting of both N£ and N20 for the six treatments of the summer experiment as
determined from measurements beneath a cover placed on the soil surface over
the time period for which N2 and ^0 were measureable. The amount of denitri-
fication ranged from 1% for uncropped, h = -70 cm treatment to 69% for the
32
-------
TABLE 4. DENITRIFICATION AS N20, N2, AND TOTAL FOR THE SUMMER
EXPERIMENT AS DETERMINED FROM MEASUREMENTS BENEATH A
COVER PLACED OVER THE SOIL
Total
Treatment
N20
(kg N ha"1) (kg N ha"1) (kg N ha'1) (% of fertilizer)
Manure,
h = -15 cm
Manure,
h = -70 cm
Cropped,
h = -15 cm
Cropped,
h = -70 cm
Uncropped,
h = -15 cm
Uncropped,
h = -70 cm
9.9
5.4
4.3
1.8
2.1
0.6
198
42
30
7
5.7
3.6
208
47
34
9
8
4
69
16
11
3
3
1
manure, h = -15 cm treatment. It is obvious that water content and carbon
level had an enormous influence on the total denitrification.
The N2 to N20 ratio as a function of time after application of the NOT
to each of the plots is given in Table 5. The average ratio calculated on
the basis of total amount of each gas produced is also given. The largest
ratio of 20.0 occurred for the manure treatment at the soil-water pressure
head of -15 cm. All other treatments gave N2 to N20 ratios of between approx-
imately 2.7 and 8.0. The ratio of 6.0 for the uncropped, h = -70 cm treat-
ment is most likely not real due to the error in measuring very small fluxes.
For very anoxic conditions it would be expected that the ratio would be large
as is demonstrated by the large ratio of the manure plot compared to the
other treatments. Without manure added to the plots the data show that there
is still considerably more N2 produced than N20.
Some N2 and N20 may have diffused below the 60 cm deep barrier of each
plot and lost. The amount of gas lost in this manner was calculated from the
N2 and N20 concentration gradient between the 30-and 60-cm depths and from
the average gaseous diffusion coefficient between the two depths. Using a
diffusion coefficient of 5 cm^ hr"1 corresponding to a soil-air content of
0.12 (Rolston and Brown, 1977) the amount of N2 and N20 diffusion below the
60 cm depth was calculated to be 10, 5.5, and 2.5 kg N ha'1 for the manure,
cropped, and uncropped plots, respectively, of the h = -15 cm water treatment.
33
-------
TABLE 5. RATIOS OF N2 TO N20 PRODUCED FROM DENITRIFICATION IN THE PLOTS OF THE
SUMMER EXPERIMENT FROM MEASUREMENTS BENEATH A COVER PLACED OVER THE SOIL
10
Treatment N2/N20 ratio
Days after NO^ application
0.25 1 2 34 56 78 10 13
Manure ,
h = -15 cm 8.8 8.3 16.4 22.5
Manure,
v. — _7f) pm 9 Q A7 9 Q An A
Cropped ,
h = -15 cm 2.8 4.3 1.9 2.2 7.0 8.9 14.6 17.7
Cropped ,
h = -70 cm 0.5 5.0 6.0
Uncropped,
h = -15 cm 8.2 3.7 2.8 14.3
Uncropped,
h - -7fl r-m 1 _ 5 1 V 1 Q A
Average
N2/N20 ratio
20.0
7 ft
7.0
3.9
2.7
A n
-------
The concentration gradients for the 30-to 60-cm depth in the plots of the
h = -70 cm treatment were very near zero so the loss of gas below the barrier
was negligible.
Soil gaseous diffusion coefficients, determined by the C02 flux method,
are given for four plots of the high-temperature experiment in Table 6. The
TABLE 6. SOIL GASEOUS DIFFUSION COEFFICIENTS
DETERMINED BY THE C02 FLUX METHOD FOR
THE SUMMER. EXPERIMENT
Treatment Date D (cm2 hr"1)
Manure,
h = -15 cm
Manure ,
h = -70 cm
Uncropped,
h = -15 cm
Uncropped,
h = -70 cm
8/26
9/16
9/22
10/6
10/7
10/14
9/3
9/21
10/11
10/5
11/11
0.09
0.009
0.02
0.05
1.02
0.84
0.10
0.07
4.07
1.70
2.14
diffusion coefficient of the cropped plots could not be determined using the
C02 flux method because of the sink for C02 in photosynthesis. Each value in
the table is the individually determined value for several different days.
The diffusion coefficients for any plot tended to decrease with time indica-
ting that the soil surface was becoming less permeable to the diffusion of
gas as more irrigation water was applied. Determinations of diffusion coef-
ficients were made more than one month after denitrification experiments were
conducted. Thus, it may be expected that the measured diffusion coefficients
would have been smaller than those during the denitrification experiment.
These measured diffusion coefficients were generally an order of magnitude
smaller than those required to approximate the measured flux as determined
from a cover placed over the soil surface. These results demonstrate the very
dynamic nature of the soil-gas duffusion coefficient and the importance of the
soil surface condition on the transport of water and gas across the soil-
atmosphere boundary.
Soil gaseous diffusion coefficients determined by the transient-state
method (Equation 5) are given by Table 7. Values of D determined by the
transient-state method are at least an order of magnitude larger than those
determined by the steady-state method. The soil-air content, e, is required
to convert fitted values of Dm from the transient-state method to values of
Dp. If only a portion of the total air space of the soil was contributing to
diffusion as would occur with trapped air isolated by water films, the effec-
tive e for diffusion would be much less than the total air space. Apparently,
35
-------
TABLE 7. SOIL GASEOUS DIFFUSION COEFFICIENTS DETERMINED BY THE TRANSIENT-
STATE METHOD FOR THE SUMMER EXPERIMENT
treatment
Manure, h = -15 cm
Manure, h = -70 cm
Cropped, h = -15 cm
Cropped, h = -70 cm
Uncropped, h = -15 cm
Uncropped, h = -70 cm
Date ]
9/15
10/6
10/15
9/16
9/21
9/28
10/12
6/28
9/14
10/7
10/14
11/9
11/10
DmCcrn2 hr-1)
90
120
110
220
200
100
60
60
80
225
240
200
280
e(cm3 cm 3) Dp (cm2 hr"1)
0.078 7.0
0.119 14.3
13.1
0.008 1.8
1.6
0.023 2.3
1.4
0.071 4.3
5.7
0.086 19.4
20.6
17.2
24.1
the effective e must be less than 10% of the actually measured values of e at
the water contents maintained in the field plots.
Gas sampling within the soil profiles demonstrated that considerable
was produced from denitrification of 15N03 fertilizer. Similar profiles were
also measured for N20. For all plots except the manure plot at h = -15 cm,
the 1%2 concentrations were nearly constant with depth or decreased gradually
near the surface. For the manure (h = -15 cm) treatment, however, the great-
est concentrations of ^-%2 an
-------
The addition of manure to the soil greatly increased the rate and total
denitrification. The addition of crop residues would also increase the deni-
trification potential. The presence of living plants greatly increased the
amount of denitrification as shown by the differences in N2 and N£0 flux at
the soil surface of the cropped and uncropped plots. The consumption of (^
through respiration of grass roots and addition of carbon to the system from
sloughed roots increased denitrification by approximately three to four times
over that of the uncropped plots at equal soil-water pressures. For the un-
cropped plot maintained at a soil-water pressure head of -15 cm, only 8 kg of
N ha"^- were lost as N£ and N20. Thus, it appears that very little denitrifi-
cation of added NOo will occur from uncropped Yolo loam even under the most
adverse soil-water conditions. The maximum rate of denitrification in the
cropped plot maintained at h = -15 cm was considerably less than that reported
by Rolston et al. (1976). The field plot used by Rolston et al. (1976) had
been cropped with perennial ryegrass for approximately two years before appli-
cation of NO^, and the soil may have been slightly wetter than that of this
study since water was occasionally pulled into the soil atmosphere samplers.
It appears that long-term cropping with grass may substantially increase the
available carbon of the soil and the denitrification potential.
The soil-water pressure also has a large effect on denitrification. By
decreasing the soil-water pressure head from -15 to -70 cm, denitrification
in the manure plots was decreased from 208 kg N ha~-*- to 47 kg N ha"-*-. De-
creasing the soil-water.pressure head from -15 to -70 cm for the cropped plots
decreased total denitrification from 34 to 9 kg N ha . Decreasing soil-
water pressure in the uncropped plots decreased denitrification from 8 to 4
kg N ha""l. The very narrow range of soil-water pressure for which denitrifi-
cation occurs for this particular soil indicates that slight manipulation of
soil-water content may be achieved to either increase or decrease denitrifi-
cation depending upon particular objectives.
Plant Uptake
Figure 20 gives the cumulative N uptake by the grass as a function of
time after the fertilizer was applied to the two cropped plots of the high-
temperature experiment. The grass of the h = -70 cm treatment took up 20% of
the applied fertilizer as compared to 11% uptake by the grass of the h = -15
cm treatment. This difference in uptake was due to the greater N loss through
denitrification and the greater rate of N movement through the root zone of
the h = -15 cm treatment. The rate of N uptake for both treatments was very
large soon after the fertilizer was applied. The rate of N uptake in the h =
-70 cm treatment also increased drastically at approximately 50 days after
fertilizer was applied. This increase in N uptake was primarily due to in-
creased dry matter production most likely due to decreasing air and soil tem-
perature.. The N uptake of the h = -15 cm treatment, however, did not increase
at 50 days as did that of the h = -70 cm treatment since the fertilizer NO^
had been either denitrified or leached from the root zone by that time. Both
plots had approximately the same rate of N uptake for the first 50 days.
37
-------
60
TIME (days)
80
100
120
Figure 20. Plant uptake of fertilizer N with time of the summer experiment.
Leaching
Figure 21 gives soil solution NO, concentrations derived from the
fertilizer as a function of soil depth at approximately 125 days after appli-
cation of the fertilizer to the h = -15 cm treatments. The large differences
in denitrification among the cropped, manure, and uncropped plots of the h =
-15 cm treatment also resulted in large differences in the concentration of
fertilizer N0~ in the soil solution of the profile. As expected, the manure
plot with the largest amount of denitrification resulted in the smallest N0~
concentrations in the soil solution. Denitrification in the uncropped plot
of only 3% resulted in the largest NOg concentration in the soil solution.
The peak of the NO" pulse for all three plots of the h = -15 cm treatment was
at approximately 140 cm for 120 days after fertilizer application. On the
other hand, for the h = -70 cm treatments, most of the NO^ was still in the
upper 60 cm of the soil profile. An estimate of evapotranspiration was
obtained from the water application rate for the h = -15 cm treatments, the
volumetric water content of the soil profile, and the rate of movement of NO^
through the soil profile. Inasmuch as the profiles were kept constantly wet,
38
-------
SOIL SOLUTION FERTILIZER N (ppm)
20 40 60 80 100
I i I
SUMMER
h =-!5cm
CROPPED, day 120
MANURE, day 133
UNCROPPED, day 127
Figure 21. Concentration of fertilizer derived N07 in the soil solution as
a function of soil depth for the h = -15 cm treatments of the
summer experiment.
it would be expected that evapotranspiration would be similar for all plots.
Applying this estimated evapotranspiration to the h = -70 cm treatments gave
a pore-water velocity, vs, for theh = -70 cm treatments of approximately 0.5
cm day""1 as compared to 1.1 cm day~ for the h = -15 cm treatments. From
these estimates of pore-water velocity, the peak of the NO^ pulse should have
been at about 55 cm at 110 days for the h = -70 cm treatment. Soil solution
samples taken at 110 days show a slight increase in NO^ concentration at the
60 cm depth of the h = -70 cm treatments. The position of the NOo pulse in
the h = -70 cm treatments can be ascertained from the soil samples taken ap-
proximately 5.5 months after the NO^ was applied. The residual soil N will
be presented in a later section.
The total leaching of NO^ from the h = -15 cm treatments was determined
from the flux of water and the NOj concentration at each of the 15 soil solu-
tion samplers as NOj moved through the soil profile. The soil-water flux was
39
-------
determined from the time of arrival of the NO^ peak at each sampler and the
average water content above that sampler. The soil-water flux used in calcu-
lating NOg recovery was determined from the pore-water velocity at each sam-
pler, the mean pore-water velocity from the 9 samplers (3 plots) at each
depth, the mean pore-water velocity from the 15 samplers of all depths of one
plot, and the mean pore-water velocity from the 43 (2 samplers not functional)
samplers of all depths of three plots. The results of determining NOg re-
covery by these methods are given in Table 8. In calculating the NC>3 leach-
ing component, the mean recovery was calculated from all depths of each plot
using the mean pore-water velocity of the 9 samplers at each depth (second
row of Table 8). The pore-water velocity of the 9 samplers at each depth
was considered the best estimate of the velocity at that depth inasmuch as
velocity would change with depth due to changing water content with depth.
Although this method was selected due to the above mentioned reason, the
recovery percentages of any of the other methods of determining pore-water
velocity are similar. However, the data indicate that the recovery percent-
ages are too large for the shallow depths and too small for the deep depths.
This discrepancy is believed to be due to the inability to accurately deter-
mine the water content at all depths and time from soil-water characteristic
curves, limited soil-water pressure head data for the deeper depths, and
nonuniform displacement of the NO^ pulse through the soil. These data demon-
strate the difficulty and uncertainty in determining the leaching of NO^ in
a field soil even in small, well-instrumented plots.
Residual Soil N
The extractable soil N derived from the fertilizer as a function of soil
depth for the h = -70 cm treatments of the high-temperature experiment is
given in Figure 22. Extractable N for the h = -15 cm treatments was very
small. Extractable soil N includes NflJ, NO^, and NO- in solution and NtiJ on
the exchange sites of the soil. The concentration of Nfll" was determined
separately and found to be less than 1 ppm (usually smaller than 0.6 ppm in
top 15-cm and much less below). The data points are mean concentrations
within 15-cm depth increments from eight samples. The extractable N in the
h = -70 cm treatments was large in the upper 60 cm of soil due primarily to
NOg which had not yet been leached out of the upper part of the profile.
