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

456 Nitrogen in the Environment

concentration is halved every 10 km from the source (or every 30 min). If there is no

precipitation, the NH

x

concentration is halved every 1,400 km (or every third day)

( Ferm, 1998 ). Ferm also estimated that, over Europe, the half life of NH

x

is 3–6 h o v e r

a distance of 65–130 km. Overall, the average transport distance of NH

x

depends upon

the atmospheric concentration of SO

2

and NO

x

( Ferm, 1998 ). Atmospheric residence

times and transport distances of NH

x

are sufficiently long for significant interaction

with terrestrial and aquatic systems far removed from the NH

3

sources.

1

10

100

NO

y

and NH

x

deposition (kg N/m

2

/year)

1000

234

Distance from source (km)

5678

NH

x

NO

y

Figure 6 . Estimated deposition patterns of NO

y

and NH

x

emitted from Western

Europe (adapted from Ferm, 1998 ; Mosier, 2000) .

When NH

x

is deposited back onto soil it re-enters the terrestrial N cycle. If it is

not taken up by plants directly as NH

3

or NH

4



, it contributes to ecosystem acidifica-

tion as protons are released when NH

4



is oxidized (nitrified). Reactive N deposition

can also decrease soil consumption of atmospheric methane ( Steudler et al., 1989 ).

NO

x

. Upon its emission into the atmosphere, NO

x

plays a central role in

tropospheric chemistry ( Williams et al., 1992 ). The NO

x

interacts in a series of

concentration-dependent cycles with atmospheric oxidants, ozone (O

3

), hydroxyl

radicals (OH) during the oxidation of carbon monoxide (CO), methane (CH

4

), and

nonmethane hydrocarbons. The oxidizing capacity of the atmosphere is regulated

by NO

x

through regulation of OH and O

3

production and loss ( Holland et al., 1997 ).

Increasing NO

x

concentrations are contributing substantially to observed increases

in O

3

concentrations in the Northern Hemisphere ( Chameides et al., 1994 ). