Hatfield J.L., Gandini A. (eds.) Nitrogen in the Environment

Подождите немного. Документ загружается.

446 Nitrogen in the Environment

200

150

100

N-input (Tg N)

1800 1900 1930 1950 1960

Year

1970 1980 1990 1996

Animal manure Crop residue Synthetic NN-fixation

Figure 1 . Estimate of global annual N input into crop production from synthetic N,

biological N-fixation, crop residue return, and animal manures ( Kroeze et al., 1999 ;

Mosier, 2000 ).

7.0

5.0

6.0

4.0

3.0

O emission estimate (Tg N)

2.0

1.0

0.0

1800 1900 1930 1950 1960

Year

1970 1980 1990 1996

Animal WMS DirectIndirect

Figure 2 . Estimates of global N

O emissions as a result of crop and livestock pro-

duction from 1800 to 1996 ( Kroeze et al., 1999 ). Direct  N

O emitted directly

from cropped fields, Animal WMS  N

O emitted from livestock excreta where

emissions are related to the waste management system used, and Indirect  N

emitted from N that was used in crop production after it was leached or eroded from

the site of application ( Mosier, 2000 ).

CH13-P374347.indd 446CH13-P374347.indd 446 5/31/2008 6:16:59 PM5/31/2008 6:16:59 PM

Exchange of Gaseous Nitrogen Compounds 447

N input into crop production, with the greater portion coming from animal pro-

duction. As a result of increased N input and increased crop production by 1996

about one-third of the N

O originated from each of the three segments of the N

estimate. In 1950, total N

O from global food production systems totaled ⬃ 3 Tg N

compared to ⬃ 6 Tg in 1996 ( Kroeze et al., 1999 ). Note that these N

O estimates

present a very different picture from the IPCC 1990 and 1992 projections. In those

estimates agricultural N input was considered as the only synthetic fertilizer, and

the livestock waste management segment was not included ( Cole et al., 1996 ). The

estimates presented here and by Mosier et al. (1998) and Kroeze et al. (1999) indi-

cate that when a more complete picture of the agricultural N cycle is depicted that

O emissions resulting from food production have been a significant part of the

global budget for centuries.

3 . NUTRIENT REDISTRIBUTION BY SOIL–ATMOSPHERE EXCHANGE

OF NITROGEN COMPOUNDS

3.1 . NH

Exchange Between the Soil, Plants, and the Atmosphere

Plant–Atmosphere NH

Exchange . Plants can serve as both a sink and a source

of atmospheric NH

( Hutchinson et al., 1972 ; Farquhar et al., 1980 ; Harper et al.,

1996 ; Schjorring and Mattsson, 2000 ). Whether NH

is emitted or absorbed

by plants depends upon the NH

compensation point of the plant. Ammonia is

absorbed when the mole fraction of NH

in the atmosphere above the plant leaf is

greater than the NH

concentration in the atmosphere around the mesophyll cells in

the stomatal cavity. Ammonia is emitted when the atmospheric NH

concentration

is lower than the leaf mesophyll cell concentration ( Farquhar et al., 1980 ).

The impact of plant NH

exchange on field crop N balance and exchange with

the regional ecosystem is not yet clear. The net NH

exchange at a location is likely

dependent upon several interacting factors, thus general simplifying conditions to

describe NH

exchange are not known. Wetselaar and Farquhar (1980) showed that

N loss from wheat ( Triticum aestivum ) was increased with increasing amount of

N-fertilizer application. They showed that the N content of plant leaves typically

declined between anthesis and maturity in most field crops, including wheat, rice

( Oryza sativa ), rye ( Secale cereale ), corn ( Zea mays ), and barley ( Hordeum vul-

gare ). They attributed much of this N loss to foliar volatilization of NH

described by

Hutchinson et al. (1972) . Both laboratory ( Farquhar et al., 1980 ; Parton et al., 1988 )

and field studies ( Harper et al., 1987 ; Harper and Sharpe, 1995 ) show that NH

com-

pensation point can vary with plant type, temperature, phenological growth stage,

time of day, and soil N availability. Because of the continuous exchange of NH

between the atmosphere and the plant, Francis et al. (1997) concluded that determin-

ing the true loss or gain in between corn plants and the atmosphere is difficult.

Schjorring and Mattsson (2000) note that the role of crop NH

emissions in agro-

ecosystem N balance is not clear. Schjorring (1995) and Schjorring and Mattsson

(2000) did not observe a general relationship between N-fertilizer application rate

CH13-P374347.indd 447CH13-P374347.indd 447 5/31/2008 6:17:00 PM5/31/2008 6:17:00 PM

448 Nitrogen in the Environment

and net plant–atmosphere NH

exchange. On the other hand, Harper et al. (1987)

suggested a direct relationship between plant N status, fertilization, and NH

loss.

They reported a relatively large loss between corn anthesis and harvest ( ⬃ 1 5 k g

-N/ha) compared to the 1–2 kg N/ha/season reported by Schjorring (1995) .

Schjorring and Mattsson (2000) observed that N loss from crop vegetation was

important only when the N harvest index at maturity (ratio between grain and shoot

N content) was less than 0.63. Net NH

losses from spring barley that had N harvest

indices at maturity of 0.8 were only 0.5–1.5 kg N/ha.

Net NH

exchange within plants, because NH

can be both absorbed and emit-

ted, can influence fertilizer

N balance studies ( Harper and Sharpe, 1995 ; Francis

et al., 1997 ). Plants grown with

N-enriched fertilizer tend to loose

and

gain

even if the net NH

flux is zero ( Sharpe and Harper, 1997 ). The actual

impact of this exchange on N-fertilizer studies is not always clear, although the NH

exchange would suggest that N-fertilizer losses estimated by

N balance would be

overestimated ( Francis et al., 1997 ). Schjorring and Mattsson (2000) point out that

overestimates of N loss by isotopic techniques can also be due to leaf drop before

flowering, NH

emissions from decaying leaves, or NH

exchange within the crop

canopy. They conclude that crops are typically a net source of NH

to the atmos-

phere on a seasonal basis of up to 5 kg NH

-N/ha. The amount of NH

lost from

crop vegetation ranges between one and four percent of fertilizer N applied and

between one and four percent of the N present in the crop ( Schjorring and Mattsson,

2000 ). As a result, agroecosystems play an important role in regional atmospheric

N gas composition and may be the dominant feature where NH

emissions from

livestock waste is small (see Oenema et al. this volume).

Absorption/Volatilization by Soil . Uptake of atmospheric NH

by soils has

been documented for more than 150 years (references and discussion in Hanawalt,

1969 ). In many areas of the world, atmospheric NH

concentrations are several times

higher than in relatively pristine environments, and in these areas the atmosphere–soil

exchange of NH

is a main supplier of ecosystem N ( Vitousek et al., 1997 ). Generally

speaking, NH

volatilization to the atmosphere is from soils that have insufficient sorp-

tion capacity to hold ammonium from NH

deposition or from ammonium fertilizer

application ( Terman, 1979 ). Soil characteristics and basic chemical factors control

the soil–atmosphere exchange of NH

. Soil NH

retention is controlled by chemical

equilibria in soil water. As a basic gas, NH

reacts readily with protons, metals, and

acidic compounds to form ions. Ammonium formed can be ionically or physically

bound to soil particles and the interaction between these processes control the soil–

atmosphere exchange of NH

. These interactions are fully described by Freney et al.

(1983) and will not be described in detail here. Briefly, the various reactions which

regulate NH

exchange with soil can be represented as follows:

Absorbed NH NH NH NH NH

(solution) 3 (solution) 3 (gas) 3 (ga



ss in atm.)

(1)

CH13-P374347.indd 448CH13-P374347.indd 448 5/31/2008 6:17:00 PM5/31/2008 6:17:00 PM

Exchange of Gaseous Nitrogen Compounds 449

Adsorption of NH

and fixation of NH



onto clay particles is typically regulated

by soil organic matter content and the type of clay minerals in the soil. Adsorption is

related to the surface area of the sorbing material and generally clay soils sorb more

than do sandy soils ( Terman, 1979 ). The rate of NH

volatilization may be con-

trolled by the rate of removal and dispersion of NH

into the atmosphere by chang-

ing the concentration of NH



or NH

in the soil solution or by displacing any of the

equilibria as in Eq. 1 . The driving force for NH

volatilization from soil solution is

the difference in the NH

partial pressure between that in equilibrium with the liquid

phase and that in the ambient atmosphere. The equilibrium vapor pressure of NH

is controlled by the NH

concentration in adjacent solutes which, in the absence of

other ionic species, is affected by NH



ion concentration and pH. At pH 9.2 a solu-

tion contains approximately equal amounts of solution NH



and solution NH

. At

pH 7.2 the solution contains ⬃ 99% solution NH



and 1% solution NH

. Thus, NH

emissions are typically higher in more basic soils. Chemical equilibria dictate that

an aqueous solution will hold less NH

with increasing temperature, so temperature

affects soil–atmosphere NH

exchange as well ( Freney et al., 1983 ).

Ammonia must be transported to the soil surface before it can be lost to the

atmosphere. As a result, NH

losses are typically reduced by subsurface application

of fertilizers ( Terman, 1979 ). Ammonia transport can be accomplished by move-

ment in liquid or gaseous phases and their relative importance depends on soil water

content. Volatilization of NH

from solution at the soil surface occurs in response to

a difference in vapor pressure between solution and ambient air. Increasing wind

speed increases the rate of volatilization by permitting more rapid transport of NH

away from the water surface ( Freney et al., 1983 ).

Factors which influence soil–atmosphere NH

exchange include soil cation

exchange capacity (CEC), soil pH, soil buffer capacity, and calcium carbonate con-

tent. As NH



is positively charged it readily reacts with the soil cation exchange

complex. The disassociation of ammonium ion releases a proton in addition to NH

Consequently as NH

loss proceeds, the solution becomes acidified and as the frac-

tion of ammoniacal N is reduced an equilibrium is reached. For this reason NH

volatilization is generally not important in soils that have a high base saturation

( Freney et al., 1983 ). A strong correlation between NH

loss and calcium carbonate

content of soils has been observed ( Fenn et al., 1981 ). The apparent stimulation of

volatilization has been related to soil clay-sized calcium carbonate content and

the formation of calcium fluoride, sulfate, and phosphate precipitates, and ammo-

nium bicarbonate ( Fenn et al., 1981 ; Terman, 1979 ).

Volatilization from Surface Residues . Since NH

emissions are readily

reduced by placing ammonium-based fertilizers below the soil surface, surface appli-

cation of fertilizers can lead to increased NH

losses ( Terman, 1979 ). Increased use

of no-till management could contribute to this increase if new fertilizer N manage-

ment tools are not developed. An example of the impact of crop residue management

on NH

loss is the practice of trash retention following green cane harvesting in sugar

cane production. As practiced in north Queensland, Australia, cane trash is left on the

CH13-P374347.indd 449CH13-P374347.indd 449 5/31/2008 6:17:00 PM5/31/2008 6:17:00 PM

450 Nitrogen in the Environment

field following harvest to retain soil moisture, prevent weed growth, and to minimize

soil erosion ( Freney et al., 1992 ). Surface application of urea onto green cane trash

was a common practice which typically resulted in large NH

loss. These losses are

driven by addition of small amounts of water from dewfall, light rain, and condensa-

tion of evaporated soil water. This water dissolves the urea, permits urea hydrolysis to

and allows NH

to be lost as the water evaporated ( Freney et al., 1992 ). Efforts

to decrease these NH

losses included fertilizer banding, supplemental irrigation, and

delaying fertilization until cane canopy was developed. Freney et al. (1991) found

that banding urea increased NH

loss while irrigating with 16 mm of water decreased

loss by about 50%. The water addition dissolved part of the urea and washed it

into the soil. A 1-m high crop canopy absorbed about 20% of the NH

emitted from

the residue surface, thus decreasing net NH

loss. Substituting ammonium sulfate for

urea reduced NH

loss to  1.8% of the N applied ( Freney et al., 1992 ).

Volatilization from Flooded Rice . Wetland rice is grown on ⬃ 125 million

hectares globally ( Neue, 1992 ) and is frequently limited by soil N supply ( Simpson

et al., 1984 ). Fertilizer N application typically, greatly increases rice yield but it

is costly to Asian farmers because of low N-use-efficiency. Recoveries of applied

N can be as low as 10% and rarely exceeds 50% ( Freney et al., 1990 ). The main

cause of fertilizer inefficiency is gaseous emissions of NH

or denitrification. The

greatest N losses are reported to occur when fertilization leads to high concentra-

tions of ammoniacal N in flood water and thus NH

volatilization ( Freney et al.,

1990 ). One method of reducing NH

emissions is to incorporate the fertilizer into

the soil after application. Although this practice generally reduces NH

emissions

it does not always lead to decreased total N loss ( Cai et al., 1986 ). Freney et al.

(1990) found that when urea was broadcast into the flood water at rice transplant-

ing, NH

loss varied from 10% to 56% of applied N at four different locations in

the Philippines. Losses were greater where temperature and wind speed were great-

est. Ammonia loss was reduced by 7–56% by harrowing after fertilizer application.

Total N loss from the basal urea application ranged from 59% to 71% of N applied.

Incorporating the urea by harrowing had little effect on total N loss. Denitrification

losses ranged from 3% to 50% of N applied. The denitrification losses were low

when NH

losses were high and vice versa ( Freney et al., 1990 ).

In China, ammonium bicarbonate and urea are the main N-fertilizer sources

( Bouwman et al., 1997 ). Surface application of ammonium bicarbonate can result in

losses of 30–70% of N applied by NH

volatilization ( Cai et al., 1986 ). Application of

urea and ammonium bicarbonate to calcareous soil in north-central China at rice trans-

planting resulted in 33% and 39% volatilization of NH

, respectively. Denitrification

losses were also large, 33% and 30%, respectively. Incorporation of fertilizer into

drained soil substantially increased N utilization by the rice crop. Fertilizer N-use-

efficiency was improved mainly through the reduction in NH

loss ( Zhu et al., 1989 ).

3.2 . Global Emission Sources of NO

and NH

. Global NH

emissions from earth surface to the atmosphere in 1990

total about 54 Tg N ( Bouwman et al., 1997 ). These emissions arise from four main

CH13-P374347.indd 450CH13-P374347.indd 450 5/31/2008 6:17:00 PM5/31/2008 6:17:00 PM

Exchange of Gaseous Nitrogen Compounds 451

sources ( Table 1 ) which include excreta from domesticated animals (21.6 Tg N),

synthetic fertilizers (9 Tg N), biomass burning (5.9 Tg N) of crops and crop decom-

position (3.6 Tg N). These data indicate that about 65% of the total global NH

emitted from agricultural systems. In animal production much of N contained in

organic compounds in animal excreta is rapidly converted to NH

which can be vol-

atilized directly from the animal production system or when the excreta is applied

to the soil. Synthetic fertilizers applied in the form of urea (about 55% of global

fertilizer N production; FAO, 1999 ) or ammonium bicarbonate (about 11% of glo-

bal fertilizer N production) are susceptible to significant NH

losses when applied

to the soil surface. Urea is typically converted to NH



within a few days of appli-

cation to the soil by the enzyme urease ( Bouwman et al., 1997 ) thereby making

volatilization a problem for surface applied urea.

Table 1.

Total global NH

emission estimates adapted from Bouwman et al. (1997) for 1990

( Mosier, 2000 ).

Source

-N Emission

Estimate (Tg N) Uncertainty of estimate

(Tg N)

Excreta from domestic animals 21.6 10–30

Excreta of wild animals 0.1 0–1

Synthetic fertilizers 9.0 4.5–1

Biomass burning 5.9 3–7.7

Soils under natural vegetation 2.4 0–10

Oceans 8.2 3–16

Fossil fuel combustion 0.1 0–0.2

Industrial processes 0.2 0.1–0.3

Human excreta and pets 2.6 1.3–3.9

Crops and crop decomposition 3.6 1.4–5.0

Total emission 54 40–70

When wood, crop residue, or fossil fuels are burned, a portion of the N contained

in these fuels is converted to NH

which is emitted to the atmosphere. Crops can

either take up NH

from the atmosphere or emit NH

to the atmosphere, depending

upon atmospheric concentrations and plant N status. When plants begin to senesce

they typically lose N content through NH

volatilization ( Bouwman et al., 1997 ).

Ferm (1998) estimated for 1990 that 74%, 12%, and 5.5% of the NH

emitted

from Western Europe came from livestock, synthetic fertilizers, and crops, respec-

tively. He also presented data indicating that NH

emissions from Europe started to

increase dramatically following the World War II. Extending Ferm ’ s information to

1990 suggests that a slowing of this rate of increase in NH

emissions has occurred,

CH13-P374347.indd 451CH13-P374347.indd 451 5/31/2008 6:17:00 PM5/31/2008 6:17:00 PM

452 Nitrogen in the Environment

during the past two decades, probably as a result of the decline in cattle population

and decrease in fertilizer use in Western Europe during this time.

The rate of increase in NH

emissions globally also should have slowed during

the past decade. Using FAO data for global domestic animal populations and syn-

thetic fertilizer use and emissions calculations from Bouwman et al. (1997) , global

emission estimates from domestic animal excreta and synthetic fertilizers from

1961 to 1994 are shown in Figure 3 . During these three decades NH

from animal

production increased 54% from 14.2 to 22.1 Tg N/year at a relatively constant rate

of 1.7% per year. During this time NH

from use of synthetic fertilizer increased

540%, 1.4 Tg N/year to a maximum of 9 Tg N/year in 1990. The rate of increase

between 1961 and 1980 was 21.4% per year but this rate of increase slowed to 1.5%

from 1980 to 1994, mirroring the rate of annual increase from animal production.

Ammonia emission (Tg N)

1961 1965 1970 1975 1980

Year

1985 1990 1994

Animal excreta Synthetic fertilizer

Figure 3 . Estimates of global NH

emissions from animal excreta and synthetic fer-

tilizer ( Bouwman et al., 1997 ; Mosier, 2000 ).

. The release of nitrogen oxides (NO

 NO  NO

) has accelerated

during the last few decades through, primarily the increase in fossil fuel combus-

tion ( Galloway et al., 1995 ; Holland and Lamarque, 1997 ). With this increase in

emissions from ⬃ 5 Tg N in 1940 to ⬃ 25 Tg N in 1995, combustion of fossil fuels

account for about 50% of the total global NO

emissions ( Table 1 ) for 1990. Of the

anthropogenic sources, fossil fuel, aircraft, biomass burning, and part of the soil ’ s

emission are most important. Although the estimates are also similar for the soil

source; 6.2 Tg ( Holland et al., 1997 ) and 5.5 from Delmas et al. (1997) , Davidson

CH13-P374347.indd 452CH13-P374347.indd 452 5/31/2008 6:17:00 PM5/31/2008 6:17:00 PM

Exchange of Gaseous Nitrogen Compounds 453

and Kingerlee (1997) estimated, from field flux measurements, total global soil NO

emissions to be about 21 Tg N.

When plant uptake of NO

at the place of NO emission is considered, the total

soil-based emission of NO

is at least 13 Tg N. The NO

emissions from the soil are

primarily a result of NO production by the microbial oxidation of ammonium (nitri-

fication) ( Williams et al., 1992 ). NO production in the soil also occurs through the

microbial reduction of nitrate (denitrification). This reaction generally occurs only

in water-saturated soils where little NO is released from the soil to the atmosphere

( Conrad, 1996 ). Davidson and Kingerlee (1997) estimate that about 5 Tg of NO

-N

is emitted annually from cultivated soils globally. Through an analysis of published

flux measurements in agricultural fields, Veldkamp and Keller (1997) esti-

mated that about 0.5% of fertilizer N applied to agricultural fields was emitted to

the atmosphere as NO. The total global N input into agricultural soils in 1990 was

about 120 Tg. This amount includes N from: (1) synthetic fertilizer ( ⬃ 78 Tg); (2)

animal excreta used as fertilizer ( ⬃ 50 Tg); (3) biological N-fixing crops (includes

pulses and soybeans but not forage crops) ( ⬃ 10 Tg); and (4) crop residues returned

to agricultural fields ( ⬃ 28 Tg) ( Mosier et al., 1998 ). Using the 120 Tg as N input

into cropped soils the fertilizer-induced NO

emissions total 0.6 Tg N/year. The

remainder of the soil sources come from natural systems ( Davidson and Kingerlee,

1997 ), all of which now have received some amount of anthropogenic N input

through atmospheric deposition in precipitation and dry fall. Including the addi-

tional soil NO

source estimated by Davidson and Kingerlee (7 Tg N), total global

emissions in 1990 were ⬃ 48 Tg N using the Holland et al. (1997) numbers

( Table 2 ).

Table 2 .

Global sources of NO

emissions to the atmosphere.

Source Holland et al.

(Tg N) Delmas et al. (Tg N)

Fossil fuel combustion 21.1 (20–22.4) 22 (15–29)

Lightning 6.0 (3–10) 2 (1–4)

Soils 6.2 (5–10) 5.5 (3.3–7.7)

Aircraft 0.4 (0.23–0.6) 0.4 (0.5–0.6)

Biomass burning 7.0 (4.4–10) 7 (3–10.4)

Stratospheric injection 0.3 (0.2–0.6) 0.5 (0.4–0.6)

Total 41.0 (35–48.8) 38.2 (23.7–53.8)

Compiled from Holland et al. (1997) and Delmas et al. (1997) , and from Mosier

(2000).

Numbers presented are the mean of emission estimates from ﬁ ve different

3-dimensional chemical transport models; numbers in parentheses are the range of

model output values.

CH13-P374347.indd 453CH13-P374347.indd 453 5/31/2008 6:17:01 PM5/31/2008 6:17:01 PM

454 Nitrogen in the Environment

Figure 4 . A cartoon depiction of the terrestrial–atmosphere exchange of NH

and

deposition of NH

(adapted from Langford et al., 1992; Mosier, 1998 ).

3.3 . Fate of NH

and NO

in the Atmosphere (NH

and NO

Deposition)

. When NH

evolves from the earth ’ s surface it is either redeposited near

the source of emission or, since it is highly water soluble, dissolved in atmos-

pheric water ( Figure 4 ). When NH

is dissolved in water, ammonium ions (NH



)

and hydroxyl ions (OH



) are formed. The reaction is reversible and the solubility

increases if acidic species are also dissolved. Since NH

is the dominant basic com-

pound in the atmosphere it reacts readily with acidic gases and particles to form

hygroscopic salts containing ammonium sulfate and ammonium nitrate. These two

reactions occur very rapidly, a few hours to a few days in the troposphere, so NH

rapidly removed from the atmosphere through deposition as NH

and NH



in pre-

cipitation particulates ( Ferm, 1998 ), or through foliar absorption of NH

( Schjorring

and Mattsson, 2000 ).

Fowler et al. (1998) showed that atmospheric NH

concentrations drop from

⬃ 9 0  g NH

at a poultry production facility to ⬃ 1 0  g NH

100 m downwind.

At 300 m distance the NH

concentration was near ambient at ⬃ 0.5  g NH

( Figure 5 ). Although redeposition and atmospheric dispersion rapidly decrease NH

concentrations as distance from a livestock facility increases, total deposition within

1-km radial distance was less than 10% of the NH

emitted ( Figure 5 ). Pitcairn

et al. (1998) extended this work and looked at relationships between NH

emissions

from livestock facilities and N deposition onto nearby woodlands. They found that



(NH

)

HNO

CH13-P374347.indd 454CH13-P374347.indd 454 5/31/2008 6:17:01 PM5/31/2008 6:17:01 PM

Exchange of Gaseous Nitrogen Compounds 455

foliar N concentration of a number of plant species were large close to the livestock

buildings and declined with distance from them. Within the woodlands, ectohydric

moss tissue N concentration increased from ⬃ 1% at ambient atmospheric NH

con-

centration to ⬃ 3.2% at NH

concentrations of  2 0  g/m

. Pitcairn et al. (1998) also

found that moss tissue N content continued to increase to ⬃ 3.7% at NH

deposi-

tion rates of 40 kg N/ha/year. Since most of NH

evolved from terrestrial systems

is redeposited within 20 km of the emission point, NH

deposition near cattle pro-

duction operations can be substantial. For example, NH

from a 90,000 head cattle

feedlot collected in acid traps located 2 km from the feedlot was 30 kg N/ha/year

greater than that collected 15 km from the feedlot ( Hutchinson and Viets, 1969 ).

The NH

evolved from the feedlot provided about 50 kg N/ha/year to agricultural

soils within a few kilometers of the feedlot. The atmospheric deposition was not

considered in fertilization budgets for crops within the area.

can be transported long distances in the atmosphere. NH

and other highly

water soluble compounds are mainly dispersed in the lower troposphere in the

so-called mixing layer. The transport distances within the mixing layer depend upon

wind speed and the deposition rate. Model estimates indicate that about 50% of emit-

ted NH

is redeposited to earth surface systems within 50 km of the emission source

( Ferm, 1998 ). When gaseous NH

reaches the top of the mixing layer (about 500 m i n

winter and 1,500 m in summer in northern Europe) ( Figure 6 ), the NH

can be trans-

ported long distances. When clouds within the mixing layer contain water, the NH

Figure 5 . Distance from poultry production buildings and atmospheric NH

con-

centration and NH

deposition (adapted from Fowler et al., 1998 ; Mosier, 2000) .

100

0 200 400 600

Distance from poultry (m)

800 1000 1200

concentration (µg N/m

)

deposition (% emitted)

% DepositedNH

Conc.

CH13-P374347.indd 455CH13-P374347.indd 455 5/31/2008 6:17:01 PM5/31/2008 6:17:01 PM