United States
Environmental Protection
Agency
Robert S. Kerr Environmental Research EPA-600/2-80-066
Laboratory April 1980
Ada OK 74820
Research and Development
&EPA
Denitrification as
Affected by
Irrigation Frequency
of a Field Soil
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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
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The nine series are:
1. Environmental Health Effects Research
2. Environmental Protection Technology
3. Ecological Research
4. Environmental Monitoring
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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-600/2-80-066
April 1980
DENITRIFICATION AS AFFECTED BY IRRIGATION
FREQUENCY OF A FIELD SOIL
by
Dennis E. Rolston
Andrew N. Sharpley
Dianne W. Toy
David L. Hoffman
Francis E. Broadbent
Land, Air and Water Resources
University of California
Davis, California 95616
Grant No. R-805550
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 endorse-
ment or recommendation for use.
ii
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FOREWORD
Environmental protection efforts dealing with agricultural and nonpoint
sources have received increased emphasis with the passage of the Clean Water
Act of 1977 and the subsequent implementation of the Rural Clean Water
Program. As part of this Laboratory's research on the occurrence, movement,
transformation, fate, impact, and control of environmental contaminants, data
are developed to assess the causes and possible solutions of adverse environ-
mental effects of irrigated agriculture.
This report addresses the denitrification process as it affects the
management of nitrogen and water in an agricultural production system. An
understanding of the complete nitrogen cycle, including denitrification, is
required to make sound management decisions regarding nitrogen use- and water
use-efficiency in irrigated agricultural systems. This research should
benefit environmental managers as they attempt to understand and solve pollu-
tion problems related to nitrogeneous compounds and wastes.
William C. Galegar M
Director
Robert S. Kerr Environmental
Research Laboratory
iii
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ABSTRACT
The amount of nitrogen (N) as nitrate (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 nitrous
oxide (NaO) and dinitrogen (N2), are evolved whenever anoxic sites develop
within the soil and when sufficient carbon (C) 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 irrigation frequency
and soil incorporation of crop residue. Field plots were intensely instru-
mented with soil atmosphere samplers, soil solution samplers, tensiometers,
neutron access tubes, and thermocouples. Two different C treatments were
established by using plots to which no crop residues had been incorporated
within one year prior to the experiment and plots to which 10 metric tons ha *
of chopped barley straw were incorporated into the top 10 cm of soil two
months prior to fertilization. Irrigation frequencies of three irrigations
per week, one irrigation per week, and one irrigation every two weeks were
established on areas cropped with perennial ryegrass. Fertilizer was applied
at the rate of 300 kg N ha"1 as KN03 enriched with 56 to 58% 15N to 1-m2
plots. The flux of volatile gases at the soil surface was measured from the
accumulation of N20 and 15N2 beneath airtight covers placed over the soil
surface for one to four hours at several times immediately after irrigation
and at less frequent intervals as denitrification fluxes decreased.
Small rates of total denitrification were measured in this well-drained
alluvial soil under normal cyclic applications of irrigation water. For
plots without C addition, the largest denitrification of only 1.5% of the
applied fertilizer was measured in the most frequently irrigated plot. For
the least frequently irrigated plot of one irrigation every two weeks, only
0.7% of the fertilizer denitrified. For plots to which C was added as straw,
denitrification was greatly increased over that of the plots not receiving
straw. The greatest denitrification also occurred for the most frequently
irrigated plots with denitrification being between 5 and 6.5% of the fertil-
izer applied. For the least frequently irrigated plot, only 1.8% of the
fertilizer was denitrified. Denitrification rates decreased to near zero
values within one or two days after irrigation. The amount of N2 produced
was much greater than N20. The N20 flux at the soil surface varied between
5 and 27% of the total denitrification over a 40 to 50 day period. N20 mole
fractions tended to be smallest Immediately after irrigation and increased as
the soil water redistributed and the soil profile became less anoxic.
iv
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The irrigation frequency of three irrigations per week gave higher N03
concentrations as measured by both soil solution and soil samples within the
root zone of the crop than those of the other two frequencies. Thus, fre-
quent, small irrigations tended to result in less leaching losses than
frequent, large irrigations.
Denitrification as measured using the 15N enrichment method compared
reasonably well with that determined using the acetylene (C2H2) inhibition
method. However, rates of denitrification as measured by the two methods at
any one sampling time varied considerably due to the lags in reduction of N£0
to N£ and to possible development of organisms which may reduce ^0 in the
presence of acetylene.
Denitrification of N03 fertilizer was simulated using a mathematical
model that included transport and plant uptake of water and N in soil. The
rate of denitrification was considered to be a function of N03 concentration,
available C concentration, degree of soil-water saturation, and temperature.
Available C concentrations were calculated from initial amounts of soil C
and additions of plant residues or animal manure. The consumption of added
C in the soil system was assumed to occur in two or three stages with dif-
ferent rate constants for each stage and C addition. A QIQ value of two was
used to correct the denitrification rate constant and C consumption constants
for temperature. Model simulations for total denitrification were compared
with measured N2 plus %0 gas fluxes during NO^ leaching in field plots of
Yolo soil at different soil-water contents, C additions, soil temperature,
and irrigation frequencies. Reasonable agreement was found between measured
and calculated rates and total amounts of denitrification for all plots.
This report was submitted in fulfillment of Grant No. R805550 by the
University of California, Davis, under the sponsorship of the U.S.
Environmental Protection Agency. This report covers a period from
January 1, 1978 to September 30, 1979 and work was completed as of January 31,
1980.
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CONTENTS
Foreword ill
Abstract iv
Figures viii
Tables xiii
Abbreviations and Symbols xiv
Acknowledgments xv
1. Introduction 1
2. Conclusions 3
3. Recommendations 6
4. Experimental Procedures 7
Field installation 7
Experimental procedures - field 10
Analytical techniques 13
Analytical quality control 14
5. Results and Discussion 15
Plot characteristics 15
N2 and NgO surface fluxes 21
N20 mole fraction 29
Plant uptake 33
Soil solution N 33
Soil residual N 35
Mass balance of N 40
Denitrification simulation model 41
Model input data 44
Comparison of calculated and measured denitrification ... 45
Management simulations 53
References 55
Publications 57
vii
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FIGUKES
Number
1 Schematic diagram of experimental location and the treatment
layout. The area for measuring denitrificatian by the
CzS.z method did not contain wood borders placed in the
soil to a depth of 60 cm
Mean bulk density as a function of soil depth at.the experi-
mental site 10
Mean percentage of organic C as a function of soil depth at
the experimental site 11
Schematic diagram of the apparatus for measuring NjO evolved
using the C2H^ inhibition method 13
Mean soil temperature at the 5-cm soil depth as a function
of time during the experimental period. The arrows and
symbols on the graph indicate the time that fertilizer was
applied to the various plots 15
/
O2 concentration as a function of soil depth for two to
three sampling times after irrigation for each of the six
plots . 16
Soil-water content for the 15- and 60-cm soil depths as a
function of time for Plots A and B. The data points
represent values determined from neutron moisture meter
data. The arrows on the figures indicate the times of
irrigation 18
Soil-water content for the 15- and 60-cm soil depths as a
function of time for Plot C. The data points represent
values determined from neutron moisture meter data. The
arrows on the figures indicate the times of irrigation .... 18
»•
Soil-water content for the 15- and 60-cm soil depths as a
function of time for Plots D and E. The data points
represent values determined from neutron moisture meter
data. The arrows on the figures Indicate the times of
irrigation 19
viii
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Number Page
10 Soil-water content for the 15- and 60-cm soil depths as a
function of time for Plot F. The data points represent
values determined from neutron moisture meter data. The
arrows on the figures indicate the times of irrigation. ... 19
11 Soil-water pressure head at the 30- and 60-cm soil depths
as a function of time for Plots A and B. Each data
point represents the mean from triplicate tensiometers
at each depth. Arrows give the times of irrigation 20
12 Soil-water pressure head at the 30- and 60-cm soil depths
as a function of time for Plot C. Each data point
represents the mean from triplicate tensiometers at
each depth. Arrows give the times of irrigation 21
13 Soil-water pressure head at the 30- and 60-cm soil depths
as a function of time for Plots D, E, and F. Each data
point represents the mean from triplicate tensiometers
at each depth. Arrows give the times of irrigation 22
14 The N20 and N2 flux at the soil surface as measured by the
accumulation of gases beneath covers as a function of
time for Plot A. The open circles are for N2 and the
closed circles are for N20. The arrows give the times
of irrigation 23
15 The N20 and N2 flux at the soil surface as measured by the
accumulation of gases beneath covers as a function of
time for Plot B. The open circles are for N2 and the
closed circles are for N.O. The arrows give the times
of irrigation 23
16 The N20 and N2 flux at the soil surface as measured by the
accumulation of gases beneath covers as a function of
time for Plot C. The open circles are for N2 and the
closed circles are for N20. The arrows give the times
of irrigation 24
17 The N'20 and N2 flux at the soil surface as measured by the
accumulation of gases beneath covers as a function of
time for Plot D. The open circles are for N2 and the
closed circles are for NaO. The arrows give the times
of irrigation ..... 24
18 The N20 and N2 flux at the soil surface as measured by the
accumulation of gases beneath covers as a function of
time for Plot E. The open circles are for N2 and the
closed circles are for N20. The arrows give the times
of irrigation 25
ix
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Number pagt
19 The N20 and N2 flux at the soil surface as measured by the
accumulation of gases beneath covers as a function of
time for Plot F. The open circles are for N2 and the
closed circles are for N.20. The arrows give the times
of irrigation 26
20 Comparison of the denitrification flux as a function of
time as measured by the N and C2H2 inhibition methods
for Plots A, B, C, G, H, and I. The denitrification flux
is the sum of N plus N.20. The broken lines and open
circles are for the C2H2 method. The closed circles and
solid line are for the 15N method. Arrows give the
times of irrigation 27
21 The N20 mole fraction as a function of time for Plots A and
G. The solid lines give the N20 mole fraction using the
15N method and the broken lines give the N 0 mole fraction
using the C2H2 method 2 30
22 The N20 mole fraction as a function of time for Plots B and
H. The solid lines give the N.O mole fraction using the
'15. "2 "
H method and the broken lines give the N20 mole fraction
using the C2H2 method 30
23 The N20 mole fraction as a function of time for Plots C^and I.
The solid lines give the N20 mole fraction using the N
method and the broken lines give the N20 mole fraction
using the CaH? method 31
24 The N20 mole fraction as a function of time using the l %
method for Plot D. The arrows give the times of irrigation . 32
25 The NzO mole fraction as a function of time using the 15N
method for Plot E. The arrows give the times of irrigation . 33
26 The N20 mole fraction as a function of time using the 15N
method for Plot F. The arrows give the times of irrigation . 34
27 Plant uptake of fertilizer N as a function of time after
fertilizer addition for all six plots, except for Plot A
for which no grass was harvested 35
28 Soil solution fertilizer N as a function of depth for Plots
A, B, and C for a sampling time midway through the ex-
perimental period (upper part of figure) and at the end
of the experimental period (lower part of figure). Plot
A was not sampled at the end of the period. The data
points represent the mean-concentration from triplicate
soil solution samplers 36
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Number Pag<
29 Soil solution fertilizer N as a function of depth for Plots
D, E, and F for a sampling time midway through the experi-
mental period (upper part of figure) and at the end of the
experimental period (lower part of figure). The data
points represent the mean concentration from triplicate
soil solution samplers 37
30 Labeled inorganic and organic N for Plots A, B, and C as a
function of soil depth at the end (63 days for B and C,
49 days for A) of the experimental period 38
31 Labeled inorganic and organic N for Plots D, E, and F as a
function of soil depth at the end (36 days) of the ex-
perimental period 39
32 Flow diagram giving the order of calculations in the simu-
lation model 42
33 Measured and calculated surface fluxes of denitrification
products (N2 + N20) as a function of time for two manure-
amended plots maintained at two different values of soil-
water pressure head, h 45
34 The dependence of the empirical water function, fy, (Eq. [1])
on relative soil-water content (water content/saturated
water content) 48
35 Measured and calculated surface fluxes of denitrification
products (Na + NaO) as a function of time for plots with
and without straw incorporation at an irrigation frequency
of three irrigations per week. The solid lines are simu-
lations based on Eq. [1]. The broken lines simply connect
measured data points. Arrows indicate time of irrigation.
Note that the scales of the ordinate are greatly different
for the "no straw" and "straw" plots 49
36 Measured and calculated surface fluxes of denitrification
products (N2 + NjiO) as a function of time for plots with
and without straw incorporation for an irrigation frequency
of one irrigation per week. The calculated lines are simu-
lations based on Eq. [1]. Arrows indicate time of irriga-
tion. Note that the scales of the ordinate are greatly
different for the "no straw" and "straw" plots 50
xi
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Number Page
37 Measured and calculated surface fluxes of denitrification
products (N2 + NzO) as a function of time for plots with
and without straw incorporation for an irrigation frequency
of one irrigation every two weeks. The calculated lines
are simulations based on Eq. [1]. Arrows indicate time of
irrigation. Note that the scales of the ordinate are
greatly different for the "no straw" and "straw" plots. . . 51
38 Three hypothetical soil-water content versus time curves
for an irrigation frequency of one irrigation per week
for plots with straw incorporation 52
xii
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TABLES
Number Pag(
1 Labeling System for the Nine Plots at Three Different
Irrigation Frequencies and Two Carbon Additions 9
2 Particle Size Distribution and Texture with Soil Depth ... 9
3 Characteristics of 15N Plots 12
4 Amount of Irrigation Water Applied and Estimated Evapo-
transpiration for 10- or 11-Day Periods During the
Experimental Period 17
5 Amounts of N^O and N2 Produced During Denitrification of
Added Fertilizer N as Measured by the C2H2 and 15N
Methods 29
6 Mass Balance of Fertilizer N in the Various Components of
the N Cycle for Each of the Six 15N Plots. Leaching was
Determined by Difference From the Other Components .... 40
7 Comparison of Measured and Calculated Denitrification From
Constant Water Plots on Yolo Loam Soil. A Value of kj
of 1.68 x 10-* g Soil Day"1 (yg C)-1 was Used for the
Manure and Uncropped Calculations. A Value of kj of
6 x 10" ** g Soil Day"1 (yg C)-1 was Used for the Cropped
Calculations 47
8 Total Denitrification (kg N ha"1) Calculated for Various
Ways of Applying NO" Fertilizer During One Irrigation
Cycle of Cropped Soil to Which Straw was Applied 43 days
Prior to Fertilization. Simulations Were Made for
Approximately 40 days After Fertilization 53
xiii
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LIST OF ABBREVIATIONS AND SYMBOLS
ABBREVIATIONS
cm
m
ml
°C
kg ha"
mg
ppm
SYMBOLS
NOs
N20
N2
ISN
C02
02
c
Na
Cu
H20
63Ni
A
x
h
6
e
— centimeter ppmv
— meter
— milliliter cm3
— degrees Centigrade g
— kilograms (103 grams) yg
per hectare hr
— milligram (10~3 grams) ET
— parts per million on a MT
weight basis
acetylene gas
Nitrogen
Ammonium
Nitrite
Nitrate
Nitrous Oxide gas
Nitrogen gas
Nitrogen -isotope of
mass 15
Carbon dioxide gas
Oxygen gas
Carbon kj
Sodium
Copper
Sulfate cs
Chloride
Water
Nickel isotope of mass t
63 kg
Potassium
Angstrom
distance (cm) gw
soil-water pressure
head (cm) C^
soil-water content
(cm3 cm"3)
saturated soil-water k^
content (cm3cm"3)
soil bulk density
(g cm"3) ko
N
w
f-T
parts per million on a
volume basis
cubic centimeters
gram
micro grams (10 6 grams)
hour
evapo transp irat ion
metric ton (103 kg)
sum of denitrification
gases (N2 + N20)
[ygN(g soil)-1]
concentration of NO3
[yg N (cm solution)"3]
concentration of water
soluble carbon [yg C
(g soil)-1]
water function for de-
nitrification
temperature function
first-order denitrifica-
tion constant [g soil
concentration of total
soil organic carbon
[yg C(g soil)"1]
time (day)
first-order constant for
soil carbon decomposition
(day-1)
water function for carbon
decomposition
concentration of total
organic carbon from straw
or manure [yg C(g soil)"1]
first-order constant for
straw or manure carbon
decomposition (day"1)
zero-order denitrification
constant [yg N day
(yg cr1]
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) and
the University of California Agricultural Experiment Station. The laborato-
ries and mass spectrometer facilities of the Department of Land, Air and
Water Resources provided strong support for this research. The senior author
also gratefully acknowledges support by the Soil Science Department, Univer-
sity of Florida, Gainesville, while on sabbatical leave. The denitrification
part of the simulation model was developed at the University of Florida in
cooperation with J.M. Davidson, P.S.C. Rao, and R.E. Jessup, during the
sabbatical leave of the senior author.
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SECTION 1
INTRODUCTION
The amount of N03 reaching the groundwater of irrigated lands is depend-
ent upon each of the components of the N cycle in soils. One of the poten-
tial losses of N from the soil system for which absolute amounts and rates
are not well known is denitrification. Volatile denitrification products,
primarily N20 and N2, are evolved whenever anoxic sites develop within the
soil and when sufficient C 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 N03 in irrigation return flow water as a
function of irrigation and cropping practices (Mehran and Tanji, 1974;
Donigian and Crawford, 1976; Shaffer et_ al., 197^; Tanji and Gupta, 1978-; 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 fertilizers is used
quite frequently such as 10 to 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 e_t 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. These studies only
evaluated the amount of denitrification under one cropping or C input system
and one soil-water content near saturation. Rolston and Broadbent (1977),
Rolston e£ al. (1978, 1979) directly measured denitrification from the fluxes
of N2 and N20 at the soil surface of small, intensely-instrumented field
plots. NO3 fertilizer was applied to plots which had a crop growing on the
soil, to plots to which manure had.been added, and to uncropped plots main-
tained at two different soil-water contents near saturation and at two dif-
ferent temperatures (winter and summer). These experiments were conducted
for constant water content conditions over the entire period that denitri-
fication measurements were made. These experiments defined the range over
which denitrification might occur and gave the potential rates and total
amounts that might be expected in field soils. However, the continual main-
tenance of high water content conditions for long time periods in the field
is generally not the normal practice which might occur during irrigation or
rainfall events. The wetting and drying cycles which would take place under
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field situations, due to either rainfall or irrigation, may drastically
change the rates and total amounts of denitrification that may occur. The
rate that a microbial population can increase from a relatively small biomass
in an air dry soil to a population which could effectively reduce N03, the
length of time that irrigation water is maintained on the soil, and the rate
of redistribution of applied irrigation water within the soil profile would
all have a very dynamic effect on the denitrification process.
Ryden et_ al. (1979) directly measured denitrification in field soils of
the Santa Maria Valley of California using the C2H2 inhibition technique.
The C^B.2 inhibition method is based upon evidence that C2B.2 completely blocks
the reduction of N20 to N2 in the denitrification sequence. Thus, all deni-
trification yields N20 which is easy to measure without the use of 15N.
The objectives of the research reported here were:
A. To directly measure fluxes of N2 and N20 gases from a field soil as
influenced by three different irrigation frequencies and two levels of
C.
B. To compare denitrification obtained directly using 15N2 and N20 gas
fluxes from 15N enriched fertilizer with denitrification measured directly
using the C2H2 inhibition method.
C. To evaluate existing N simulation models to determine if such models
could simulate the dynamic denitrification process that occurs during and
after normal irrigation cycles and to develop or Improve existing models to
adequately consider denitrification.
The research was conducted on small 1-m2 field plots because of the
large cost of NO^ fertilizer tagged with high enrichments of the stable
isotope 15N. The experiments were conducted at three different irrigation
frequencies of three irrigations per week, one irrigation per week, and one
irrigation every two weeks with the same, total amount of water applied to
each plot. The plots also had two C levels; one in which no plant materials
(residues) were added to the soil for more than one year prior to the experi-
ment and a second in which 10 metric tons per hectare (MT ha'1) of chopped
barley straw were added approximately two months prior to fertilization.
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SECTION 2
CONCLUSIONS
The following major conclusions were obtained from this research:
1. The results of this research demonstrate that denitrification rates
and total amounts are generally small for normal irrigation practices in
fairly well-drained, alluvial soil.
The greatest amount of fertilizer N lost through denitrification was
only 1.5% of the total N applied (300 kg N ha"1) for .the situation where
plant materials had not been incorporated into the soil for greater than one
year prior to the experiment. For the plots to which straw was incorporated
two months prior to fertilization, the greatest denitrification loss was
still only 6.5% of the total fertilizer applied (300 kg N ha'1). It is
expected that these values would be similar for other well-drained, loam
soils of similar C levels. Little denitrification is expected in sandy
soils. Approximately two or three times the denitrification measured here
might be expected in clay soils. The presence of hardpans, impeding layers,
textural discontinuities or high water tables in the soil profile would all
tend to increase the amount of denitrification over that given in this
report.
2. Denitrification rates were largest immediately after the first irri-
gation and decreased for subsequent irrigations.
Denitrification fluxes tended to decrease quickly within one to two days
after irrigation. The soil-water pressure head values for one to two days
after irrigation corresponded fairly closely with those from experiments of
Rolston et al. (1978) for constant water content plots. This very rapid
decrease in denitrification fluxes soon after irrigation was most likely due
to rapid redistribution of the soil water deeper into the soil profile
resulting in oxygen (02) diffusing into the soil pores and a decrease in the
amount of anoxic soil volume.
3. The presence of added organic C greatly increases denitrification
rates and total amounts due to the availability of C derived from the added
crop materials.
The effect of C in the denitrification process is very important,
especially that from crop or manure additions. However, simulations using
the denitrification model indicate -that soil C levels or organic matter
levels can be increased by two or three times with only slight increases in
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denitrification. This is due to the fact that only a small proportion of the
total organic C is available for denitrification.
4. In general, the plots irrigated frequently, with small amounts of
water,- resulted in the greatest amount of denitrification.
Those plots receiving irrigation only once every two weeks resulted in
very small amounts of denitrification and were much smaller than the more
frequently irrigated treatments. This phenomenon of greatest denitrification
under the most frequently irrigated plots is partially due to the initial
distribution of the added N(>3 fertilizer during the first irrigation. NOa
fertilizer was applied uniformly during the first irrigation so that the NO^
band was distributed over a much narrower depth interval for the frequently
irrigated experiments than that of the less frequently irrigated experiments.
Another important factor affecting the amount of denitrification for the
least frequent irrigations was the fact that the soil profile was fairly dry
at the initiation of each irrigation. There may have been some time lag in
the development of anoxic conditions and microbial activity. However, the
water applied to the initially dry profiles redistributed very quickly with
little time available for the development of anoxic conditions conducive to
denitrification.
5. Total denitrification for plots without straw additions compared
reasonably well for the 15N and €2^2 inhibition methods, although the rates
measured at any one day were very much different between the two methods of
directly measuring denitrification gases.
These differences in rates at any one time period were attributed to the
lag in reduction of %0 to N2 for the 15N method and possibly to the develop-
ment of organisms which could reduce ^0 in the presence of
6. The NzO mole fraction was generally small immediately after irriga-
tion and then increased as redistribution of soil water resulted in less
anoxic conditions within the wetted soil zone.
The mole fraction as measured by the 15N and C2H2 methods compared
reasonably well. There was some indication that the %(> mole fraction tended
'to decrease with subsequent irrigations possibly due to the effect of N03
concentration on the inhibition of ^0 reduction.
7. The addition of plant materials such as barley straw resulted in a
decrease in the N20 mole fraction over those experiments without the addition
of barley straw.
This again would be expected since greater anoxic conditions would
develop in the plots to which straw was .added than those without straw,
resulting in more favorable conditions for %(> reduction to N2»
8. The data on %() mole fraction demonstrate that the proportion of
produced during denitrification was very dynamic and variable.
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Mole fractions varied from zero to one for treatments without C addi-
tions and varied from nearly zero to 0.4 or 0.5 -for plots with C additions.
The overall N2<3 mole fraction throughout all irrigation cycles varied from
0.04 to 0.27.
9. The frequently irrigated plots with small applications of water re-
sulted in higher NO3 concentrations in the root zone than those plots with
less frequent, larger applications of water.
The most frequently irrigated plots also resulted in greater plant up-
take of fertilizer N, most likely due to higher NO3 concentrations in the
root zone, and soil-water contents potentially more conducive to plant
growth. The most frequently irrigated plots also tended to lose less fer-
tilizer by leaching than that in the least frequently irrigated plots. A
water management program using small irrigations several times per week would
tend to increase denitrification. However, the increase in denitrification
may be more than compensated by less leaching and more plant uptake of
applied N.
10. The denitrification simulation model was able to reasonably predict
rates and total amounts of denitrification with a minimum amount of model
calibration.
First order kinetics with respect to NO3 concentration gave the best
prediction of denitrification rates and total amounts for all plots. This
does not mean that denitrification per se followed first-order kinetics due
to the fact that diffusion of N03 to anoxic zones may be the primary
mechanism resulting in a better fit using first-order than zero-order
kinetics. The model is very sensitive to soil-water content, which is
expected as previous data indicated that the very dynamic nature of denitri-
fication is dependent upon the amount of water in the soil. It may be better
to use an Q£ diffusion and consumption component to directly describe the
anoxic volume development. However, it is expected that this would also be a
very sensitive parameter and the necessary input data to do such a calcula-
tion ss& complex and not available. The amount of organic C derived from
manure and straw additions, which is available for denitrification, is still
somewhat uncertain and needs to be researched.
-------
SECTION 3
RECOMMENDATIONS
The following recommendations for efficient management of water and
nitrogen under irrigated conditions can be proposed from this research:
1. To decrease potential M>3 leaching and pollution of groundwater,
small and frequent applications of irrigation water should be made instead
of larger, less frequent applications. However, increased labor, equipment,
and energy costs must be considered before recommending this management
technique as a viable alternative to present irrigation practices.
2. To increase N-use efficiency of applied N fertilizers (decrease
denitrif ication) , incorporation of organic materials should be made at least
two months prior to NO 3 additions.
3. Future research should be directed at understanding the dynamic
effects of C from crop and manure incorporation into the soil on denitrif ica-
tion rates and total amounts. The addition of C greatly increases denitrif i-
cation, yet there is very little information on the proportion of the applied
crop or manure C which is available for denitrification as a function of time
after incorporation.
4. In simulation modeling of the denitrification process in field
soils, the use of a water function based on relative soil-water saturation is
the most useful and easily-determined parameter indirectly accounting for the
degree of anoxic soil development. Some means of accounting for degree of
anoxic soil development is essential in simulation of denitrification. The
applicability of the soil-water function developed in this report to other
soils needs further research.
-------
SECTION 4
EXPERIMENTAL PROCEDURES
FIELD INSTALLATION
Six plots for the 15N method and three plots for the C^^z method were
established on Yolo loam soil, a member of the fine-silty, mixed, non-acid,
thermic, Typic Xerorthents family, at Davis, California. The Yolo loam soil
is a deep, well-drained, alluvial soil in the Sacramento Valley. The soil is
similar to other soils of extensive acreage. The schematic diagram of the
experimental location and the treatment layout is given in Figure 1. Each of
1
Three Irrigations
per week
PLOT A
No Straw
I5N Method
PLOT D
10 Mt ho"'
Straw Added
I5N Method
1 -|
PLOT 6
No Straw
C2H2 Method
One Irrigation
per week
PLOTB
No Straw
I5N Method
PLOT E
10 Mt ha"'
Straw Added
I5N Method
PLOTH
No Straw
C2H2 Method
1
One Irrigation
per two weeks
PLOTC
No Straw
I5N Method
PLOTF
10 Mt ha"'
Straw Added
I5N Method
I~
PLOT I
No Straw
C,H- Method
Lll__J
Figure 1. Schematic diagram of experimental location and the treatment lay-
out. The area for measuring denitrification by the C2H2 method
did not contain wood borders placed in the soil to a depth of 60
cm.
-------
the six, 1-m2 plots (A through F) was established with a 60-cm deep redwood
barrier around the outside edges of each undisturbed block of soil. Redwood
barriers were installed by digging a trench around the 1-m2 areas, slipping
the redwood over the undisturbed block of soil, and backfilling the trench on
the outside of the redwood. The space between the wood barrier and the soil
on the inside was sealed by pouring melted paraffin into the small crack be-
tween the soil and the wood. Each of the six plots was instrumented with
tensiometers, soil solution samplers, soil atmosphere samplers, thermo-
couples, and a neutron access tube. Triplicate soil atmosphere samplers were
installed at the 2-, 5-, 15-, 45- and 60-cm soil depths. Triplicate samplers
designed to function as tensiometers or solution extractors were installed at
30-, 45-, 60-, and 90-cm depths. Duplicate thermocouples were installed at
the 5-cm depth. Soil solution samplers consisted of porous cups glued to
polyvinyl chloride tubing. Soil atmosphere samplers consisted of 0.1 cm
inside diameter nylon tubing glued into a 5-cm long, 0.25-7-cm I.D. 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 was glued into a milled plastic tip. For all samplers, the
volume of the sampling tubes was very small (less than 1.0 cm?). Soil solu-
tion samples were obtained by evacuating bottles connected to samplers. Soil
atmosphere samples were obtained by withdrawing 1 ml of gas with glass
syringes. All gas samples were analyzed within a few hours after sampling.
In addition to the six plots with redwood barriers down to the 60-cm
depth, three plots were also established to evaluate the C2H2 method for
directly measuring denitrification.
The plots were irrigated by three different irrigation frequencies.
Irrigation frequencies were three irrigations per week, one irrigation per
week, and one irrigation every two weeks. All plots received the same amount
of water which was Intended to be 15% greater than evapotranspiration (ET).
The plots were irrigated with a spray irrigation system which consisted of
spray nozzles on a traveling boom. The irrigation system applied water at a
rate of 0.54 cm hr"1 to Plots A, D, and G; 0.63 cm hr"1 to Plots B, E, and
H; and 0.71 cm hr"1 to Plots C, F, and I.
In order to establish different C treatments within each of the three
irrigation frequencies, three plots were used for which no C additions such
as plant residues or weeds were incorporated for one year prior to the ex-
periment. Three plots of each irrigation frequency had 10 ME ha 1 of chopped
barley-straw added to the soil approximately two months prior to the initia-
tion of denitrification experiments. Chopped straw was mixed in the top 10
cm of the soil surface. All plots and the surrounding buffer areas were
planted with perennial ryegrass (Lolium perenne). The grass was planted on
the plots approximately two months prior to the initiation of denitrification
experiments. The €2^-2 inhibition plots did not have straw additions. Table
1 gives the plot labeling system and the irrigation frequency and C treat-
ments for the plots.
Particle size analyses and texture as a function of soil depth for the
Yolo loam soil are given in Table 2, and the average bulk density at the
field site is given as a function of depth in Figure 2. The bulk density was
8
-------
TABLE 1. LABELING SYSTEM FOR THE NINE PLOTS AT THREE DIFFERENT
IRRIGATION FREQUENCIES AND TWO CARBON ADDITIONS
Plot
A
B
C
D
E
F
G
H
I
Denitrification
method
15N
15N
15N
15N
15N
15N
^2^2
C2H2
C2H£
*
Carbon
addition
0
0
0
10 MT ha"1
10 MT ha'1
10 MT ha"1
0
0
0
Irrigation
frequency
3 per week
1 per week
1 per 2 weeks
3 per week
1 per week
1 per 2 weeks
3 per week
1 per week
1 per 2 weeks
Chopped barley straw incorporated into the top 10 cm of soil.
TABLE 2. PARTICLE SIZE DISTRIBUTION AND 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
20
22
24
Texture
Loam
Loam
Loam
Loam
Loam
Silt Loam
Silt Loam
-------
1.10
BULK DENSITY (g cm"3)
1.20 1.30 1.40
1.50
Figure 2. Mean bulk density as a function of soil depth at the experimental
site.
greatest near the soil surface with a minimum at the 120-cm depth. Bulk
density was determined on triplicate 7.6 cm long, 7.6 cm diameter undisturbed
soil cores for each depth. The percentage of organic C as a function of soil
depth is given by Figure 3. Organic C was determined on soil samples taken
during the experiment conducted by Rolston ejt al. (1978, 1979).
EXPERIMENTAL PROCEDURES - FIELD
After the plots had gone through several irrigation cycles and the
grass was well established, KNOs solution was applied uniformly to the plots
throughout one complete irrigation. Dry NO^ fertilizer was also applied to
the surrounding border area. The total amounts of fertilizer and the 15N
enrichment of the fertilizer applied to each plot are given in Table 3.
Immediately after irrigation, 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 red-
wood border. Samples of the atmosphere beneath the cover were taken after
two to four hours with the lid in place and analyzed for 15Na and N20. Soil
atmosphere samples from within the soil profile were also taken soon after
10
-------
ORGANIC CARBON (%)
0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8
1
o
I
1-
Q.
UJ
O
-1
O
CO
w
20
40
60
80
100
i9n
1 1 1 1 1 1 1 1
/
/
\
xx/P
PX
Q
1
J
Q
o/
1 1 1 1 1 1 1 1
Figure 3. Mean percentage of organic C as a function of soil depth at the
experimental site.
applying the fertilizer. Soil atmosphere samples were taken in 1-ml aliquots
and N20, Q^t SD-^- ®2 analyzed by gas chromatography in the laboratory. An-
other 0.5 to 1 ml of gas was taken to determine *%2 with the mass spec-
trometer. Gas samples from the profile and samples from.beneath the cover
were taken several times per day for a few days after irrigation and at less
frequent intervals until the next irrigation cycle. The volume of the
chambers placed over the plots are also given in Table 3. By using the
volume of the chambers, the 15N enrichment of the applied fertilizer, the
precision of measuring 15N£ by the mass spectrometer, and the time period
that covers remained over the plots for each sampling period, a minimum
detection limit for 15N2 of 0.1 to 0.2 kg N ha"1 day : was determined. Thus,
for any flux smaller than this limit, it is uncertain whether those values
are real or not. The minimum detection limit for N20 was at least two orders
of magnitude smaller than that for 15N2«
For measurement of denitrification with the C2H2 inhibition method, the
three main plots (G, H, I) were divided into six sub-plots (0.05 m2) which
were bounded by 25 cm deep, acrylic plastic barriers, protruding 10 cm above
the soil surface. The sub-plots were separated by at least two meters.
On three of the sub-plots C2H2 flowed slowly (one liter hr"1 for one hour)
into the soil profile through six, 1-m long, perforated, acrylic plastic
11
-------
TABLE 3. CHARACTERISTICS OF 15N PLOTS
Volume of Fertilizer % 15N excess Starting
Plot Area (m2) Chamber (m3) (kg N/ha) of fertilizer date
A
B
C
D
E
F
1.0
1.0
1.0
1.0
1.0
1.0
0.0289
0.316
0.276
0.0265
0.0269
0.0349
281
284
282
288
288
287
58.7
58.7
58.7
59.8
59.8
55.9
7/3/78
7/4/78
7/10/78
8/28/78
8/29/78
9/4/78
tubes. The chambers for measuring N20 flux were placed over the soil one
hour after the C2H2 flow had stopped. The six sub-plots, three with and
three without C2H2, were subjected to the same three irrigation frequencies
as those plots to which 15N was applied. (Table 1 and Figure 1.)
After two complete irrigation cycles, KN03 solution equivalent to 300 kg
N ha"1 were uniformly applied as for the 15N method, to a 1-m2 area, en-
closing each plot. Consequently, NO^ fertilizer was also applied to the
surrounding border area. Six hours after applying the fertilizer solution,
an airtight cover was placed over the plots and the enclosed air space (7.5
liters) above the soil was slowly but continuously_swept by drawing air
through the chamber at a flow rate of 25 liters hr l for three hours. The
gas swept from the cover was passed through dehydrite and ascarite to remove
H20 and C02, respectively, and finally through a 5 A molecular sieve trap
which quantitatively adsorbed N20 (Hahn, 1972; Ryden e£ al., 1979). A
schematic diagram of the apparatus for measuring N20 evolved using the C2H2
inhibition method is given by Figure 4. The recovery of N20 from the 5 A
molecular sieve was carried out as described by Ryden e_t al. (197Q). The
minimum detectable flux of N20 using the molecular sieve trap was approxi-
mately 0.005 kg N ha"*1 day"1.
Soil solution samples were taken at two times during the experiment.
The grass of the plots was cut periodically and the total clippings were
dried for analyses. Soil .samples were taken midway through the experimental
period and at the end of the experimental period for Plots A, B, and C. Soil
samples were taken only at the end of the experimental period for Plots D, E,
and F. Soil samples were taken in 15-cm increments down to 120 cm. The
samples consisted of ten separate holes taken with a Veihmeyer tube within
the 1-m2 plots. The samples were combined to give two samples at each depth
for analyses.
12
-------
COVER
ACETYLENE
ASCARITE
DEHYDRITE
VACUUM
PUMP
5A MOLECULAR
SIEVE
Figure 4. Schematic diagram of the apparatus for measuring N20 evolved using
the C2H2 inhibition method.
ANALYTICAL TECHNIQUES
Oxygen, N2, and C2H2 were analyzed by gas chromatography with a thermal
conductivity detector. The concentration of N20 in the gas samples was
determined by chromatography using a hot 63Ni electron capture detector as
described by Rasmus sen et al. (1976) . The isotopic composition of N in gas
samples was determined on samples scrubbed for 02, C02, and H20 vapor and
directly injected into the mass spectrometer. Details for determining iso-
topic composition of N by mass spectrometry is given by Rittenberg (1948).
Soil samples were analyzed for extractable (inorganic) and digestible
(organic) N and soil solution samples were analyzed for NH^, NO 3, and N02.
A soil sample was extracted with 1.0 N KC1 and the solution analyzed by the
magnesium oxide-devarda alloy reduction technique. The extraction procedure
removed solution NHi^, N02, NOa, and exchangeable NH^. The NH^ and N02 con-
centrations in all soil and soil solution samples were negligible. The
KJeldahl method was used to determine the total digestible N in soil and
plant samples. Two-gram samples of soil were digested with 36 N H2SOtf and
salts (I&SO^, CuSOi}, and selenium) for approximately 17 hours to convert the
N to NHjj. The same procedure was used for the plant digests except that 0.25
13
-------
g of plant material were used and the digestion time was 6 hours. The N in
the digest 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 were given by Bremner (1965).
The soil for organic C 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
furnace. The C02 produced was collected in a Nesbit tower containing
ascarite. The tower was weighed before and after the burn to determine the
amount of C02 trapped. Detailed procedures for determination of organic C
in soil were given by Allison (1965). There was no difference in the % C
between a soil sample that had been extracted with KC1 and a sample that had
not been extracted.
ANALYTICAL QUALITY CONTROL
To Insure accuracy of the results all analytical methods were checked
periodically with standard samples. For gas chromatography and mass
spectrometer analyses, samples of standard gas were analyzed at least every
twenty samples. Chemical techniques for determining inorganic and organic N
in soil and soil solution samples and plant N were tested by evaluating
standard samples at least every 30 samples. In addition, duplicate soil,
soil solution, and plant samples were always used. If one duplicate varied
by more than 5% from the other, samples were rerun. Also, blanks (deionized
water) were run every 15 samples to check for contamination.
14
-------
SECTION 5
RESULTS AND DISCUSSION
PLOT CHARACTERISTICS
Temperatures at the 5-cm soil depth as a function of time during the
experimental period are given by Figure 5. The arrows indicate the time that
fertilizer was applied to particular plots. Plots G, H, and I were conducted
at the same time as Plots A, B, and C. Soil temperature remained relatively
constant during most of the measurements on Plots A, B, C, 6, H, and I. How-
ever, on Plots D, E, and F, the soil temperature tended to decrease with time
later in the summer.
40 60
TIME (days)
too
Figure 5. Mean soil temperature at the 5-cm soil depth as a function of time
during the experimental.period. The arrows and symbols on the
graph indicate the time that fertilizer was applied to the various
plots.
15
-------
The 62 concentration as a function of soil depth for two or three
sampling times for the six 15N plots, are given in Figure 6. These data
represent typical measurements after irrigation. It can be seen that for
02 CONCENTRATION (%)
10 15 20
15 20 25
• t • I hr
o t• 21hr
f39hr
Figure 6. 02 concentration as a function of soil depth for two to three
sampling times after irrigation for each of the six plots.
'Plots A, B, and C (no C additions) that 02 concentrations were relatively
high, even within a few hours after irrigation. Oxygen concentrations did
not decrease below 10% at any depth within the profile. The effect of the
straw addition is demonstrated by the low 02 concentrations near the soil
surface for Plots D, E, and F. The lowest 02 concentrations tended to occur
immediately after irrigation. There was a slight increase in 02 concentra-
tion as the soil -profile drained and water was used by the crop. The concen-
trations of 02 in Plot F did not drop below 10%. The small decrease in 02
was probably due to the fact that irrigation was made only every two weeks.
Therefore, the water infiltration and redistribution in the dry profile was
relatively rapid with little opportunity for depletion of 02 within the soil
profile. Although 02 concentration within the soil profiles is not a good
indication of denitrification due to the fact that the samples are taken
primarily from large pore sequences, these data indicate that one should
16
-------
expect more denitrification in Plots D, E, and F than in Plots A, B, and C
due to the low Q£ concentrations for those plots'to which straw had been
added.
Table 4 gives the amount of irrigation water applied and the estimated
ET for 10 or 11 day periods during the experiment. The ET was estimated from
pan evaporation data taken from a grassed area near the experimental plots.
The crop ET was estimated from the pan evaporation data and a crop coeffi-
cient factor which was determined over many years of experiments relating pan
evaporation to ET of grass using lysimeters near the experimental site. For
most time periods during the experiment, the amount of irrigation water
applied was greater than the estimated ET. The objective was to apply
approximately 15% more water by irrigation than was evapotranspired.
TABLE 4. AMOUNT OF IRRIGATION WATER APPLIED AND ESTIMATED EVAPOTRANSPIRATION
FOR 10- OR 11-DAY PERIODS DURING THE EXPERIMENTAL PERIOD
Irrigation Estimated
water evapotranspiration
7/1
7/11
7/21
8/1
8/11
8/21
9/1
9/11
9/21
10/1
Dates
- 7/10
- 7/20
- 7/31
- 8/10
- 8/20
- 8/31
- 9/10
- 9/20
- 9/30
- 10/10
Total
applied (cm)
5.7
5.7
6.0
6.0
6.0
5.2
4.6
4.0
4.0
4.0
51.2
(cm)
5.0
4.8
4.9
5.3
5.6
4.1
3.3
5.2
3.2
3.0
44.4
The soil-water content, 9 (cm3cm"3), for the 15- and 60-cm depths of the
six 15N plots are given as a function of time in Figures 7, 8, 9, and 10.
Zero time is initiation of irrigation. The water content data for Plots A
and B, Plot C, Plots D and E, and Plot F are given by Figures 7, 8, 9, and
10, respectively. The arrows on each figure indicate the time of irrigation
17
-------
.42
.38
§ .34
-' .30
-------
.38
.36
'§.34
£.32
(D
H .40
1
a: .38
Hi
I .36
.32 -
i i i i r
•—• I5cm
o—o 60cm
V:
I
Plot E
•—• 15cm
o-~-o 60cm
I
J_
Figure 9.
4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 3'4 36 38
TIME (days)
Soil-water content for the 15- and 60-cm soil depths as a function
of time for Plots D and E. The data points represent values de-
termined from neutron moisture meter data. The arrows on the
figures indicate the times of irrigation.
.42
6
o
'.36
I
|.34
i .32
=1.28
O
-------
for each treatment. As expected, the water content at the 15-cm depth was
greatly dependent upon rate of drainage, ET, and irrigation application. The
water content at the 15-cm depth increases immediately after irrigation to
nearly the saturated water content value and then slowly decreases due to
drainage and crop use until the next irrigation. As expected, the water
content of the 60-cm depth was less variable and remained fairly constant
with slight increases in water content after each irrigation. The magnitude
and rate of change of 6 at the 15-cm depth was strongly dependent upon irri-
gation frequency as shown in the figures.
The soil-water pressure head, h (cm of water), at the 30- and 60-cm
depths as a function of time are given for Plots A and B, Plot C, and Plots
D, E, and F, by Figures 11, 12, and 13, respectively. The arrows on each
figure indicate the time of irrigation. The 30- and the 60-cm tensiometers
u
UJ
X
K
CO
in
bJ
K
0.
K
111
I
-2OO -
-240 -
8 10 \Z 14
TIME (days)
16 18 20 22 24 26 28 30 32 34 36
Figure 11. Soil-water pressure head at the 30- and 60-cm soil depths as a
function of time for Plots A and B. Each data point represents
the mean from triplicate tensiometers at each depth. Arrows give
the times of irrigation.
responded fairly quickly to each irrigation for Plot A (irrigated three times
per week). The 30-cm depth tensiometer did not decrease below h = -40 cm
during the measurement period. For Plots B and C, however, the 30-cm
tensiometers dropped down to h - -240 cm and h = -600 cm, respectively.
Plots D, E, and F did not show as great a decrease in soil-water pressure
head due most likely to decreasing ambient temperatures resulting in less
20
-------
100
x"
u
~ -100
-200
IU
ct
Q.
UJ
i
o
CO
-300
-400
-500
-600
4
10 12 14 16 18 20 22 24 26 28 30
TIME (doys)
Figure 12. Soil-water pressure head at the 30- and 60-cm soil depths as a
function of time for Plot C. Each data point represents the mean
from triplicate tensiometers at each depth. Arrows give the
times of irrigation.
ET than that anticipated. The experiments described by Rolston et al. (1978,
1979) demonstrated for Yolo loam soil, that denitrification became very small
after soil-water pressure heads became less than -70 cm of water. Thus, one
would expect from the data in Figures 11, 12, and 13 for h vs. time for all
six plots, that denitrification would generally occur for only one or two
days after irrigation when h was greater than -70 cm. The soil water re-
distributes rather rapidly in this well-drained, alluvial soil resulting in
decreases in h within a few days after irrigation. Thus, one would expect
that the amount of time available for denitrification is relatively small
compared to the entire cropping season as long as restrictive layers do not
result in a buildup of water at some depth. There is a limited amount of h
data for Plot F because all tensiometers were switched over to soil solution
extractors in order to get a sample of the soil solution before the end of
the experimental period.
N2 AND N20 SURFACE FLUXES
The N20 and N2 fluxes at the soil surface as measured by the accumula-
tion of gases beneath the covers are given as a function of time for the six
15N plots in Figures 14, 15, 16, 17, 18, and 19. The N2 flux is given by the
open circles and broken lines, whereas the N20 flux is given by the solid
circles and solid lines. It is apparent that many of the data points for N2
21
-------
PlotE
• •30cm
o—o 60cm
4 6
8 10 12 14
TIME (days)
18 20
Figure 13. Soil-water pressure head at the 30- and 60-cm soil depths as a
function of time for Plots D, E, and F. Each data point repre-
sents the mean from triplicate tensiometers at each depth.
Arrows give the times of irrigation.
flux fall below the minimum detection limit of 0.1 to 0.2 kg N ha"1 day"1
for Figures 14, 15, and 16. Thus, the % flux is highly uncertain for the
three plots (A, B, C) which did not receive C additions. Due to an unfortu-
nate accident with Plot A, within one day after fertilizer application, the
cover over the plots was left unshaded and high temperatures built up beneath
the cover with considerable damage to the grass. ET and water movement for
Plot A was thus expected to be much different from that of the other plots.
22
-------
0
^r 1 \
PLOT A
oN,
1
1
i
i
8 10 12 14
16 18 20
TIME (days)
22 24 26 28 30 32 34 36
Figure 14. The N£0 and N£ flux at the soil surface as measured by the accu-
mulation of gases beneath covers as a function of time for Plot
A. The open circles are for N2 and the closed circles are for
The arrows give the times of irrigation.
.40
.35
.30
.25
.IS
.10
.OS
o
1 1 1
-
-
-
-
•
M
1 1 t 1
f.
\ i
1
1
yfl.
/V>
i i i i i i i i i i i i i i i i i i
PLOT B
\ ' N.°
\
I 1 l \ I '
N |li A h
\ \ o.
-------
0.8
0.6
as
0.4
0.3
0.2
0.1
I I
PLOT C
o Nt
.N,0
I
M
A
8 10 12 14 16
Figure 16. The N20 and N2 flux at the soil surface as measured by the accu-
mulation of gases beneath covers as a function of time for Plot
C. The open circles are for N2 and the closed circles are for
N20. The arrows give the times of irrigation.
2.4
Plot D
o—o N.
i I I
'°-4
i v i T V » » » y v v yvy yvv » » _
16 18 ZO
TIME (days)
26 26 30 32 34 36 38
Figure 17. The N20 and N2 flux at the soil surface as measured by the accumu-
lation of gases beneath covers as a function of time for Plot D.
The open circles are for N2 and the closed circles are for N20.
The arrows give the times of irrigation.
24
-------
12
10 T
x
<
o
LU
O
I
'I
I-'1
/
1
O
+
I
I
Plot E
o ° N,
N20
I 1
ll I-1.
JL
_L
J.
6 8 10 12 14 16 18 20 22 24 26 28 30
TIME (days)
32 34 36
Figure 18. The N20 and N2 flux at the soil surface as measured by the accumu-
lation of gases beneath covers as a function of time for Plot E.
The open circles are for N2 and the closed circles are for N20.
The arrows give the times of irrigation.
NO3 was apparently leached from the top part of Plot A by 22 days (Figure 14)
with the result that denitrification essentially ceased by Day 22. For Plots
B and C, however, small amounts of denitrification were measured up to between
40 and 50 days after .fertilizer application, although rates were very small as
irrigation progressed. In general, the flux of N2 was much greater than the
flux of N20.
The N2 and N20 flux for Plots D, E, and F was greatly increased over that
of Plots A, B, and C due to the addition of barley straw. There was a tend-
ency for denitrification to approach zero much sooner for Plots D, E, and F
than that for the plots which did not receive C. This may be due to differ-
ences in the amount of water movement through the soil profile with leaching
of NO3 from the upper part of the-soil profile where low 02 and high C values
were maintained. Even with the addition of a relatively large amount of crop
residue into the soil profile, the denitrification rates were relatively small
compared to rates observed by Rolston et a.1. (1978) for plots maintained
uniformly wet for long time periods.
A comparison of the total denitrification gas flux as a function of time
measured by the 15N and C2H2 methods is given by Figure 20. The total
25
-------
14 16 18 20 22 24 26 28 30
TIME (days)
32 34
Figure 19. The N20 and N2 flux at the soil surface as measured by the accumu
lation of gases beneath covers as a function of time for Plot F.
The open circles are for N£ and the closed circles are for
The arrows give the times of irrigation.
denitrification gas flux for the 15N method is the sum of 15N2 and N20 gas
fluKes. The total denitrification gas flux for the C2H2 method is only ^0
flux since reduction of N20 to N2 was inhibited. The denitrification flux
plotted in Figure 20 is the total of N2 and ^0 for Plots A, B, C, G, H, and
I at any sampling time. The pattern of gas flux produced during denitrifica-
tion was similar for both methods with peak flux occurring shortly after
•application of water. It was observed, however, that denitrification at any
one time during the repeated irrigation cycles was not equal for both methods.
For Plots B and H the flux of gas produced during denitrification as measured
by the C^2 method was initially greater than that measured by the 15N method.
For example, 0.42, 0.66, and 0.46 kg N ha 1 of denitrification gas was evolved
in the presence of C2H2 and 0.26, 0.52, and 0.27 kg N ha'1 of denitrification
gases were evolved in the absence of C2H2 using the 15N method during the
first three days after irrigation for the first three applications, respec-
tively. Following this, however, the opposite was true when only 0.1 and 0.06
kg N ha 1 of denitrification gases were evolved in' the presence of C2H2 and
0.28 and 0.19 kg N ha"1 of denitrification gases were evolved in the absence
of C2H2 by the 15N method in the first two days after the fifth and sixth
irrigation applications, respectively. For Plots A and G, the amounts of
denitrification gases evolved in the first two days after the initial
26
-------
C2H2 method
H metho(j
14 18 22 26
TIME (days)
30 34 38 42
Figure 20.
Comparison of the denitrification flux as a function of time as
measured by the 15N and C2H2 inhibition methods for Plots A, B,
C, G, H, and I. The denitrification flux is the sum of N2 plus
N20. The broken lines and open circles are for the C2H2 method.
The closed circles and solid line are for the 15N method. Arrows
give the times of irrigation.
irrigation in the presence and absence of C2H2 were 0.93 and 0.72 kg N ha *,
respectively. In the same time period, after the tenth irrigation, the
amounts of denitrification gases evolved in the presence and absence of C2H2
were 0.12 and 0.33 kg N ha~*, respectively. For Plots C and I, the amounts of
denitrification gases evolved in the first two days after the initial irriga-
tion in the presence and absence of C2H2 were 0.82 and 0.61 kg N ha *, re-
spectively. For a two day period after the third irrigation, the amounts of
denitrification gases evolved in Plots C and I in the presence and absence of
C2H2 were 0.35 and 0.43 kg N ha"1, respectively.
The .data of Figure 20 suggest that the production of denitrification
gases during the initial stages of denitrification can be increased by the
presence of C2H2. This increase results from the fact that N20 was converted
to N2 in the absence of C2H2 leading to a delay in evolution of N2 compared
to N20 from the field soil. Furthermore, after a certain period of time, the
presence of C2H2 can result in a decrease in production of N20- compared to
N20 and 15N2 in the absence of C2H2-. Part of the reason for this behavior may
be that 02 concentrations were slightly reduced in the presence of C2H2. Also
27
-------
Yeomans and Beauchamp (1978) using soil incubation studies, reported that C2H2
is effective in inhibiting N20 reduction for a limited time only in the con-
tinued presence of C2H2 such that N20 could eventually be converted to N2.
It is possible that several applications of C2H2 at the same site in the field
soil in order to measure variations in N20 flux at frequent intervals could
facilitate the growth of organisms capable of reducing N20 in the presence of
C2H2. The results of the present study indicate that such a population may
have developed when N20 flux in the presence of C2H2 became lower than that of
N20 and N2 in the absence of C2H2. This occurred in Plots G, H, and I after
13, 15, and 17 C2H2 applications, respectively. The differences between
treatments may have resulted from the increased variation in the soil moisture
content as irrigation frequency decreased, with a subsequent decrease in soil
microbial activity.
It is interesting to note for the denitrification flux comparisons of
Figure 20 for Plots A and G that the fluxes were comparable at 22 days.
However, it would be expected that fluxes would not be the same for the 15N
and C2H2 methods since the grass of Plot A was not transpiring, whereas in
Plot G the grass was transpiring. One would possibly expect differences in
water movement and differences in the residence time of NO^ in the active zone
where denitrification was occurring for these two areas. However, gas fluxes
were similar indicating that the differences in residence time may not have
been that different with or without the grass.
The total amounts of gases produced during denitrification of applied
fertilizer N as measured by the C2H2 and 15N methods are presented in Table 5.
Although denitrification flux at any single time as measured by the two
methods was greatly different, only a slightly different total amount of
denitrification gases was measured by the two methods., The denitrification
of fertilizer in the presence of C2H2 for the three treatments (1.4, 1.2, and
1.0% for Plots G, H, and,!, respectively) was slightly greater than that
using 15N (1.5, 1.1 and 0.7% for Plots A, B, and C, respectively).
The total denitrification as measured by the 15N method for Plots D, E,
and F which had received straw are also given in Table 5. It is obvious from
Table 5 that the addition of the straw greatly increased denitrification over
that without straw addition. However, the total amount denitrified from the
,. straw treatments was still not very large compared to the total amount of
fertilizer N applied. This indicates that denitrification fluxes under normal
irrigated conditions where the soil profile was not kept continuously wet, is
rather small, at least for deep, well-drained alluvial soils such as Yolo.
The data of Table 5 show that the least amount of denitrification occurred
for the irrigation frequency of one irrigation every two weeks. This small
amount of denitrification is due primarily to the fact that the soil is
relatively dry for an extended time period and that when irrigation water is
applied, infiltration and redistribution of the soil water occurs rapidly
resulting in only a very short time period when the soil is anoxic enough for
denitrification to occur. The effect of infrequent irrigation is also to move
the fertilizer.N. into the lower part of the root zone, resulting in less N©3
in the upper part of the soil where high C and high water contents may occur
simultaneously. For the other two irrigation frequencies, the 15N and C2H2
methods show that the largest amount of denitrification occurred for the most
28
-------
TABLE 5. AMOUNTS OF N20 AND N2 PRODUCED DURING DENITRIFICATION OF ADDED
FERTILIZER N AS MEASURED BY .THE C2H2 AND 15N METHODS
Denitrifi cation (kgN ha"1)
Plot
15N
C2H2
Method
A
B
C
D
E
F
Method.
G
H
I
N20
1.1
0.6
0.3
1.8
0.8
1.0
1.0
0.8
0.7
N2
3.0
2.6
1.6
13.1
17.6
4.0
3.5
2.6
2.0
Total
4.1
3.2
1.9
14.9
18.4
5.1
4.3
3.4
2.7
N20
(N20 + N2)
0.27
0.19
0.16
0.12
0.04
0.22
0.23
0.24
0.26
Loss of fert.
N as total
denit. (%)
1.5
1.1
0.7
5.2
6.4
1.8
1.4
1.2
1.0
frequently irrigated plot of three irrigations per week. The soil was kept
fairly wet for long time periods and by adding small, frequent amounts of
water, the W^ tended to remain in the upper portion of the soil profile for
longer time periods resulting in more denitrification. For Plots D and E,
the irrigation frequency of one irrigation per week (Plot E) gave the
greatest amount of denitrification. However, the differences between Plots D
and E are small and there is some indication from the water content data
(Figure 9) that an impeding layer or a hardpan existed in Plot E which tended
to keep water contents higher in the profile for longer time periods creating
more anoxic conditions. These results indicate that very frequent irriga-
tions tend to result in the largest amount of denitrification, whereas infre-
quent irrigations result in the least amount of denitrification.
N20 MOLE FRACTION
The various proportions of N20 and N2 produced during denitrification is
of great interest due to the potential that N20 may be contributing to the
depletion of the ozone layer of the lower stratosphere. Figures 21 through
26 give N20 mole fraction as a function of time for the nine plots of this
experiment. Figures 21, 22, and 23 give the N20 mole fraction from both the
29
-------
T
T
15
CM
N
1.0
0.8
0.6
CM 0.4
0.2
N method
C2H2 method
Plots A ond G
Fll v y v v v v v v
\
0 2 4 6 8 10 12 14 16 18 20 22
TIME (days)
Figure 21. The N20 mole fraction as a function of time for Plots A and 6.
The solid lines give the N20 mole fraction using the 15N method,
and the broken lines give the N20 mole fraction using the C2H2
method.
1.0
4-
O.
0,0.6
0.4
M
0.2
• I5N method
o C2H2 method
i i I
Plots B ond H
/
M.
f: 1.
/i
10 15 20 25 30 35
TIME (days)
40
45
Figure 22. The N20 mole fraction as a function of time for Plots B and H.
The solid lines give the ^0 mole fraction using the 15N method
and the broken lines give the N20 mole fraction using the C2H2
method.
30
-------
CM
1.0
0.8
0.6
0.2
0
I
Figure 23.
1
N method
0 C2H2 method
Plots C and I
1
I
1 -
10
15
20
25
30
35
40
TIME (days)
The N20 mole fraction as a function of time for Plots C and I.
The solid lines give the N20 mole fraction using the 15N method,
and the broken lines give the N20 mole fraction using the C2H2
method.
15N2 and the C2H2 methods. For the frequently irrigated plots (Plots A and
G) , the N20 mole fraction was quite dynamic due to the frequent irrigation
applications. The N20 mole fraction varied from nearly zero to one during
different irrigation cycles. The mole fraction as measured by the two dif-
ferent methods compared reasonably well. For Plots B and H (Figure 22) there
was a general tendency for a decrease in the N20 mole fraction with increas-
ing time. This may be due to the effects of high NO^ concentration initially
which tends to inhibit N20 reduction, and therefore, would result in high N20
mole fractions shortly after fertilizer application. The C2H2 method
compared reasonably well with the 15N method except toward the end of the
sampling. After the first two irrigation cycles, both sets of data indicate
that the mole fraction tended to be relatively small immediately after
irrigation and then increased as the soil profile dried or became less
anoxic. This would be expected since under less anoxic conditions there is a
decreased potential for N20 reduction to N2.
Similar behavior is demonstrated by the N20 mole fraction for Plots C
and I (Figure 23) demonstrating that the N20 mole fraction tended to increase
from the low value immediately after irrigation to higher values as the pro-
file dried.
For Plots D, E, and F (Figures 24, 25, and 26), which were the plots to
which C was added as chopped barley straw, the N20 mole fractions tended to
31
-------
M
*
1.0
0.8
I I T
Plot D
' » V VTV T^T VTV TTV W »
V
10
15
TIME (days)
V
20 20 30 35
Figure 24.
The N20 mole fraction as a function of time using the 15N method
for Plot D. The arrows give the times of irrigation.
be much lower than those measured for the plots without C addition. This
again would be expected since much more anoxic conditions developed in plots
to which straw was added than those without straw resulting in better condi-
tions for N20 reduction to N2. There does seem to be a general decrease in
N20 mole fraction with time for Plots D and F. Plot E did not show that be-
havior. The data for Plot E, however, definitely showed the increase in N20
mole fraction within each irrigation cycle. In fact, there is essentially no
NaO produced very shortly after irrigation to result in mole fractions near
zero.
The data on N20 mole fraction demonstrate that the proportion of N20
produced during denitrification was a very dynamic and variable property.
Mole fractions varied all the way from zero to one for treatments without C
additions and varied from nearly zero to 0.4 or 0.5 for plots with C addi-
tions. A time-averaged N20 mole fraction would be about 0.2 or 0.3 for those
plots without C additions and approximately 0.1 for those plots with C addi-
tions. The overall N20 mole fraction calculated from the data in Table 5
varied from 0.04 for Plot E to 0.27 for Plot A.
32
-------
PLOT E
1.0
CM
0.8
I I
4
1 I
CM
0.6
I
15 20 25
TIME (days)
30 35
Figure 25.
The N20 mole fraction as a function of time using the 15N method
for Plot E. The arrows give.the times of irrigation.
PLANT UPTAKE
Figure 27 gives the plant uptake of fertilizer N as a function of time
after fertilizer addition for Plots B, C, D, E, and F (no uptake for A).
Plots B and C for the experiments without C addition compared reasonably well
in total uptake versus time. In a similar fashion, Plots D, E, and F showed
similar N uptake. The cooler temperatures later in the summer appeared to
have an effect on Plots D, E, and F with less uptake than that of Plots B and
C. Plot D took up more N than the other two plots, most likely due to much
better water conditions from frequent small irrigations.
SOIL SOLUTION N
The soil solution fertilizer N within the six 15N plots for two sampling
times are given by Figures 28 and 29. Figure 28 gives the data for Plots A,
B, and C for a sampling time midway through the experimental period and for a
sampling time shortly before termination of the experiment. Data for Plots
D, E, and F are given by Figure 29 for a sampling time midway through the
period and near the end of the experimental period. Data points given here
are the mean soil solution NO^ concentrations derived from the fertilizer
from triplicate solution extractors at each depth for each plot. Both
figures show that the two plots which received irrigation once per week and
once every two weeks had similar soil solution NO3 concentrations. The two
33
-------
1.0
0.8
0.6
I 1
Plot F
v
M
0.2
i
i
10
15 20 25
TIME (days)
30
35
Figure 26. The N20 mole fraction as a function of time using the 15N method
for Plot E. The arrows give the times of irrigation.
plots which received irrigation water three times per week, however, behaved
very differently with much higher concentrations of fertilizer remaining in
the upper part of the profile especially for the sampling midway through the
experimental period (the top part of both figures). This demonstrates that
the frequent, small irrigations tended to keep the NO3 in the upper part of
the soil profile whereas the infrequent, large irrigations tended to move the
NOs deeper into the soil. Part of this was due to the fact that the initial
distribution of the fertilizer was somewhat different due to the fact that
the fertilizer was applied uniformly during the first irrigation. Therefore,
all the fertilizer was applied in a very small pulse for Plots A and D,
whereas for Plots C and F, all the fertilizer was applied in one large irri-
gation which would tend to distribute the fertilizer over a deeper depth.
These figures also show that, even for the sampling period midway through the
experiment, that fertilizer NO3 concentrations at the 90-cm depth were al-
ready quite high for Plots B, C, E, and F, indicating that probably large
amounts of fertilizer N would be leached below the grass root system for
these plots. By the last sampling time near the end of the experiment, the
fertilizer N(>3 in the soil solution for Plot D began to decrease at the 30-cm
depth due to denitrification and continual leaching to deeper soil depths.
As demonstrated by Rolston ^it al. (1979), the variability in the NOg
concentration of the triplicate samples at any particular soil depth was
quite high. Standard deviations were sometimes as great as 150% of the mean,
with 60% of the mean being fairly common.
34
-------
60
50
i
D
2 40
o>
uj 30
^^
^f
I-
% 20
\-
_J
Q_
10
20 40 60
TIME (days)
80
Figure 27. Plant uptake of fertilizer N as a function of time after fertil-
izer addition for all six plots, except for Plot A for which no
grass was harvested.
SOIL BESIDUAL N
The labeled inorganic N (fertilizer derived N) for the six plots at the
end of the sampling period are given by Figures 30 and 31. Each data point
represents the mean from ten Individual soil samples at each depth combined
to_two samples for analyses. The labeled inorganic N represents primarily
NOa-N whereas the labeled organic N is simply organic N which had been
Immobilized by microorganisms or by live or dead plant roots. The effect of
the three different irrigation frequencies are also demonstrated here on the
leaching of NO3 through the soil profile. In Plot A, a. relatively high NOs
peak occurred between 60 and 75 cm after approximately 60 days. Plot A
received small frequent irrigations. For Plots B and C which received irri-
gations less frequently, the high peak did not occur and the NO3 concentra-
tions were relatively uniform with depth. Relatively high concentrations
still existed at the 120-cm soil depth, indicating that substantial NO^ was
potentially leached below 120 cm. The labeled organic N within the soil
profile was predominantly due to live or dead plant material. The result of
extreme damage to the grass of Plot A was very low labeled organic N in the
upper part of the profile, whereas Plots B and C had high organic N in the
top 30 cm of soil. However, the organic N values continued to be measurable
35
-------
down to 120 cm in this profile, indicating that roots extended fairly deep or
that there was some immobilization of added N by microorganisms.
SOIL SOLUTION FERTILIZER N (ppm)
.0 20 40 60 80 100 120 140 160 180200
Plots A,B,C
• A- 24days
oB- 37 days
* C- 30 days
Plots B,C
° B-51 days
* C-44 days
,-/ ,o
Figure 28. Soil solution fertilizer N as a function of depth for Plots A, B,
and C for a sampling time midway through the experimental period
(upper part of figure) and at the end of the experimental period
(lower part of figure). Plot A was not sampled at the end of the
period. The data points represent the mean concentration from
triplicate soil solution samplers.
36
-------
SOIL SOLUTION FERTILIZER N (ppm)
^0 20 40 60 80 100 120 140 160 180 200
E
o
Q.
UJ
Q
10
20
30
40
50
60
70
80
90
0
10
20
30
40
50
60
70
80
90
Plots D, E, F
• D-17 days
o E-15days
*• F -15 days
Plots D.E.F
• D-31 days
0 E - 29 days
* F-31 days
Figure 29. Soil solution fertilizer N as a function of depth for Plots D, E,
and F for a sampling time midway through the experimental period
(upper part of figure) and at the end of the experimental period
(lower part of figure). The data points represent the mean con-
centration from triplicate soil solution samplers.
A similar behavior of organic and inorganic N is demonstrated by Figure
31 for the plots receiving straw additions. As for Plot A, the plot receiv-
ing frequent, small irrigations (Plot D) demonstrated a peak in NO3 concen-
tration between 30 and 45 cm, indicating less leaching of the applied
37
-------
LABELED INORGANIC N (ju,q Ng"' soil)
0 10 20 30 40
30
60
.-. 90
E
E 120
Q.
UJ
O 0
O
Cfl
30
60
90
120
I
• Plot A
o Plot B
Plot C
T
I I I I I I
• Plot A
o Plot B
4 Plot C
I I I I i I I
10
20
30
-I
40
LABELED ORGANIC N (>*g Ng soil)
Figure 30. Labeled inorganic and organic N for Plots A, B, and C as a func
tion of soil depth at the end (63 days for B and C, 49 days for
t A) of the experimental period.
fertilizer through the profile than for the other plots. Aldfcugh a definite
NO^ peak occurred for both Plots A and D, the magnitude of the peak was
greater in A than in D due to no plant uptake of N in A and very little de-
nitrification in A. As was the case for Plots B and C, Plots E and F showed
very little difference In NO^ due to irrigation treatment. The labeled
organic N was similar for all three plots, with high, labeled organic N in
the top 15 cm and a rapid decrease to fairly low levels deeper in the profile.
38
-------
LABELED INORGANIC N (/*g Ng 'soil)
0 10 20 30 40
• Plot D
o Plot E
Plot F
• Plot D
o Plot E
Plot F
120
0 10 20
LABELED ORGANIC N
30 40
Ng soil)
Figure 31. Labeled inorganic and organic N for Plots D, E, and F as a func-
tion of soil depth at the end (36 days) of the experimental
period.
The labeled inorganic N values demonstrate that leaching of N03 was
decreased by small, frequent irrigations. However, as shown under the
section on gas fluxes, the frequent, small irrigation treatments resulted in
the greatest amount of denitrification loss. Thus, although NO^ leaching may
be less, frequent irrigations would result in more denitrification. A bal-
ance would have to be drawn between denitrification losses and leaching
39
-------
losses. These data show that over the time period of this experiment that
although denitrification was increased on the frequently irrigated plots, the
increased denitrification was not as great as the leaching that occurred in
the infrequently irrigated plots. Thus, frequent small irrigations would
result In maintaining high NO3 in the upper part of the soil profile, and
would, thus, be more accessible for plant uptake.
MASS BALANCE OF N
Table 6 gives the amounts of fertilizer N for the various components of
the N cycle. The amount of fertilizer, the amount remaining in the soil, the
TABLE 6. MASS BALANCE OF FERTILIZER IN THE VARIOUS COMPONENTS OF THE N CYCLE
FOR EACH OF THE SIX 15N PLOTS. LEACHING WAS DETERMINED BY DIFFER-
ENCE FROM THE OTHER COMPONENTS
Plots
Component
Fert. applied
Soil digests
Soil extracts
Plant uptake
Denitrification
Leaching
(by difference)
A
281.0
10.8
149.8
—
4.1
116.3
B
284.0
114.9
68.8
45.8
3.2
51.3
C
282.0
77.7
60.7
46.7
1.9
95.0
D
l-i -| — 1
288.0
95.3
82.3
21.0
14.9
74.5
E
288.0
65.7
54.4
11.9
18.4
137.6
E
287.0
82.9
77.9
13.0
5.1
108.1
amount taken up by the plant, and denitrification were measured directly.
Due to the difficulties in estimating the leaching component even in small
plots such as those used by Rolston et al. (1979), leaching was estimated by
difference from the other measured components. The residual soil N in the
upper 120 cm of soil was determined with reasonable accuracy. There could be
some question about the accuracy or the ability to measure all of the deni-
trification gases produced. However, the C2H2 and 15N methods gave nearly
the same total denitrification indicating that the flux of denitrification
gases below the borders of the 15N plots was insignificant. ; Thus, it seems
that although some errors in denitrification fluxes could easily have been
made, it appears that the numbers given for denitrification are reasonable.
The determination of leaching by difference in Table 6 shows that con-
siderable N was lost below the 120-cm depth for these experiments. These
data are somewhat confusing for Plot A since the calculation gives greater
leaching for Plot A than for Plots B or C. However, the soil solution N03
40
-------
values and the residual soil NOa values showed considerable NOs remaining in
the soil profile for Plot A, whereas Plots B and.C had much less NOs re-
maining in the upper 120 cm of soil. Plant uptake was zero and very little
N remained in the soil as labeled organic N for Plot A. Thus, the N not
taken up by the plant and not immobilized as organic N was apparently avail-
able for leaching, resulting in 116 kg leached out of 281 kg applied. For
Plots D, E, and F, however, Plot D, which was the frequent, small irrigations,
resulted in the least amount of leaching. For Plot D, substantial N03 re-
mained in the upper 120 cm of the profile, considerable N was immobilized in
the organic fraction, and plant uptake of applied N was high. Nearly one-
half of the applied N was leached from the plot with an irrigation frequency
of one irrigation per week, and something less than one-half was leached from
the plot with an irrigation frequency of once every two weeks.
This amount of leaching seems excessive if the amount of irrigation was
no greater than 15% of the ET. Evidence exists from the denitrification
modeling section that the amount of irrigation water applied was greater than
115% of actual ET. The plots may have been using less water than the esti-
mated ET due to frequent cuttings of the grass and the effect of placing
covers over the plots for two to four hours per day on each sampling day.
This effect of more leaching than anticipated will be discussed further in
the section on the denitrification simulation model.
DENITRIFICATION SIMULATION MODEL
The mathematical equations used to describe the transient behavior of
water and N in soils are similar to those presented by Davidson et al.
(1978). The numerical procedures used to solve these equations, however,
were different in that plate theory rather than finite difference techniques
were employed. The numerical scheme used in this report and verification of
the model are also presented by Rao et^ al^, (1980). A flow diagram giving the
order of calculations in the simulation model is given by Figure 32.
To verify the denitrification portion of the N simulation model de-
scribed by Rao jet jal. (1980), the experimental results of Rolston et al.
(1978) and those of this report were used. The field experiments used 15N
tagged NO 3 fertilizer to measure N2 and ^0 gas emission from the soil
surface during denitrification. To simulate denitrification, a first-order
reaction with respect to NO3 and C concentration was assumed. It was also
assumed that the time required for the N2 and N20 gases to diffuse from the
site of production to the soil surface was small relative to the time scale
of the experiments. Thus, the model contained no gaseous diffusion component.
The effect of soil temperature on denitrification was accounted for by using
a Qio (temperature coefficient) value of 2. The effect of anoxic conditions
on denitrification was accounted for through a water function which was based
upon degree of soil-water saturation. The rate of denitrification was
calculated from:
p f - ki 9 *v f T Sr <*
41
-------
ROOT GROWTH
(Maximum Rooting Depth &
Root Density in Soil)
POTENTIAL E,T, DEW*)
(Penman Model or Input)
POTENTIAL NITROGEN UPTAKE
DEPWND (Empirical)
PLANT WTER UPTAKE
(Holz-Remson Model)
N TRANSFORWTIONS
MftNURE-N AND SOIL~N
Mineralization, Immobilization
Nitrification, Denitrification
PLANT NITROGEN UPTAKE
(Empirical Model)
WTER TRANSPORT
(Semi-Empirical Approach)
SOLUTE TRANSPORT
AWONIIM-N AND NITRATC-N
(Chromatographic-Plate Theory)
CARBON TRNfttWriONS
MANURE-C AND SOIL-C
(Calculate Available Carbon
For Input Into Denitrification
Model)
Figure 32. Flow diagram giving the order of calculations in the simulation
model.
42
-------
where G is sum of denitrif ication gases (N2 + N20) , C is the concentration
of NO^, (L.. is the concentration of water-soluble carbon, fy is the water
function, fT is the temperature function, p is the soil bulk density, 6 is
the volumetric soil-water content, and kj^ is the first order denitrif ication
rate constant.
The water-soluble carbon, C^, in Eq. [1] has been shown by Burford and
Bremner (1975) to correlate significantly with denitrification. For soil
organic matter, the water extractable C was calculated from the following
relationship (Burford and Bremner, 1976; Reddy et^ a^. , 1979):
(^ = 24.5 + 0.0031 Cg [2]
where C^ is water extractable C concentration and Cg is total soil organic C
concentration. The total soil organic C decomposition rate was assumed to
be a first-order reaction:
where t is time, kg is the first-order constant for C decomposition, and &,
is a function describing relative respiration as a function of relative
soil-water content:
gw = 1.67 (6/9s) for 0.1 <. (6/9s) <. 0.6 [4a]
gw = 1.75 -1.25 (9/0s) for 0.6 <. (9/6s) <. 1.0 [4b]
adapted from Reddy et al. (1979) where 0g is the saturated soil-water content
(cm3 cm"3) . Equations [4a] and [4b] are specific for the Yolo soil but may
be reasonable for other fine textured soils. This function gives a maximum
decomposition (g^ = 1) at a soil-water potential of 0.33 bar (9/6 = 0.6).
Relationships between water extractable C (or C available for dentrifi-
cation) and total organic C in manure or plant residue are not readily avail-
able. Thus, it was assumed for this study that the C in the manure or plant
residues could be divided into a portion which was readily decomposed
(Fraction I) and totally available for dentrif ication and a portion which was
slowly decomposed (Fraction II) and only partially available. The latter
portion was assumed to follow the same relationship as that for soil organic
C (Eq. [2]). The percentages of C in Fractions I and II and the rate con-
stants for various manures and plant residues are presented by Reddy et al.
(1979). The decomposition of manure or plant residues can be described by:
5 k± % fT Ci
where the subscript i refers to Fraction I or II. The value of C. for
Fraction I enters directly into the denitrification equation (Eq. [1]). The
43
-------
value of C. for Fraction II is considered to be the same as soil C and is
substitutes for Cg into Eq. [2], Thus, the total "soil" C for cases where
manure or plant residues are added to soil is the sum of Fraction II C from
the manure or residue and the soil C. The decomposition of manure C of
Fraction I can be more adequately described by two subtractions, each having
different rate constants (Reddy et_ aJL., 1979).
MODEL INPUT DATA
The input data for the denitrification model were obtained from Rolston
£t jl. (1978), and the data of this report on Yolo loam soil at Davis,
California. Rate constants for the decomposition of C in soils are presented
by Reddy et_ al. (1979).
The field experiment of Rolston jjt al. (1978) consisted of six, 1-m2
field plots maintained at two soil-water contents near water saturation and
at three C levels established by applying manure (3.4 x 10** kg ha l in the
top 10 cm of soil) to some plots, cropping some plots with perennial ryegrass,
and leaving some plots uhcropped. These experiments were conducted during
the summer and the winter to obtain two temperature levels. Steady state
soil-water contents were maintained in the soil profile during the denitri-
fication process by small but frequent irrigations each day. These field
experiments by Rolston et al. (1978) will subsequently be referred to as
"constant water" plots throughout this report. The constant water plots were
used to develop the empirical water function, fw, in Eq. [1] by forcing the
calculated denitrification to be the same as that measured for the two plots
at different soil-water contents. The water function is further described in
the "Comparison of Calculated and Measured Denitrification" section of this
report. After the water function was developed and the denitrification rate
constant for the two plots determined, the same water function and rate
constant k^ were used to calculate denitrification for the other ten constant
water experiments. The effect of the crop root system through the additional
C it added and 02 depletion which resulted also increased denitrification.
The rate constant required for cropped plots was approximately four times
greater than that for the uncropped plots.
The water function and the denitrification rate constant determined from
the constant water plots were subsequently used to calculate denitrification
for field experiments described in this report. These plots will subsequent-
ly be referred to as the "irrigation frequency" plots in this report.
Denitrification in the constant water plots and irrigation frequency
plots was determined by measuring the flux of N20 and N2 gases at the soil
surface after the addition of 15N03 fertilizer. The uptake of 15N by the
grass as a function of time was used as input data in the denitrification and
N transport model.
Soil-water content and pressure head were measured at frequent intervals
in all plots and these data were used to check the calculated soil-water
contents predicted by the model, especially in the irrigation frequency
plots. For the irrigation frequency plots, it became immediately apparent
that the predicted soil-water contents versus time after each irrigation
44
-------
were smaller than those measured values. For four plots, it was necessary to
decrease the estimated ET by 50 to 85% in order to attain a reasonable com-
parison of calculated and measured water contents within the soil profile.
The ET may have been underestimated due to placement of covers over the plots
for up to eight hours on some days and to decreased transpiration from short,
clipped grass.
Soil-water characteristic curves at various depths for the Yolo soil
were taken from Rolston and Broadbent (1977) and LaRue e_t al. (1968). The
relationship between hydraulic conductivity and soil-water content was
taken from LaRue et al. (1968) for a Yolo loam field site within 100 m of
the plots used for direct measurement of denitrification.
COMPARISON OF CALCULATED AND MEASURED DENITRIFICATION
Comparisons of the measured and calculated denitrification flux as a
function of time for two constant water plots with manure during the summer
(23°C) are given by Figure 33. The solid circles are measured values of
80
2 50
X40
30
O
r-20
10
0
Total (kgNha-')10
• 218
— 206
• 209 8
6
h= -15cm
Summer
* Manure
Tola I (kgN ha1)
Measured • 47
Calculated-1storder— 57
-0*brder--50
70cm
"024 6 8 10 12 14 16 18 20
TIME (days)
Figure 33. Measured and calculated surface fluxes of denitrification
products (N£ + N£0) as a function of time for two manure-amended
plots maintained at two different values of soil-water pressure
head, h.
45
-------
the N2 and N20 flux, and the solid line is the calculated denitrification
flux assuming first-order kinetics as derived by Eq. [1]. The rate coeffi-
cient, klf used for the calculations in Figure 33, was 1.68 x ICT1* g soil
day 1 (ug C) 1. Since N03 concentrations within the soil profile were gen-
erally large in all plots, it might be_assumed that denitrification followed
zero-order kinetics with respect to NO^ concentration rather than first-order
kinetics as given by Eq. [1], For zero-order kinetics, the denitrification
rate is given by:
Where ko is the zero-order denitrification constant and the other functions
and coefficients are the same as in Eq. [1]. The broken line in Figure 33
is the calculated denitrification rate assuming zero-order kinetics (Eq.
[6]). The zero-order rate coefficient, kg, used for the calculations in
Figure 33 was 0.046 ug N day l (ygC) 1. The zero-order model does not
predict the large denitrification rate that occurred immediately after the
NOs was applied. The first-order equation describes these large initial
rates better than does the zero order case.
The calculated denitrification rates given in Figure 33 were developed
using the water function, f,,, in Figure 34. The water function was developed
by forcing the calculated amounts of denitrification for the indicated period
in the two plots shown in Figure 33 to be approximately equal to the mea-
sured values. The water function in Figure 34 is an empirical relationship
which explicitly implies a relative degree of anoxic development for these
field plots. The water function provides a simple way of accounting for the
change in 02 diffusion and storage in the soil as the soil-water content
changes. Denitrification becomes essentially zero below 80% of the saturated
water content value. The maximum potential for denitrification would occur
at saturation where all pores are completely filled with water, and the
diffusion of 02 is limited to diffusion through water.
Total denitrification, as determined by integrating the flux versus time
data, is also given in Figure 33. Comparisons of total denitrification for
all 12 of the constant water plots are given in Table 7. The same water
function presented in Figure 34 was used to calculate denitrification for all
plots. Also, the same denitrification rate constant was used for all plots
except those cropped with grass. The constant required to describe denitri-
fication from plots cropped with grass was approximately 3.6 times greater
than that for the other plots due to the effect of the root system in con-
suming 02 and in adding soluble C to the soil.
The denitrification rate constant (6 x lo"1* g soil day"1 (ugC)""1),
determined for the cropped plots of the constant water experiments and the
water function of Figure 34, were subsequently used in calculating denitri-
fication for the six irrigation frequency plots of this report. Figure 35
gives the surface flux of N20 plus N2 for the plots receiving three irriga-
tions per week (1. 15 ET) . The arrows at the top of the figure indicate when
the irrigation was made. The top and bottom sections of Figure 35 are for
plots without and with added straw, respectively. Note that the scales of
46
-------
TABLE 7. COMPARISON OF MEASURED AND CALCULATED DENITRIFICATION FROM CONSTANT
WATER PLOTS ON YOLO LOAM SOIL. A VALUE OF kx OF 1.68 x 10'^ g SOIL
DAY"1 (ygC)"1 WAS USED FOR THE MANURE AND UNCROPPED CALCULATIONS.
A VALUE OF kx OF 6 x 10"^ g SOIL DAY'1 (ygC)"1 WAS USED FOR THE
CROPPED CALCULATIONS
Denitrificatlon
Temperature
°C
23
23
23
23
23
23
8
8
8
8
8
8
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
Manure, h = -8 cm
Manure, h = -50 cm
Uncropped, h = -8 cm
Uncropped, h = -50 cm
Cropped, h = -8 cm
Cropped, h = -50 cm
Measured
kg N
218
47
40
9
10
4
33
30
0.4
0.4
19
2
Calculated
ha""1
206
57
47
8
15
2
52
0
3
0
21
1
47
-------
1.0
z
O
u°-6
z
o:
^ 0.2
o
0.5 0.6 0.7 0.8 0.9 1.0
RELATIVE WATER CONTENT, Q/QS
Figure 34. The dependence of the empirical water function, fy, (Eq. [1]) on
relative soil-water content (water content/saturated water
content).
the ordinate are greatly different for the top and bottom sections of the
figure (Figures 36 and 37 also). It is also important to recall that the
minimum detection limit for N£ flux was in the neighborhood of 0.1 to 0.2 kg
N ha"1 day"1. Thus, many of the data points for the top section of each
figure are highly uncertain. The data points are the measured denitrifica-
tion flux and the solid lines are calculated denitrification rates using the
simulation model assuming first-order kinetics. The total measured and
calculated denitrification are also given in each section for each plot.-
The data in Figure 35 illustrate that both denitrification rate and total
denitrification were described reasonably well using the model.
Figures 36 and 37 give the denitrification flux as a function of time
for the plots irrigated once per week and once every two weeks, respectively.
Again, the data in Figures 36 and 37 illustrate that the calculated denitri-
fication compares reasonably well with measured rates and total amounts of
denitrification.
48
-------
0.8
0.7
0.6
05
0.4
10.3
7ro
^0.2
D)
~ 0.1
x
il 0
^,4.0
3.2
2.4
1.6
OB
0
V V vVV V V V VVV YVV vVv
Plot A Total(kgNha1)
No Carbon added
—• Measured 4.1
— Calculated 3D
Plot D Total (kgN ha'1)
Straw added
• Measured 143
— Calculated 12.8
0 8 16 24
Time (Days)
32
40
Figure 35. Measured and calculated surface fluxes of denitrification
products (N2 + N20) as a function of time for plots with and
without straw incorporation at an irrigation frequency of three
irrigations per week. The solid lines are simulations based on
Eq. [1]. The broken lines simply connect measured data points.
Arrows indicate time of irrigation. Note that the scales of the
ordinate are greatly different for the "no straw" and "straw"
plots.
49
-------
O8
0.6
0.4
02
O)
x
0
10
8
6
r~r
r
Rot B Total (kgNha1)
No Carbon added
• Measured 312
— Calculated 3.7
- • • 1
4-^ 1 1 1 1 h
1 I
Plot E Total (kgNhS1)
Straw added
Measured ia4
Calculated 22.3
0 8 16 24 32
Time (Days)
40
48
Figure 36. Measured and calculated surface fluxes of denitrlfication
products (N2 + N20) as a function of time; for plots with and
without straw incorporation for an irrigation frequency of one
irrigation per week. The calculated lines are simulations based
on Eq. [1]. Arrows Indicate time of irrigation. Note that the
scales of the ordinate are greatly different for the "no straw"
and "straw" plots.
50
-------
0.6
0.5
0.4
Q3
0.2
0.1
0
6.0
5.0
i
4.0
3.0
2.0
1.0
Rot C Total (kg N ha"1)
No Carbon added
Measured 1.9
Calculated 20
•
H H
1 1
Plot F Total (kgNha')
Straw added
Measured 5.1
• Calculated 3.9
A.
•. • . /•^. ,« i« . ••«,
0 8 16 24
Time (Days)
32
40
48
Figure 37. Measured and calculated surface fluxes of denitrification
products (N2 + N£0) as a function of time for plots with and
without straw incorporation for an irrigation frequency of one
irrigation every two weeks. The calculated lines are simulations
based on Eq. [1]. Arrows indicate time of irrigation. Note that
the scales of the ordinate are greatly different for the "no
straw" and "straw" plots.
51
-------
Simulations of denitrification were sensitive to the empirical water
function in Figure 34. Figure 38 gives three hypothetical water content
(15 cm depth) versus time curves for the one irrigation per week irrigation
frequency plot with straw added. Line B in Figure 38 is the soil-water
content calculated by the model for Plot E (Figure 36). Lines A and C are
0.01 cm3 cm"3 larger or smaller, respectively, than the soil-water content
represented by curve B. For differences in soil-water content of 0.01
cm3 cm"3 (Figure 38), the calculated denitrification was different by
approximately a factor of two.
'E
u
IE
u
o
(J
ro
0.42
0.40
0.38
0.36
Q34
0.32
9
TO
I
X 41
li. 3
q, 2
0
Total Denitrification
kgNha-'
A - 37
B - 22
C - 12
Figure 38. Three hypothetical soil-water content versus time curves for an
irrigation frequency of one irrigation per week for plots with
straw incorporation.
The sensitivity of denitrification to the soil-water function makes it
difficult to accurately simulate denitrification for field situations.
Measured water contents in the field frequently vary by as much or more than
the ± 0.01 cm3 cm"3 considered in Figure 38. For a site adjacent to the
plots of Rolston et al. (1978) and those of this report, Simmons et al.
52
-------
(1959) measured standard deviations of ± 0.02-0.03 cm3 cm"3 for 16 soil-
water content measurements (at one depth) from a 1 ha field. Thus, it would
be desirable to have a function which accounted for the degree of anoxic
development in the soil which was not as sensitive as the empirical water
function given in Figure 34. On the other hand, a function which accounted
for diffusion of 02 in the macropores and diffusion of 02 through water films
or into aggregates could be equally sensitive to the diffusion rate of 02 in
the water, the size of the microsites, and the consumption of 02 by micro-
organisms and roots. It is probable that the sensitivity demonstrated in
this model due to the empirical water function is indeed real. Therefore,
one would expect that denitrification would vary substantially from spot to
spot in a field. In fact, the concept of microsites as sites of denitrifica-
tion requires that denitrification be sensitive to the amount of soil water
and the diffusion of 02 to zones of high microbial activity. It is not known
whether the water function developed for these Yolo loam soil field sites can
be extrapolated to other soils. Considerably more research is needed on
other soil types to determine whether soil-water content or 02 diffusion is
the most sensitive and which procedure could be more easily extrapolated to
other situations.
MANAGEMENT SIMULATIONS
The simulation model described and used in this manuscript can be used
to calculate potential denitrification losses for various soil-water, soil,
and crop management situations. For example, total denitrification for six
hypothetical cases involving the possibilities of applying NOs fertilizer with
irrigation water are given in Table 8. All input data for the simulations
TABLE 8. TOTAL DENITRIFICATION (kg N ha"1) CALCULATED FOR VARIOUS WAYS OF
APPLYING NOa FERTILIZER DURING ONE IRRIGATION CYCLE OF CROPPED
SOIL TO WHICH STRAW WAS APPLIED 43 DAYS PRIOR TO FERTILIZATION.
SIMULATIONS WERE MADE FOR APPROXIMATELY 40 DAYS AFTER FERTILIZA-
TION
Fertilizer timing
Applied uniformly Applied during Applied during
Irrigation during entire 1st 1/3 of last 1/3 of
frequency irrigation irrigation irrigation
3 Irrigations 10.7 13.8 14.3
per week
1 Irrigation 4.6 2.8 5.4
per two
weeks
53
-------
are the same as those used in Figures 35 and 37 (straw addition) with the
exception of when the NO^ fertilizer was applied. Simulations of denitrifi-
cation were made for applying NOs fertilizer uniformly during the entire
first irrigation, during the first one-third of the first irrigation, and
during the last one-third of the first irrigation. For each of these three
timings of fertilizer application during irrigation, two irrigation frequen-
cies of three irrigations per week and one irrigation every two weeks were
used. The calculations given in Table 8 demonstrate that the fertilizer
application time did not affect denitrification significantly for the fre-
quent irrigation system. This was due primarily to the fact that only small
amounts of water were applied at any one time and the NO^ resided at about
the same position in the soil profile regardless of whether it was applied
during the first one-third or the last one-third of the irrigation cycle.
Denitrification was calculated to be slightly greater by applying fertilizer
during one-third of the cycle than for the case where the fertilizer was
applied uniformly throughout the first irrigation period (Table 8). This is
primarily due to the increased NO3 concentration in the narrow band when the
same quantity of fertilizer is applied in one third the water.
The computed values in Table 8 suggest, however, that the timing of
fertilizer application may be more important for the infrequent irrigation
system. If the fertilizer were applied during the first one-third of the
first irrigation for an infrequent irrigation program, the NO3 will be pushed
deeper into the soil profile during successive irrigations and less denitri-
fication occurs than that calculated for a uniform application during the
irrigation. If the fertilizer were applied during the last one-third of the
first irrigation, the NO3 remains in the upper part of the soil profile and
is susceptible to denitrification. The calculated denitrification for this
case was only slightly greater than that for the case where the fertilizer
was applied uniformly during the irrigation process.
Other management simulations demonstrate that increasing the soil
organic C level by three or four times would result in only a 10 to 20%
increase in denitrification. This is due to the fact that only a small part
of the soil organic C is water soluble or available for denitrification.
54
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REFERENCES
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C.A. Black (ed.). Agronomy, 9:1367-1378.
Bremner, J.M. 1965. Inorganic forms of nitrogen. In: Methods of Soil
Analysis, Part 2, C.A. Black (ed.). Agronomy, 9:1179-1237.
Burford, J.R., and J.M. Bremner. 1975. Relationships between the denitrifi-
cation capacities of soils and total, water-soluble and readily
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Donigian, Jr., A.S., and N.H. Crawford. 1976. Modeling pesticides and
nutrients in agricultural lands. EPA-600/2-76-043. U.S. Environmental
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Fried, M., K.K. Tanji, and R.M. Van De Pol. 1976. Simplified long term
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Hahn, J. 1972. Improved gas chromatographic method for field measurement of
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LaRue, M.E., D.R. Nielsen, and R.M. Hagan. 1968. Soil water flux below a
ryegrass root zone. Agron. J., 60:625-629.
Mehran, M., and K.K. Tanji. 1974. Computer modeling of nitrogen transforma-
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of nitrogen and phosphorous behavior in cropped land areas receiving
organic wastes. EPA-600/ -80. U.S. Environmental Protection Agency,
Athens, Georgia, (in review)
Rasmussen, R. A., J. Krasnec, and D. Pierott. 1976. Nitrous oxide analysis
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Reddy, K.R., R. Khaleel, and M.R. Overcash. 1979. A nonpoint source model
for land areas receiving animal wastes: V. Carbon transformations.
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isotope analysis, p. 31-42. E.W. Wilson, A.O.C. Nier, and
S.F. Reinmann (eds.). Preparation and Measurement of Isotopic Tracers.
J.W. Edwards, Ann Arbor, Mich.
Rolston, D.E., M. Fried, and D.A. Goldhamer. 1976. Denltrification measured
directly from nitrogen and nitrous oxide gas fluxes. Soil Sci. Soc.
Am. J., 40:259-266.
Rolston, D.E., and M.A. Marino. 1976. Simultaneous transport of nitrate and
gaseous denitrification products in soil. Soil Sci. Soc. Am. J.,
40:860-865.
Rolston, D.E., and F.E. Broadbent. 1977. Field measurement of denitrifica-
tion. EPA-600/2-77-233. U.S. Environmental Protection Agency, Ada,
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Rolston, D.E., D.L. Hoffman, and D.W. Toy. 1978. Field measurement of
denitrification: I. Flux of N2 and N20. Soil Sci. Soc. Am. J.,
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Shaffer, M. J., R. W. Ribbens, and C. W. Huntley. 1977. Prediction of
Mineral Quality of Irrigation Return Flow. Volume V. Detailed Return
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\
van Veen, H. .1977. Behavior of nitrogen in soil. A computer simulation
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56
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PUBLICATIONS
The following manuscripts have to date resulted from this research:
Rolston, D.E., and S. Cervelli. 1978. Denitrification as affected by
irrigation frequency and applied herbicides. Combined FAD/IAEA Advisory
Group and Research Coordination Meeting on Isotopic Tracer-Aided Studies
of Agrochemical Residue-Biota Interactions in Soil and Water, Vienna,
Austria.
Sharpley, A.N., and D.E. Rolston. 1980. Comparison of the acetylene
inhibition and *5N methods for the direct field measurement of
denitriflcation loss from soils. Soil Sci. Soc. Am. J. (submitted)
57
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
REPORT NO.
EPA-600/2-80-066
3. RECIPIENT'S ACCESSION NO.
4. TITLE AND SUBTITLE
DENITRIFICATION AS AFFECTED BY IRRIGATION FREQUENCY
OF A FIELD SOIL
5. REPORT DATE
April 1980 issuing date
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
D. E. Rolston, A. N. Sharpley, D. W. Toy, D. L. Hoffman,
and F. E. Broadbent
8. PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORGANIZATION NAME AND ADDRESS
Land, Air and Water Resources
University of California
Davis, California 95616
10. PROGRAM ELEMENT NO.
1HB617
11. CONTRACT/GRANT NO.
R-805550
12. SPONSORING AGENCY NAME AND ADDRESS
Robert S. Kerr Environmental Research Laboratory
Office of Research and Development
U.S. Environmental Protection Agency
Ada, Oklahoma 74820
13. TYPE OF REPORT AND PERIOD COVERED
Final
14. SPONSORING AGENCY CODE
EPA/600/15
15. SUPPLEMENTARY NOTES
16. ABSTRACT
The influence of irrigation frequency on dentrification was studied on a Yolo loam
field profile at Davis, California. Two carbon treatments were also established by
using plots with and without incorporated crop residues. Irrigation frequencies of
three irrigations per week, one irrigation per week, and one irrigation every two
weeks were established on areas cropped with grass. Fertilizer was applied as KNOs
enriched with 15N to 1-m2 plots. The flux of volatile gases at the soil surface was
measured from the accumulation of N20 and 15N£ beneath airtight covers placed over the
soil surface for 1 to 4 hours at several times after irrigation. For plots with and
without addition of crop residue, the largest denitrification was only 6.5 and 1.5^
of the applied fertilizer (300 kg N ha"1), respectively. Denitrification from the
least frequently irrigated treatments was less than that in the most frequently irri-
gated treatments. The N20 flux at the soil surface varied between 5 and_27% of the
total denitrification over a 40 to 50 day period. Denitrification of NOs fertilizer
was simulated using a mathematical model that included transport and plant uptake of
water and nitrogen in soil. Reasonable agreement was found between measured rates
and total amounts of denitrification with those calculated from the model.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lDENTIFIERS/OPEN ENDED TERMS C. COSATI Field/Group
Nitrogen cycle
Nitrogen isotopes
Nitrous oxide (N20)
Soil water
Fertilizer
Irrigation
Denitrification
Irrigation return flow
Nitrate leaching
Gas fluxes
Simulation modeling
02/A,c
18. DISTRIBUTION STATEMENT
RELEASE TO PUBLIC
19. SECURITY CLASS (ThisReport)
Unclassified
21. NO. OF PAGES
74
20. SECURITY CLASS (Thispage)
Unclassified
22. PRICE
EPA Form 2220-1 (9-73)
58
(, U.S. GOVERHUEHT PRINTING OFFICE: 1980-657-146/5652
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