The digestible soil N derived from the fertilizer as a function of soil
depth for the six plots of the high-temperature experiment is given in Figure
23. Digestible soil N is that portion of the soil N existing as live or dead
plant roots, microbial biomass, and fixed NfiJ. Fixed NHj is expected to be
small for Yolo loam soil. The data points are mean concentrations within
15-cm depth increments from eight samples. Digestible soil N was greatest
near the surface due to plant roots and microorganisms. The cropped and
manure plots resulted in the greatest digestible N, although the uncropped
plots also had digestible N in the upper part of the profile. "Digestible N
in the manure and uncropped plots was due in part to microbial immobilization,
although it was difficult to keep small weeds and moss from growing on the
manure and uncropped plots which would immobilize N in roots. Although the
manure plots had very little plant growth, considerable N was immobilized.
40
-------
TABLE 8. PERCENTAGE RECOVERY OF FERTILIZER N LEACHED IN THE h = -15 CM PLOTS OF THE SUMMER EXPERIMENT
CALCULATED BY VARIOUS WAYS OF DETERMINING THE SOIL-WATER VELOCITY, v . THE MEAN RECOVERY OF
ALL DEPTHS FOR EACH PLOT IS GIVEN IN PARENTHESES S
Method of determining
pore-water velocity
Cropped
60 90 120 150 180
% Recovery
Manure
60 90 120 150 180
Uncropped
60 90 120 150 180
From v of each
•o sampler
132 57 68 61 44
(72.4)
11 14 12 11 10 104 121 78 66 56
(11.6) (85.0)
From mean v of all
S
samplers at each depth
(9 samples)
From mean v of all
O
depths of one plot
(15 samples)
From mean v of all
depths of 3 plots
(43 samples)
75 55 64 57 44
(59.0)
86 64 68 57 44
(63.8)
80 60 63 53 41
(59.4)
14 14 12 12 10 100 114 76 66 57
(12.4) (82.6)
13 14 11 10
(11.4)
14 15 12 11
(12.2)
110 129 77 65 55
(87.2)
108 122 76 62 54
(84.4)
-------
SOIL EXTRACTABLE FERTILIZER N (JJLQ N g" soil)
.0 10 20 30 40 50 60
T
\
Summer
h = -70cm
• Cropped
o Manure
* Uncropped
Figure 22. Soil extractable N from fertilizer as a function of soil depth,
5.5 months after fertilizer application, of the summer experiment,
42
-------
-I
SOIL DIGESTIBLE FERTILIZER N (/tgNg soil)
0 10 20 0 10 20 30 40 50 60
Summer
• Cropped
° Manure
Uncropped
h=-70cm
IOOT-h = -l5cm
Figure 23. Soil digestible N from fertilizer as a function of soil depth,
5.5 months after fertilizer application of the summer experiment.
Mass Balance
Table 9 gives the N balance for each of the components of the N cycle.
Denitrification was determined by difference from fertilizer application,
plant uptake, leaching, and residual soil N. Denitrification as determined
43
-------
TABLE 9. MASS BALANCE OF FERTILIZER N FOR THE SUMMER EXPERIMENT
Residual
in soil Denitrification
Gaseous loss Plant
N£ below 60 cm Leaching Uptake Extractable Digestible Direct Difference
Treatment (kg N/ha) (kg N/ha) (kg N/ha) (kg N/ha) (kg N/ha) (kg N/ha) (kg N/ha) (kg N/ha) (kg N/ha)
Manure,
h = -15 cm 9.9 198 10 37 — 1 25 218 237
Manure,
h = -75 cm 5.4 42 0 — — 157 62 47 79
Cropped,
h = -15 cm 4.3 30 5.5 175 34 0 52 40 35
Cropped,
h = -75 cm 1.8 7 0 — 62 64 132 9 39
Uncropped,
h = -15 cm 2.1 5.7 2.5 252 — 1 27 10 25
Uncropped,
h = -75 cm 0.6 3.6 0 — — 254 23 4 16
-------
by difference is compared with denitrification determined from the flux of N£
and N£0 at the soil surface and any gaseous loss below the 60-cm barrier.
The amount of fertilizer applied to each plot is given by Table 2.
The direct method for determining denitrification compares reasonably
well with the difference method. The direct method gave 19 kg N ha~ less
denitrification than did the difference method for the manure, h = -15 cm
treatment. A difference of only 19 kg N ha"-*- could easily be accounted for
by an underestimation of the gaseous flux on hot afternoons since measurements
were generally taken mid-morning, by small denitrification rates below the
detection limits for N£ over several weeks, and errors in measuring leach-
ing and residual soil N. Denitrification calculated directly was slightly
greater (5 kg N ha~l) than that of the difference method for the cropped, h =
-15 cm treatment. This difference may be explained by an overestimation of
the leaching component since all samplers, including those at the 60-cm depth,
were used in determining NO^ leaching. It would be expected that plant roots
could take up some additional N below 60 cm and thus, result in an overestima-
tion of the leaching.
For all other plots, denitrification as determined directly was between
12 and 30 kg N ha~l less than that determined by difference. The substantial
differences between the two methods for the h = -70 cm treatments may be
explained by small rates of gaseous loss below the minimum detection limits
over several weeks and elevated rates on hot afternoons which were not mea-
sured. These speculations seem especially reasonable since the NO, pulse
remained in the upper 60 cm of the profile for several months. There may
also be some error in the residual soil N component of the difference method.
The sensitivity of the direct method may be increased by increasing the ^N
enrichment; of the added fertilizer, increasing the time that covers remain over
the soil surface, decreasing the volume of the chamber placed over the soil
surface, or by calculating gas flux from diffusion coefficients and concentra-
tion gradients within the soil. Although larger concentrations are possible
from soil air samples, the latter method is plagued with uncertainty in
measuring both the diffusion coefficient and the concentration gradient
(Rolston, 1977).
LOW-TEMPERATURE EXPERIMENT
Plot Characteristics
Temperature at the 5-cm soil depth as a function of time after fertilizer
was applied to the plots of the winter experiment is given by Figure 24. The
average soil temperature during most of the denitrification activity was
approximately 8-10°C. There was a gradual increase in soil temperature ap-
proximately two weeks after the fertilizer was applied to the first plot.
The labeled arrows in the figure give the fertilizer applications to each of
the six plots. The fertilizer was applied to plots at a temperature of 8-10°
C except for the cropped plot of the dry treatment at which the temperature
was 10-13°C.
The soil-water pressure head, h, for the 5- and 15-cm depths as a function
of time after fertilizer application for the winter experiment is given by
45
-------
o
o
LU
a:
ID
a:
Ld
o_
2
LU
0
10
20 30 40
TIME (days)
50 60
Figure 24. Mean soil temperature at the 5-cm soil depth as a function of
time after fertilizer was applied to the plots of the winter
experiment. The arrows give the fertilizer application for each
plot with the symbols given in Table 2.
Figure 25. The soil-water pressures during the fertilizer application were
approximately -6 to -8 cm for the wet treatment, and approximately -50 cm for
the dry treatment. The arrows in the figure give the times that the fertili-
zer was applied to each plot. The pressure heads are mean values of the
three plots of each soil-water treatment. Values of the soil-water pressure
head as a function of depth are given by Figure 26. As with ,the summer ex-
periments, the soil-water pressure heads became smaller with depth. In a
similar manner, the soil-water content data as a function of depth for the
two water treatments are given in Figure 27. These data were>obtained from
neutron moisture probe data, and the data points are mean values for the three
plots of each water treatment. Water contents determined from neutron meter
data compare closely with those from tensiometers and soil-water characteris-
tic curves. The soil-air content, e, as a function of depth can be calculated
and is plotted in Figure 28 for the two soil-water treatments. These values
46
-------
-120
< -100 h
1 1—
WINTER
—•— 5cm depth
-°- 15cm depth
10
20
30
TIME (days)
40
50
Figure 25. Mean soil-water pressure head at the 5- and 15-cm soil depth as
a function of time after fertilizer application to the two water
treatments of the winter experiment. The arrows give the fertili-
zer application for each plot.
47
-------
SOIL-WATER PRESSURE HEAD h (cm H20)
0
^ 30
S 60
a. 90
UJ
Q
, 120
150
180
0 -40 -80 -120 -160 -200
h-\
- \
»
\WET
i
A
WINTER
DRY
Figure 26. Mean soil-water pressure head as a function of soil depth for the
two water treatments of the winter experiment.
48
-------
SOIL-WATER CONTENT 6 (cm3 cm'3)
0.30 0.32 0.34 0.36 0.38 0.40 0.42 0.44 0.46
0
Figure 27. Mean soil-water content as a function of soil depth for the two
water treatments of the winter experiment.
49
-------
SOIL-AIR CONTENT € (cm3 cm"3)
ff.O 0.04 0.08 0.12 0.16 0.20 0.24 0.28
160 -
180
Figure 28. Mean soil-air content as a function of soil depth for the two
water treatments of the winter experiment.
50
-------
of e are average values for the three plots of each water treatment. The h =
-8 cm and h = -50 cm treatments result in only very small differences of soil-
air content in the surface soil.
Soil Gases
The amount of 02 in the soil as a function of soil depth for the six
plots of the winter experiment is given by Figure 29. The Q£ concentrations
10
12
02 CONCENTRATION (%)
14
Figure 29. Oxygen concentration as a function of soil depth in the plots of
the winter experiment. The symbols are given in Table 2.
were measured during the time that the denitrification experiments were con-
ducted. Because of decreased biological activity at low temperature, all
plots exhibited relatively uniform Q£ concentrations with depth. The only
major deviation among plots was the manure, h = -8 cm treatment. The minimum
value attained in that particular plot was approximately 13% at the 5-cm
depth. All other plots had Q£ values between 18 and 21%. Although Q£ concen-
tration within the profile gave only a qualitative estimate of anoxic develop-
ment, it is obvious from Figure 29 that anoxic development in the profiles was
smaller than those of the summer experiment.
51
-------
The initial concentration of N20 within the six plots of the winter
experiment before application of NO^ fertilizer is given by Table 10. The
TABLE 10. CONCENTRATION OF N20 (PPMV) WITHIN PROFILES BEFORE APPLICATION OF
FERTILIZER OF THE WINTER EXPERIMENT. AMBIENT CONCENTRATION OF
N20 IN AIR IS APPROXIMATELY 0.30 PPMV
Soil
Depth (cm)
2
5
10
15
20
30
60
90
h
Cropped
0.6
0.6
0.6
0.8
0.7
2.1
= -8 cm
Manure
11.0
12.8
15.1
15.6
8.5
6.9
2.2
1.4
Uncropped
0.7
0.7
0.6
0.8
1.1
0.7
0.7
0.8
h = -50 cm
Cropped Manure Uncropped
0.5 19.0 0.5
0.5 0.5
18.0 0.5
0.6 0.5
11.8 0.6
1.0 8.0 0.7
0.8
0.8
N£O concentrations in all plots were above ambient (0.3 ppm) indicating that
conditions were not reduced enough to cause the small amount of ^0 produced
to be further reduced to N2. This observation is entirely different from
that of the cropped plots of the summer experiments where the small amount of
N£O produced was reduced to N£ resulting in N£0 concentrations below ambient.
The manure plots of the winter experiments had initial concentrations of N£0
as large as 20 ppm. All other plots were slightly above the ambient concen-
tration with very little difference between plots.
Representative N20 concentration profiles from denitrification of added
fertilizer as a function of soil depth at 7 days after fertilizer application
for the h = -8 cm treatments are given by Figure 30. The concentration pro-
files demonstrate that considerable differences in ^0 production occurred
among the cropped, manure, and uncropped plots, although the differences are
not nearly as great as those observed during the summer experiments. Also,
the N20 concentration profiles were generally much smoother and uniform with
depth than were those of the summer experiment. Similar data for the plots
of the h = -50 cm treatment at 7 days after application of the fertilizer are
shown by Figure 31. There was much less production of N£0 in the h = -50 cm
treatments than in the h = -8 cm treatments, and the concentration profiles
52
-------
N.O CONCENTRATION (ppm v)
0
10
Q_
liJ
O
O
CO
20
30
50
100
150
WINTER
200
Day 7
h =-8cm
• CROPPED
o MANURE
* UNCROPPED
I I
Figure 30. Nitrous oxide concentration with depth on one day in the h
cm treatment of the winter experiment.
= -8
53
-------
N.O CONCENTRATION (ppm v)
0
10
20
30
10
a.
UJ
Q
O
cn 20
30 •-*
WINTER
Day 7
h= -50cm
• CROPPED
° MANURE
* UNCROPPED
Figure 31. Nitrous oxide concentration with depth on one day in the h = -50
cm treatment of the winter experiment.
were fairly uniform with depth. The largest N£0 production occurred in both
the h = -8 and -50 cm water treatments of plots to which manure had been
added in the top 10 cm of soil. *
Similar representative N£ profiles derived from the fertilizer at 7 days
after application of the fertilizer are given in Figures 32 and 33. Concen-
tration profiles for ^%2 8as follow the same pattern as that of N£0 with
differences in N£ production among the cropped, manure, and uncropped plots.
However, substantial denitrification occurred only in the manure plots.
54
-------
N,
-i
FROM FERTILIZER (mg N liter" soil air)
0 0.2 0.4 0.6 0.8 1.0
I
WINTER
Day 7
h=-8cm
• CROPPED
o MANURE ~
* UNCROPPED
Figure 32. Concentration of 15N2 with depth on one day in the h = -8 cm
treatment of the winter experiment.
55
-------
FROM FERTILIZER (mg N liter soil air)
^ 0 0.05 0.10 0.15 0.20
E
o
a.
UJ
a
en
10
30
I
WINTER
Day 7
h=-50cm
• CROPPED
o MANURE
A UNCROPPED
I
Figure 33. Concentration of N£ with depth on one day in the h = -50 cm
treatment of the winter experiment.
Gas Fluxes
Flux of N£0 derived from the fertilizer as a function of time for the
manure and cropped plots is given by Figure 34. These fluxes were determined
from the accumulation of ^0 beneath the cover placed over the soil surface
for 1- or 2-hour intervals. The numbers below each of the soil-water pressure
head treatments are total ^0 produced as determined from the area beneath
each of the flux versus time curves. All data were corrected for the concen-
tration increase beneath the cover using Equations (6), (7), and (8).
4
As with the NoO flux data of the summer experiments, the flux of gases
from denitrification occurred immediately after addition of the fertilizer.
However, the ^0 flux increased at a much smaller rate than that of the
summer experiments. In all cases the total amount of denitrification which
occurred as N£0 was very small. Similar data from the flux of ^0 from the
fertilizer as a function of time for the uncropped plots are given by Figure
56
-------
o
•O
IT
LJ
N
tr
UJ
U_
2
O
-------
0.03
o
TJ
Q02
o:
UJ
N
o
cr
o
eg
0.01
1 1 T
UNCROPPED
h = -8cm
-i
1
0
10
20
TIME (days)
30
40
Figure 35. Flux of ^0 calculated from measurements beneath a cover with
time for the uncropped plots of the winter experiment.
35. Again, very little denitrification as N£0 occurred in the uncropped plots
with the total amount being only slightly different than that which occurred
in the cropped plots. Denitrification occurred for a much longer time period
than for the summer experiments, although concentrations and fluxes were
small.
\
Data for the flux of N2 derived from the fertilizer as a function of
time for the manure and cropped plots are given by Figure 36. These fluxes
58
-------
o
•o
z
o>
N
oc
LJ
O
o:
u.
3.0
-r- 2.0
1.0
0
0.8
0.6
0.4
0.2
0
T
MANURE
h=-8cm
CROPPED
20
TIME (days)
30
Figure 36. Flux of N2 calculated from measurements beneath a cover with
time for the manure and cropped plots of the winter experiment.
were determined from the accumulation of N2 beneath the cover placed over the
soil surface and corrected for the decrease in the concentration gradient as
N£ accumulated. The numbers below each of the soil-water pressure head treat-
ments are total N£ produced. Denitrification was largest soon after applica-
tion of the fertilizer and decreased to relatively small values by approxi-
mately 20 days. As determined by the criterion used for measuring the N2
beneath the cover over the soil, the minimum value which we considered signi-
ficantly different than background or machine noise was approximately 1 kg N
ha"1 day"1. Thus, all of the N2 fluxes given in Figure 36 except for the
manure, h = -8 cm treatment were either at or below what is considered as the
minimal detectable limit. Thus, the uncertainty of the denitrification in
this very small flux range is quite large. There was very little difference
in denitrification in the manure plots of the h = -8 and -50 cm treatments,
although there was considerable uncertainty in the fluxes at these small
59
-------
rates of denitrification. Data for the N2 flux of the uncropped plots are
not given inasmuch as all fluxes were so near zero that no denitrification
was considered to have occurred as N2.
The data given in Table 11 are the total amounts of denitrification
TABLE 11. DENITRIFICATION AS N20, N2, AND TOTAL FOR THE WINTER
EXPERIMENT AS DETERMINED FROM MEASUREMENTS BENEATH A
COVER PLACED OVER THE SOIL
Total
N20 N2
Treatment (kg N ha'1) (kg N ha~l) (kg N ha'1) (% of fertilizer)
Manure,
h = -8 cm
Manure,
h = -50 cm
Cropped,
h = -8 cm
Cropped,
h = -50 cm
Uncropped ,
h = -8 cm
Uncropped,
h = -50 cm
4.2
3.8
0.7
0.1
0.4
0.2
27.1
26.4
17.6
1.9
0.0
0.0
31.3
30.2
18.3
2.0
0.4
0.4
11
10
6
1
* 0
= 0
consisting of both N2 and N20 for the six treatments of the winter experiment.
The amount of denitrification ranged from zero for the uncropped plots to 11%
of the total fertilizer added for the manure, h = -8 cm treatment. The water
content or carbon level had very little influence on the total amount of
denitrification at these low temperatures.
The N2 to N20 ratio as a function of time after application of the NO,
to each of the plots is given in Table 12. The average ratio^calculated on
the basis of total amount of each gas produced is also given. *The largest
ratios of 25 and 19 occurred for the cropped treatments at h = -8 and -50 cm,
respectively. These ratios may be somewhat uncertain inasmuch as the flux of
N2 was below that flux considered to be significant.
Soil gaseous diffusion coefficients, determined by the C02 flux method,
are given for four plots of the winter experiment in Table 13. Diffusion
60
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TABLE 12. RATIOS OF N2 TO N20 PRODUCED FROM DENITRIFICATION IN THE
PLOTS OF THE WINTER EXPERIMENT FROM MEASUREMENTS BENEATH
A COVER PLACED OVER THE SOIL
Treatment N2/N20 ratio
Days after NO^ application
0.25 12456 78 9
Manure,
h = -8 cm 6.2 2.5 3.8 4.3 7.5 11.9 12.7
Manure,
h = -50 cm 3.8 6.9 8.2 12.9 4.0 10.0
Cropped,
h = -8 cm 30.9 20.0 11.3 12.2
Cropped,
h = -50 cm 25 20 48.6 31.4
TABLE 12 (continued)
Average
Treatment N2/N20 ratio N2/N20 ratio
Days after NO^ application
12 13 14 15 16 20 21 28
Manure,
h = -8 cm 15.8 15.0 18.8 6.5
Manure,
h = -50 cm 6.8 4.8 4.8 6.9
Cropped,
h = -8 cm 13.3 51.5 34.5 25.1
Cropped,
h = -50 cm 16.2 19.0
61
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TABLE 13. SOIL-GAS DIFFUSION COEFFICIENTS FOR THE WINTER EXPERIMENT
Treatment
Date
Dp(cm2 hr"1)
Date of Fertilizer
Application
Manure
h = -8 cm
Manure
h = -50 cm
Uncropped
h = -8 cm
Uncropped
h = -50 cm
2/2
4/5
4/14
4/15
4/28
3/10
4/7
4/12
12/10
12/15
12/17
2/3
5/16
5/17
11/9
11/10
12/1
12/13
12/14
0.84
0.40
0.24
0.10
0.25
2.43
3.29
2.55
2.79
1.59
1.56
0.44
0.30
0.41
5.40
7.06
3.53
2.84
1.77
1/12
1/17
1/10
1/12
coefficients for N2 are approximately 1.25 times larger than those of C02.
Each value in the table was determined on different days. Diffusion coeffi-
cients generally decreased with time in a similar manner to those of the
summer experiments due to changing soil surface conditions. Diffusion coef-
ficients were used in conjunction with measured N2 and N20 concentration
gradients (five samplers at each depth) to calculate the denitrification flux
from Equation (1). Values of the diffusion coefficients required in Equation
(1) in order to result in the same amount of denitrification as determined
62
-------
from beneath the cover were approximately equal to measured values of the
h = -50 cm treatments (Table 13). For instance, a value of Dp equal to 4
cm2 hr for the h = -50 cm treatments resulted in the calculated flux to be
equal to the measured gas flux. Values of Dp required in order that the
calculated flux for the h = -8 cm treatments be equal to the measured flux
were larger than those measured (Table 13) . For instance, a value of Dp equal
to approximately 2.7 cm2 hr"1 for the h = -8 cm treatments resulted in the
calculated flux to be equal to the measured flux from gas accumulation beneath
covers. The underestimation of denitrification flux from Equation (1) was
due primarily to the general decrease in diffusion coefficients with time and
the resultant uncertainty in knowing the value of Dp during denitrification.
However, additional gaseous transport mechanisms such as mass flow due to
pressure fluctuations of a barometric and wind turbulence source cannot be
absolutely neglected. The very dynamic nature of diffusion coefficients,
strongly dependent upon surface soil conditions, requires that the measure-
ment of Dp be made several times as close as possible to the time period of
denitrification. Although the temporal variability of the gaseous diffusion
coefficient results in some uncertainty in calculating flux of gases from
denitrification, Equation (1) can be used to determine denitrification if
accurate measurements of diffusion coefficients and concentration gradients
are made. The advantage of calculating flux from Equation (1) over measur-
ing flux from accumulation of gas beneath a cover is the increased sensitiv-
ity due to larger concentrations of ^%2 within the soil than that from
accumulation beneath a cover.
Measurements of - an(* ^0 within the soil profiles and beneath the
lids placed over the soil surface demonstrated that very little denitrifica-
tion of added NOg occurred at the low temperatures of approximately 8-10°C.
However, there was some denitrification in the manure and cropped plots with
rates being very small. The addition of manure to the soil greatly increased
denitrification over that of the cropped plots. Low amounts of denitrifica-
tion were due to low microbial activity which was reflected in relatively
constant and high values of D£ within the soil profile. Differences in deni-
trification due to soil- water treatment were not great due to the overall
small rates of denitrification and uncertainty in the measured fluxes at
small denitrification rates.
Plant Uptake
Figure 37 gives the cumulative N uptake by the grass as a function of
time after the fertilizer was applied to the two cropped plots of the winter
experiment. The grass of the h = -50 cm treatment took up 47% of the applied
fertilizer at 115 days after application compared to 35% for the h = -8 cm
treatment. The rate of N uptake was similar for both treatments until approx-
imately 50 days after application, at which time the NO^j pulse was being
leached below the major root zone in the h = -8 cm treatment. The N uptake
was much greater for the grass of the winter experiment than that of the
summer experiment. The difference in uptake was due to a much greater dry
matter production during the winter than in the summer because of more favor-
able growing conditions.
63
-------
0»
LU
CL
20 40 60 80 100 120 140
TIME (days)
Figure 37. Plant uptake of fertilizer N with time during the winter
experiment.
64
-------
Leaching
Figure 38 gives soil solution NO^ concentrations derived from the ferti-
Or
20 if-
4O
SOIL SOLUTION FERTILIZER N (ppm)
40 80 120 160 200 240
T
T
T
T
WINTER
h=-8cm
• CROPPED (DAY 79) -
o MANURE (DAY 79)
UNCROPPED (DAY 81)
Figure 38. Concentration of fertilizer derived NO^ in the soil solution as
a function of soil depth for the h = -8 cm treatments of the
winter experiment.
lizer as a function of soil depth at approximately 80 days after application
of the fertilizer to the h = -8 cm treatments. Since denitrification was
small for all three plots, the greatest difference in the leaching component
was due to the large plant uptake. Thus, the NO^ concentrations in the
cropped plot were much less than in either the manure or the uncropped plots.
The NOg pulse moved faster through the plots of the winter experiment than
through those of the summer experiment.
The total leaching of NO^ from the h = -8 cm treatments was determined
from the flux of water and the NOo concentration at each of the 15 soil solu-
tion samples of each plot using the same approach as described for the summer
65
-------
experiments. The recovery percentages using various ways of averaging the
soil-water velocity are given in Table 14. The NO^ leaching component was
calculated using the mean soil-water velocity of the 9 samplers at each depth
since the soil-water velocity tended to change slightly with depth. All
approaches for averaging the soil-water velocity resulted in recovery of much
more fertilizer N than was applied for the manure and uncropped treatments
and more than is reasonable for the cropped plot. The mean recovery deter-
mined from all depths using the mean soil-water velocity of the 9 samplers at
each depth was 56, 138, and 182% for the cropped, manure, and uncropped plots,
respectively. The greater recovery as leaching than that applied as fertili-
zer may be explained by an overestimation of the soil-water velocity or flux
of water in the field profiles. The average soil-water flux (vs 0) was cal-
culated from values of vg determined by the time the NO^ peak reached a par-
ticular depth. Values of the flux for the 60-, 90-, 120-, 150-, and 180-cm
depths were 0.54, 0.58, 0.55, 0.52, and 0.49 cm day"1, respectively. The
water applied by the sprinkler was 0.51 cm day"1 for the first 3 months after
fertilizer application and 0.56 cm day"1 for the next 2 months. Thus, for
the shallower depths, the calculated water flux within the profiles was
greater than the application rate. For the deeper depths and longer leaching
time, the water flux within the profile was less than that applied due to
increasing temperatures and greater evaporation. The overestimation of the
water flux within the profile would result in an overestimation of the NOg
leaching also.
One explanation for the overestimation of the water flux within the soil
is an overestimation of the soil-water velocity due to anion-exclusion or
immobile water. Rolston and Marino (1976) determined for packed soil columns
of Yolo soil with a similar water flux that approximately 8% of the water was
not contributing to NO^ transport. However, if anion exclusion or Immobile
water was the reason for the overestimation of the water flux, a much larger
portion of the water must not be contributing to NOo transport in the field
soil than that of laboratory columns in order to decrease the water flux suf-
ficiently. The variability of NO^ concentrations at triplicate samplers of
each depth also contributed to the uncertainty in determining the NO^ leach-
ing component from flux of water and concentration of N07.
The recovery percentages of fertilizer N determined from concentration
versus depth (Figure 38) was 50, 120, and 109% for the cropped, manure, and
uncropped treatments, respectively. This approach more reasonably estimated
leaching loss for these experiments than did estimations of water flux and
concentration. Although this approach is not dependent upon knowledge of the
water flux, it is plagued with averaging concentrations across any particular
depth. Thus, it seems that anion exclusion, immobile water, or other unknown
mechanisms may indeed be causing a considerable overestimation of the water
flux and the resulting overestimation of the NO^ leaching in t^ie field
profiles.
Potential error in the leaching component from lateral movement of NO^
below the 60-cm deep barrier around each plot exists and would reduce NO^
recovery. This possible error was minimized by placing soil-solution samplers
near the center of each 1-m2 plot. This error does not appear to be very
large in relation to the uncertainties in determining the water flux and the
66
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TABLE 14. PERCENTAGE RECOVERY OF FERTILIZER N LEACHED IN THE h = -8 CM PLOTS OF THE WINTER EXPERIMENT
CALCULATED BY VARIOUS WAYS OF DETERMINING THE SOIL-WATER VELOCITY. THE MEAN RECOVERY OF ALL
DEPTHS FOR EACH PLOT IS GIVEN IN PARENTHESES
Method of determining
pore-water velocity
Cropped
Recovery
Manure
60 90 120 150 180 60 90 120 150 180
Uncropped
60 90 120 150 180
en From vs of each
sampler
From mean vs of all
samplers at each depth
(9 samples)
From mean vs of all
depths of one plot
(15 samples)
From mean vg of all
depths of 3 plots
(45 samples)
49 97 66 50 50 78 124 133 152 155 113 165 165 206 185
(62) (128) (167)
44 98 61 44 32 89 137 143 148 172 114 150 167 233 246
(56) (138) (182)
50 103 67 51 40
(62)
44 91 59 45 36
(55)
83 120 131 142 176
(130)
89 129 141 153 190
(140)
107 132 154 225 253
(174)
113 140 163 238 268
(184)
-------
variability in NO^ concentrations at any particular depth.
Residual Soil N
The extractable soil N derived from the fertilizer as a function of soil
depth for the h = -50 cm treatments of the low-temperature experiment is
given in Figure 39. Extractable N for the h = -8 cm treatment was negligible.
-I
SOIL EXTRACTABLE FERTILIZER N (>*gNg soil)
0 20 40 60 80 100 120
120
WINTER
h=-50cm
• Cropped
o Manure
* Uncropped
Figure 39. Soil extractable N from fertilizer as a function of soil depth,
4 months after fertilizer application in the winter experiment.
The data points are mean concentrations within 15-cm depth increments from
eight samples. The extractable N in the h = -50 cm treatments j/as large in
the upper 60 cm of soil due primarily to NO^ not yet leached from the upper
portion of the profile.
The digestible soil N derived from the fertilizer as a function of soil
depth for the six plots is given in Figure 40. As with the summer experi-
ments, digestible soil N was greatest near the soil surface due to plant roots
68
-------
-I
SOIL DIGESTIBLE FERTILIZER N Ug N g soil)
0 20 40 0 20 40 60 80 100
E
o
gj 60
a
_j
o
T
90
ieo
_ h=-8cm
WINTER
• Cropped
o Manure
* Uncropped
h=-50cm
Figure 40. Soil digestible N from fertilizer as a function of soil depth, 4
months after fertilizer application in the winter experiment.
and microorganisms. The manure and uncropped plots had negligible plant
growth, so the digestible N was primarily due to microbial immobilization of
fertilizer NO^.
Mass Balance
Table 15 gives the N balance for each of the components of the N cycle.
Denitrification was determined by difference from fertilizer application,
plant uptake, leaching and residual soil N. Denitrification was determined
directly from N£ and ^0 gas flux at the soil surface and any gaseous loss
below the 60-cm barrier. The amount of fertilizer applied to each plot is
given by Table 2.
The total denitrification as determined by the direct method was sub-
stantially less than that determined by difference in the h = -50 cm treat-
ments. The direct method resulted in 41 to 64 kg N ha"1 less than that
69
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TABLE 15. MASS BALANCE OF FERTILIZER N FOR THE WINTER EXPERIMENT. THE NUMBERS IN PARENTHESES WERE
OBTAINED BY CALCULATING LEACHING FROM CONCENTRATION VERSUS SOIL DEPTH. THE MINUS SIGN
INDICATES GREATER RECOVERY THAN FERTILIZER APPLIED
Residual
in soil
Denitrification
Gaseous loss Plant
.N20 N£ below 60 cm Leaching Uptake Extractable Digestible Direct Difference
Treatment (kg N/ha) (kg N/ha) (kg N/ha) (kg N/ha) (kg N/ha) (kg N/ha) (kg N/ha) (kg N/ha) (kg N/ha)
Manure,
h = -8 cm
4.2
Manure,
h = -50 cm 3.8
Cropped,
h = -8 cm
Cropped,
h = -50 cm
0.7
0.1
Uncropped,
h = -8 cm % 0.4
Uncropped,
h = -50 cm 0.2
27.1
26.4
17.6
1.9
0.0
0.0
1.5
1.3
0.5
406
(353)
168
(150)
118
137
535
(320)
186
22
3
39
70
71
209
37
33 (-62)
32 73
19 (-38)
2 66
0 (-28)
0 48
-------
determined by difference after approximately 4 months. These differences are
consistent with those measured in the summer experiment for the dry treatment
and are explained by small rates of gaseous loss below the minimum detection
limits for 15N2- After 4 months, most of the NO^ remained in the upper 45 cm
of soil in the h = -50 cm treatments. Temperatures gradually increased over
the 4-month period. Thus, denitrification may have occurred at small rates
since NO, remained in the portion of the profile most conducive to anoxic
development. The difference in denitrification between the direct and differ-
ence methods for the manure and cropped plots seems consistent with the
difference observed in the summer experiment over comparable time periods of
4 to 5 months. However, the amount of denitrification as calculated by
difference in the uncropped treatment is much greater than that determined in
either of the uncropped plots of the summer experiment.
Although total denitrification of 48 kg N ha"-'- in an uncropped plot
maintained at h = -50 cm seems unrealistic compared with the results of the
summer experiment, it has been shown that small changes in soil-air content,
carbon level, and diffusion of oxygen through the soil surface can drastically
change denitrification rates. A small change in the rate of denitrification
occurring over several weeks can greatly increase total denitrification.
These results for the h = -50 cm treatments suggest that the sensitivity
of the direct method of measuring the volatile denitrification products should
be increased in order to measure small fluxes over long time periods. However,
small fluxes over a long time may not be as important during normal irrigation
or rainfall events due to generally rapid wetting and drying of the upper
soil profile where most of the denitrification occurs. Research on rates of
denitrification during and after irrigation cycles should provide further
insight into the many interacting factors resulting in denitrification in
field soils.
The comparison of denitrification by the direct and difference methods
for the h = -8 cm treatments of the winter experiment was difficult due to
recovery of much greater than 100% of the fertilizer by the difference method.
