TAMPA BAY AREA
PHOTOCHEMICAL OXIDANT STUDY

Date ^	I?78

Final Report
Appendix C

EMISSION RATE FOR BIOGENIC NOx

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Mission ratF(:

F0« biogenic no



US Ik??F£2a^~..
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Afclaata, €e®rgia S0365

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Tingey

Terrestrial Ecology Branch
Corvallis Environmental Research Laboratory
U.S. Environmental Protection Agency
Corvallis, Oregon 97330

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ABSTRACT

A literature review of biogenic sources of N0x was conducted to determine
their emission rates into the atmosphere. N0x are some of the products of
microbial denitrification, chemical decomposition of nitrites and the oxida-
tion of organic nitrogen compounds. There appears to be no significant
emission of N0x from either oceans or freshwaters. Biogenic emission rates
for NO and N02 from soil range from 0.015 to 0.02 kg NO km-2 hr-1 and 0.01 to
0.2 kg N02 km-2 hr-1. Submerged soils, sediments, marshes and swamps could be
sources of N0x but emission data are not available. There is no significant
evidence of N0x emission from living vegetation. During decay, decomposition
and ensiling of vegetation, N0x can be formed. Although the emission rates
are not known, they are probably not significant.

The estimates of N0x emission rates from the above biogenic (soil)
sources were computed for a global basis and then compared closely to pre-
viously estimated natural global emissions. Also, the background atmospheric
concentrations of N0x are similar to those levels predicted from biogenic
emission rates.

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INTRODUCTION

Several authors have estimated global biogenic N0x emissions. Robinson
and Robbins (1970) estimated global natural emissions of N0x at 768 x 109 kg
N02 which was approximately 15 times the anthropogenic emissions (53 x 109 kg
N02). McConnell (1973) suggested that the natural sources of N0X were 4 times
the anthropogenic sources. Galbally (1975) estimated biogenic N0X emissions
for the northern hemisphere at 1 x 109 kg N02 yr-1 and the anthropogenic
emissions at 0.5 x 109 kg N02 yr-1. Estimates of biogenic emissions for N0x
for Ohio and surrounding states, ranged from 1.7 to 4.1 x 108 kg N02 yr-1 and
the anthropogenic emissions ranged from 2.5 to 33.3 x io8 kg N02 yr-1 (RTI,
1974). These calculated N0x emissions were based on atmospheric concentra-
tions and a theoretical balance of nitrogen compounds. At present there is no
general agreement on the relative contributions of the biogenic and anthro-
pogenic emissions. Most biogenic emission rates were derived from the amounts
of N0x needed to balance nitrogen cycles or were deduced theoretically.

The objectives of this literature review were to gather information on
possible biogenic sources of N0x, to determine biogenic emission rates and to
discuss the factors affecting N0x emissions. The nitrogen cycle is discussed
to suggest possible biogenic sources of nitrogen oxides. The biogenic sources
and emission rates of NO are divided into the following categories: 1)

A

water: ocean and freshwaters; 2) soil; 3) flooded soil, sediments, swamp and
marsh; and 4) vegetation. The biogenic emission rates determined from the

2

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literature were compared to other emission estimates and atmospheric concen-
trations.

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NITROGEN CYCLE

Nitrogen is found in five major sinks in the biosphere: primary rocks,
sedimentary rocks, the deep-sea sediment, the atmosphere, and the soil-water
pool. Approximately 98% of the earth's nitrogen is in primary and sedimentary
rocks, while the atmosphere contains about 2%; the deep-sea sediment and soil-
water pool, together contain less than a percent of the global nitrogen (Burns
and Hardy, 1975).

In the atmosphere, N2 is the major nitrogen constituent while nitrogen
oxides (N20, NO, N02), N03, N02, NH3, and NH4 are present in the ppm range or
less. The soil-water pool can contain a large amount of dissolved N2. N20,
NO, and N02 can also occur in soils for short time periods under specific
environmental conditions.

A simplified nitrogen cycle showing nitrogen transformations in the soil-
water pool and transfers between the soil-water pool and the atmosphere is
shown in Figure 1. Nitrogen enters the soil-water pool through biological
nitrogen fixation, industrial fixation, precipitation and application of
fertilizers. Biological fixation reactions occur either in free-living
organisms (i.e. Azotobacter) or in symbiotic plant-microbial associations
(i.e. Rhizobium). Plants and microorganisms utilize NH4, N03, and N02 from
the soil-water pool and nitrogen undergoes a multitude of chemical and bio-
logical transformations.

In nitrification (Figure 1), an aerobic process, ammonium is oxidized to
ni trate.

4

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nhJ

NO3 NOj

Fig 1. Terrestrial nitrogen cycle showing nitrogen
transfer and transformations. Values given
are metric tons (N) x 10® yr-1 (Burns and
Hardy, 1975; Hardy and Havelka, 1975).

5

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NHt ¦+ NOg -> NO3

Oxidation occurs mainly by the autotrophic bacteria of the Nitrobacteriacea.
The genus Nitrosomonas oxidizes ammonium to nitrite and the genus Nitrobacter
oxidizes nitrite to nitrate. Other microorganisms, including certain bac-
teria, molds and fungi, are capable of limited oxidation of nitrogen, but
their contribution to nitrification is limited. The rapid nitrification of
ammonium is important agriculturally, since fertilizer ammonium can be rapidly
oxidized to nitrate and then lost from the soil by denitrification, leaching
and chemical decomposition (Hauck, 1971).

In denitrification (Figure 1), an anaerobic metabolic process, nitrate is
reduced sequentially to NO, N20 or N2.

NO3 NO2 -» NO ¦* N20, N2
The reduction is carried out by a diverse group of bacteria but the non spore
formers such as Pseudomonoas, Micrococcus, and Achromobacter and spore-forming
species of Baci11 us are the principal denitrifiers.

BIOGENIC SOURCES OF NITROGEN OXIDES

WATER
Oceans

The nitrogen cycle in the oceans is complex in respect to the large
geographical translocations of nitrogen and in species composition of micro-
and macro-organisms (Dudgale, 1969). Theoretical models of nitrogen circu-
lation in oceans and mass balances for marine nitrogen cycles indicate that
inorganic nitrogen is removed or lost from the oceans (Yoshinari, 1976;
Dugdale, 1969). Denitrification is a significant factor in loss of nitrogen
from the marine environment, especially in oxygen-deficient waters. Goering

6

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and Cline (1970) determined that denitrification in raw seawater occurred in
two stages. First, nitrate was reduced to nitrite, and second, the nitrite
was further reduced, presumably to N2. Nitrate15-N added to water samples
from the oxygen-deficient layer in the tropical Pacific resulted in the
production of nitrite and N2. Denitrification rates varied from 7-150 pg
N/l/day depending on sample depth (Goering, 1968). Estimated denitrification
losses in oxygen-deficient waters of the Black Sea (7 x 109 - 2 x 1011 g
N yr-1) and Cariaco Trench (1 x io10 g N yr-1) were not significant compared
to those in the eastern tropical North Pacific (1 x 1013 g N yr-1) (Goering et
al. , 1973).

Patriquin and Knowles (1974) examined shallow-water marine sediments from
several locations for denitrifiers and found that most of the bacteria that
reduced nitrate to nitrite also reduced nitrite to gaseous nitrogen. Bac-
terial isolates varied considerably in the rate of N2 production and N20
reduction, and in accumulation of N20. Barbaree and Payne (1967) demonstrated
that N20 and very small quantities of NO were present transiently in the
atmosphere over reaction mixtures containing cell-free extracts of Pseudomonas
perfectomarinus cells.

The above data support the conclusion that denitrification is an active
process in the oceans and that both N2 and N20 are products but there is no
evidence of N0x production in oceans.