This discrepancy was attributed to the inability to accurately measure leach-
ing losses of NOg from the soil profile. In addition to considerable vari-
ability in NOg concentrations at individual depths, the determination of the
soil-water flux was. not sufficiently accurate to determine realistic amounts
of leaching in any plot. Sampling variability of the residual soil N may
also have attributed to failure in obtaining a mass balance. The most realis-
tic method of determining the leaching component for the winter experiment
was from concentration versus soil depth. Although this approach does not
depend upon an accurate value for the soil-water flux, it does depend upon
average concentrations at particular depths which may also be quite uncertain
due to enormous spatial variability.
These results demonstrate the difficulty and potential uncertainty in
determining denitrification by difference. Although uncertainties also exist
for the direct determination of N20 and ^%2 fl"* from the soil surface,
especially at small fluxes, rates and total denitrification are consistent and
easily measured using "N fertilizer. The direct method is the only way that
rates of denitrification from applied fertilizer can be practically measured.
71
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REFERENCES
1. Biggar, J. W., and D. R. Nielsen. Spatial variability of the leaching
characteristics of a field soil. Water Resources Research, 12:78-84,
1976.
2. Bremner, J. M. Inorganic forms of nitrogen. In: Methods of soil
analysis. Part 2., C. A. Black (ed.). Agronomy 9:1179-1237, 1965.
3. Delwiche, C. C., and D. E. Rolston. Measurement of small nitrous oxide
concentrations by gas chromatography. Soil Sci. Soc. Am. J., 40:324-327,
1976.
4. Donigian,Jr., A. S., and N. H. Crawford. Modeling pesticides and nutri-
ents in agricultural lands. EPA-600/2-76-043. U.S. Environmental
Protection Agency, Athens, Georgia, 1976. 318 pp.
5. Fried, M., K. K. Tanji, and R. M. Van De Pol. Simplified long term
concept for evaluating leaching of nitrogen from agricultural land. J.
Environ. Quality, 5:197-200, 1976.
6. Mehran, M., and K. K. Tanji. Computer modeling of nitrogen transforma-
tions in soils. J. Environ. Quality, 3:391-396, 1974.
7. Rolston, D. E. Application of gaseous-diffusion theory to measurement
of denitrification. In: Nitrogen and the Environment, D. R. Nielsen
and J. G. Mac Donald, eds. Academic Press, New York, N.Y., 1977-
(in press)
8. Rolston, D. E., M. Fried, and D. A. Goldhamer. Denitrification measured
directly from nitrogen and nitrous oxide gas fluxes. Soil Sci. Soc. Am.
J., 40:259-266, 1976.
9. Rolston, D. E., and M. A. Marino. Simultaneous transport of nitrate and
gaseous denitrification products in soil. Soil Sci. Soc. Am. J., 40:860-
865, 1976.
10. Rolston, D. E., and B. D. Brown. Measurement of soil gaseous diffusion
coefficients by a transient-state method with a time dependent surface
condition. Soil Sci. Soc. Am. J., 41:499-505, 1977. *
11. Shaffer, M. J., R. W. Ribbens, and C. W. Huntley. Detailed return flow
salinity and nutrient simulation model, Vol. V. of Prediction of mineral
quality of irrigation return flow. Draft rept. to U.S. Environmental
Protection Agency, Ada, Oklahoma, 1976. 242 pp.
72
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12. Tanji, K. K., and S. K. Gupta. Computer simulation modeling for nitrogen
in irrigated croplands. In: Nitrogen and the Environment, D. R. Nielsen
and J. G. Mac Donald, eds. Academic Press, New York, N.Y., 1977.
(in press)
13. van Veen, H. Behavior of nitrogen in soil. A computer simulation
model. Ph.D. Thesis, Wageningen, The Netherlands, 1977.
73
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PUBLICATIONS
Rolston, D. E., and B. D. Brown. Measurement of soil gaseous diffusion
coefficients by a transient-state method with a time dependent surface
condition. Soil Sci. Soc. Am. J., 41:499-505, 1977.
Rolston, D. E. Application of gaseous-diffusion theory to measurement of
denitrification. In: Nitrogen and the environment, D. R. Nielsen and
J. G. Mac Donald, eds. Academic Press, New York, N.Y., 1977 (in press)
Rolston, D. E., D. A. Goldhamer, D. L. Hoffman, and D. W. Toy. Field measured
flux of volatile denitrification products as influenced by soil-water
content and organic carbon source. In: Proceedings of National Conference
on Irrigation Return Flow Quality Management, Fort Collins, Colorado,
1977. pp. 55-61.
74
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
1. REPORT NO.
EPA-600/2-77-233
2.
3. RECIPIENT'S ACCESSION NO.
, TITLE AND SUBTITLE
Field Measurement of Denitrification
5. REPORT DATF
November 1977 issuing rlate
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
Dennis E. Rolston and Francis E. Broadbent
8. PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORGANIZATION NAME AND ADDRESS
University of California
Davis, California 95616
10. PROGRAM ELEMENT NO.
1HB617
11. CONTRACT/GRANT NO.
R804259
12. SPONSORING AGENCY NAME AND ADDRESS
Robert S. Kerr Environmental Research Lab. - Ada, OK
Office of Research and Development
U.S. Environmental Protection Agency
Ada, Oklahoma 74820
13. TYPE OF REPORT AND PERIOD COVERED
Final 12/1/75 - 7/31/77
14. SPONSORING AGENCY CODE
EPA/600/15
15. SUPPLEMENTARY NOTES
16. ABSTRACT
Denitrification from a Yolo loam field profile was studied in relation to the
influence of soil-water content, organic carbon source, and temperature. Field plots
were intensely instrumented with soil atmosphere samplers, soil solution samplers,
and tensiometers. The two soil-water pressure treatments were -0.01 and -0.05 bars
in the topsoil. Three levels of soil carbon were studied by evaluating plots cropped
with ryegrass, uncropped plots, and plots amended with manure. Experiments were
conducted at soil temperatures of 8 and 23°C. Fertilizer was applied as KN03 en-
riched with 1%. The flux of volatile gases at the soil surface was measured from
the accumulation of N20 and 15N2 beneath an air-tight cover placed over the soil
surface for 1 or 2 hours per day. Denitrification at 23°C ranged from 73% of the
fertilizer N for the manure treatment at -0.01 bar to 1% for the uncropped treatment
at -0.05 bar. At 8°C, denitrification ranged from 11% for the manure treatment at
-0.01 bar to zero for the uncropped plots. The N20 flux at the soil surface varied
between 5 and 26% of total denitrification.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lDENTIFIERS/OPEN ENDED TERMS
c. COSATI Field/Group
Nitrogen cycle
Nitrogen isotopes
Nitrogen oxide (N20)
Soil water
Fertilizer
Irrigation
Denitrification
Irrigation Return Flow
Nitrate Leaching
Gas Fluxes
02/A,C
18. DISTRIBUTION STATEMENT
RELEASE TO PUBLIC
19. SECURITY CLASS (ThisReport)
Unclassified
21. NO. OF PAGES
91
20. SECURITY. CLASS (This page!
Unclassified
22. PRICE
EPA Form 2220-1 (9-73)
75
ft U.S. GOVBtHMEHTPRHraiG OFFICE 1978— 757-140/6628
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