Freshwater

As in the oceans, denitrification in freshwaters is a significant factor
in nitrogen loss. The nitrogen cycle and the fate of nitrogen in freshwaters

has been reviewed recently (Keeney, 1973;	Brezonik, 1973).

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In lakes, denitrification occurs mainly in the oxygen depleted hypolimnic
layers where inorganic nitrogen levels are at a minimum (Vollenweider, 1968).
Denitrification rates in Smith Lake, Alaska, in water one meter below the ice
were measured at 15 pg N/liter/day. Molecular nitrogen appeared to be the
only significant product and NO and N20 were not detected (Goering and
Dugdale, 1966). Brezonik (1973) concluded that denitrification did not appear
to be significant in Florida lakes. Of the 55 north central lakes sampled in
Florida, only four developed anoxic conditions at the bottom and the nitrate
concentrations in Florida lakes are typically low. Nitrogen is the only
product of denitrification released in significant quantity, and seldom is NO
or N20 detected (Payne, 1973).

In addition to denitrification, there is the possibility of chemical
decomposition of nitrite to produce gaseous nitrogen products. In some lakes,
concentrations of polyphenolic substances (tannins, lignins, humic acid) are
high and pH of water is acidic. Under these conditions the reaction of ni-
trous acid with organics could be important as a source of nitrogen oxides
(Brezonik, 1973).

SOIL

The soil is an open system from which various nitrogen forms volatilize
into the atmosphere. As early as 1944, it was discovered that nitrous oxide
was one of the constituents of soil air (Kriegel, 1944). Adel (1946, 1951)
suggested that N20 was produced in the soil and was the source of atmospheric
N20. Arnold (1954) confirmed N20 losses from the soil and Wijler and Delwiche
(1954) found that N20 was the major initial product of denitrification under
some soil conditions, but also identified NO as an additional product. More

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recently, in addition to losses from denitrification, investigators have found
that NO and N02 are produced by chemical reactions in soil involving HN02 and
nitrite. (Reuss and Smith, 1965; Nelson and Bremner, 1970; Bollag, et al.,
1973).

Denitri fication

The term "denitrification" implies the gaseous loss of nitrogen to the
atmosphere, usually as N2, N20 or NO, through some biological agency and it is
the subject of two recent reviews (Payne, 1973; Delwiche and Bryan, 1976). In
the absence of oxygen, microorganisms use nitrate or its reduction products as
electron acceptors for the oxidation of some organic compound as an energy-
yielding reaction. The intermediates and products oxf denitrification have
been studied in cultures and in soil experiments, but the pathways are varied
and not presently elucidated. One possible pathway is as follows:

NO3 ¦» N02- -> NO N20 -» N

Many genera of bacteria are able to reduce nitrate to nitrite and of
those a limited number are able to further reduce nitrite to N20 or N2.

Studies have determined that NO is a specific product of nitrite reduction;
nitrous oxide (N20) results from NO reduction and is the terminal denitrifi-
cation product of several bacterial strains (Renner and Becker, 1970; Payne,
Riley and Cox, 1971).

Factors Affecti ng Denitri fication. Denitrification is influenced by
factors such as soil oxygen concentration, redox potential, pH, soil organic
matter content and temperature. In general, conditions that directly or
indirectly decrease the soil oxygen content or increase microbial activity
increase denitrification. When soil oxygen level decreased from 8.5% to 1%,

9

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N20 production increased, but as the oxygen level decreased to zero the N20
was reduced to N2 (Cady and Bartholomew, 1961). Although it was not measured,
a similar response would be expected for NO. In a closed soil-plant system
(Stefanson, 1972) N2 was the major component of denitrification but in the
absence of plants the main product was N20. Over the temperature range of 15
to 35°C, increasing the temperature 10°C doubled the rate of denitrification
(Stanford et al., 1975). Bailey (1976) reported that as soil temperature
increased the rate of N2 production increased and NO production decreased.
Denitrification capacities of 17 surface soils were significantly correlated
with total organic carbon content and very highly correlated with water-
soluble organic carbon or mineralizable carbon (Burford and Bremner, 1975).
Nitric oxide was detected in the atmosphere of several soils incubated at 20°C
for 7 days, but the amount represented not more than 0.1% of the nitrate lost.
As the soil pH increased from 5 to above 7, the ratio of N20 to N2 decreased
(Burford and Bremner, 1975); below pH 7.0, N20 was the main product (Wijler
and Delwiche, 1954). The pH dependency of.NO production should be similar to
N20. In general, low temperature, low pH, and marginal anaerobic conditions
favor the production of N20 relative to N2 (CAST, 1976).

Chemodenitrification or Nonbiological Chemical Decomposition

In addition to microbial denitrification there is increasing evidence
that non biological reactions produce nitrogen gas or nitrogen oxides under
some circumstances (Delwiche and Bryan, 1976; Porter, 1971). Non biological
loss of nitrogen may result from "side-tracking" during nitrification and
denitrification processes. This can occur when an intermediate in the process
(i.e. N02) is produced more rapidly than it can be oxidized or reduced bio-

10

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logically and undergoes chemical decomposition (Lance, 1972). There are
several non biological reactions that could release NO into the atmosphere.

Nitrosation denotes the addition of the nitroso group (-N = 0) to an
organic molecule, and is brought about by HN02 and other compounds to form an
organic complex (0 = N - X). The nitroso groups formed are labile and react
further with the nitrosating agent to form gaseous products.

HN02 -» NO- + OH
NO- + R->N = 0- R
N + 0- R + A-»N0 + A- R
Stevenson, et al. (1970) showed evidence that NO, N20, and N2 could be pro-
duced by nitrosation of humic and fulvic acids, lignins, and aromatic sub-
stances at pH 6.0 and 7.0 in the absence of oxygen. Steen and Stojanovic
(1971) found that NO was volatilized from a calcareous soil when high con-
centrations of urea were nitrified with concurrent accumulation of nitrite and
assumed that nitrosation between nitrous acid and organic matter was the main
pathway by which NO was formed.

Wullstein and Gilmour (1964, 1,966) reported that nitrite reacted with
certain reduced transition metals in sterile, moderately acid systems to yield
NO as a primary gaseous product. In their proposed scheme, N02 and NO reacted
with metallic ions to form complexes which were either stable or reactive.
The metals could also react directly with N02 or NO without forming complexes
or intermediates to form NO or N2. They concluded that copper and manganese
ions in the soil were primarily responsible in reacting with nitrite. Nelson
and Bremner (1970) found that Cu+, Sn2+ and Fe2+ were the only metallic ca-
tions that promoted nitrite decomposition and that soils normally do not

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contain sufficient amounts of these cations under conditions suitable for
chemodenitrification to be significant in nitrite decomposition.

Reuss and Smith (1965) showed that N2 and N20 are formed by chemical
decomposition of nitrite in soil and also showed that N02 was evolved when
nitrite was added to acid soils. Bremner and Nelson (1968) found that N2 and
N02 and small amounts of N20 were formed when nitrite was added to neutral and
acid soils. They suggested that the reactions between soil organic constitu-
ents and nitrite were responsible for the formation of N2 and N20, while self-
decomposition of HN02 was responsible for the formation of NO and N02. In the
steam-sterilized raw humus samples incubated with nitrite, NO was the pre-
dominant gaseous reaction product (Nommick and Thorin, 1971). Nelson and
Bremner (1970) found that the formation of N02 by decomposition of nitrite in
pH 5.0 solution was not promoted by organic and inorganic soil constituents,
and concluded that most of the N02 evolved was formed by self-decomposition of
HN02. The amount of N02 formed was inversely related to soil pH, but pH had
little effect on the amount of nitrite converted to nitrate. These findings
led to the conclusion that the self-decomposition reaction of HN02 was best
represented by the equation:

2HN02 -> NO + N02 + H20

Measurements of NO and N02 emissions from soil and biological and non
biological reactions are difficult and only limited data are available (Table
1). Soil emission rates were determined by directly trapping liberated gases
at the soil surface. Based on the limited number of studies, NO is produced
at lower rates (0.004 to 0.02 kg NO km-2 hr-1) than N02 (0.01 to 0.2 kg N02
km-2 hr-1). These rates are temperature dependent and would be expected to
increase as the temperature increases (Bailey, 1976).

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Table 1. Nitrogen Oxide Flux From Soil



Soil or

N

Type N

kg/km2/hr.

Reference

Medi urn

Addition

Oxide

Emission Rate



low humic

nh4 no3

NO

.015-.02

Getmanets, 1972

Ordinary Chernozem

urea







Chernozem

nh4 no3

NO

.006

Borisova, et al

Podzol

100 Kg.



.004

1972



N/ha







top soil from pine,

none

no2

pine 0.125

Kim, 1973

oak, sod stand





oak 0.07



(sandy loam)





sod 0.11



Sod- Podzolic

nh4 no3

no2

0.01-0.2

Makarov, 1969







comment: varv









ation during









the growing









season



Sod - Podzolic

urea

no2

0.03-0.05

Makarov and

medium clay loam

240 Kg/ha





Ignatova, 1964

raw humus

KN03

*(N0+N02)

.018

Mahendrappa,

in spruce

nh4 no3



.029

1974

stand

Ca (N03)2



.035



(feather

A1 (N03)3



. 104



moss)









raw humus

nh4 no3

*(N0+N02)

.012

Mahendrappa,

(sphagnum

Ca (N03)2



.018

1974

moss)

A1 (N03)3



.035



* emission rate calculated as N02

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FLOODED SOIL, SEDIMENTS, SWAMP AND MARSH

Flooded soil or sediments have characteristics that are unique and sep-
arate them from arable soil, such as the interruption of gaseous exchange
between air and soil (Patrick and Mikkelsen, 1971). The restriction of oxygen
diffusion results in an oxidized layer of up to one cm. in the soil-water
interface, but below this layer the oxygen content declines rapidly. A second
characteristic of flooded soil is a change in microbial forms from aerobic to
facultative anaerobic organisms to anaerobic bacteria. Retarded metabolic
processes are reflected in reduced organic matter decomposition and a lowered
nitrogen requirement for decomposition. In terms of physiochemical changes,
the pH of flooded soils tends to change toward neutrality, redox potentials
are low (-300 mv) and there is an increase in the amount of ions in the soil
solution (Ponnamperuma, 1972).

In submerged soil, inorganic nitrogen is present almost exclusively as
NH4 because the lack of oxygen prevents the nitrification of NH4 to N03.
However, the N03 formed in the aerobic layer at the sediment-water interface
diffuses downward to the anaerobic layer where denitrification occurs. As in
soils, denitrification (biological and non-biological) is the major source of
nitrogen oxides from flooded soils, sediments, swamps and marshes. Oxygen
content, pH, redox potential, temperature, nitrate content and organic matter,
content of submerged soils are factors that affect the amount and products of
denitrification.

Denitrification as measured by nitrate and nitrite reduction rates and N2
formation is significant in submerged soil and sediments (Chen, Keeney,

Konrad, Holding, Graetz, 1972; Chen, Keeney, Graetz and Holding, 1972; Reddy
and Patrick, 1975; Engler and Patrick, 1974; Goering and Dugdale, 1966). The

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disappearance of nitrate in sediments and submerged soils is usually accom-
panied by the production of N2 but recently several workers have shown other
denitrification products. Lake sediments incubated with nitrate and nitrite
produced N20 in addition to N2 (Macgregor and Keeney, 1973; Chen, Keeney,
Konrad, Holding, Graetz, 1972). In the decomposition of nitrite in flooded
soils, N2 was produced at all pH's but NO and N20 were produced only at pH 6.0
and below (Van Cleemput, et al. 1976). The addition of a soil sterilant
(HgCl2) increased the rate of NO production. The important conclusions from
this study were that under acid conditions significant amounts of N2, N20 and
NO were formed. The production of NO under acid conditions with and without a
sterilant suggested the self-decomposition of nitrous acid similar to what
occurs in arable soils. However, the data are not adequate to calculate
emission factors.

VEGETATION

Plant leaves and roots can absorb both reduced or oxidized forms of
nitrogen from the environment and a relatively large concentration of nitrogen
compounds are found in plants. However, there is no direct evidence that any
of this nitrogen is emitted into the atmosphere as N0^. In contrast during
decomposition N0x can be emitted into the atmosphere but its significance is
not known. During the ensiling process high concentrations of nitrogen oxides
can be emitted. In an unventilated silo or enclosure, NO and N02 can reach
hazardous levels and such levels have accounted for several fatalities
(Commins, et al., 1971; Scaletti, et al., 1960). In decomposition of plant
products only a small fraction of total nitrogen losses from the system are
attributed to denitrification. In an eastern mature hardwood forest, an

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estimated 3.6% of the total nitrogen flux was lost through denitrification
processes (Mitchell, et al., 1975). Estimated denitrification rates of 0.17,
1.61 and 0.08 kg N ha-1 yr-1 were measured for branches, logs and litter (H
layer), respectively (Todd, et al., 1975). It is conceivable that under some
conditions small amounts of nitrogen oxides are emitted during litter decom-
position. However, data on the emission rates from decomposition are lacking.

ESTIMATES OF GLOBAL N0X EMISSION
The source of N0x in the atmosphere is both anthropogenic and biogenic.
Estimates of anthropogenic emissions of N0x on a global basis are in good
agreement as shown in Table 2. But the biogenic emissions are varied and are
much more difficult to identify, measure, or estimate.

The biological and chemical transformations of nitrogen compounds in the
soil appear to be the major source of N0x- The estimates of NO^ emission
rates from soil (Table 1) range from 0.01 - 0.2 kg N km-2hr-1. This rate
computed on a global scale gives an emission from 3 to 58 x 109 kg N yr-1 and
compares closely to the estimated global N0x emissions by Galbally (1975) and
Soderlund and Svensson (1976).

The background concentrations of NO and N02 reported by Rasmussen, et
al. (1975) is about 0.3 - 2.5 pg m-3 and 2 - 2.5 pg m-3, respectively.
Soderlund and Svensson (1976) reported background concentrations of NO .to

X

range from 0.5 - 7.5 pg m3 (as N02) depending on geographical location. If it

is assumed that background NO is in a steady state condition, then NO

X	X

deposition must be balanced by N0x emission. The total wet and dry deposition
of N0x for the terrestrial system was estimated at 32 to 83 x 109 kg N yr-1
and for the aquatic system at 11 to 33 x io9 kg N yr-1 (Soderlund and

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Svensson, 1976). Peterson (1977) estimated that N0x is removed by wet and dry
deposition processes at about 0.04 kg N km-2hr-1. This value is similar to
soil NO^ emission rates given in Table 1. Therefore, background concentra-
tions of N0X can be explained in large part by biogenic N0x emissions.

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Table 2. Estimates of Global N0x Emission

Sources of N0x(109 kg N yr-1)	References

Anthropogenic

Biogenic



16

234

Robinson and Robbins, 1970

18

* 72

McConnell, 1973

15

20

Galbally, 1975

19

8-25

Soderlund and Svensson, 1976



3-58

estimated from data in Table 1.

* natural sources estimated at 4 or more times the anthropogenic
sources.

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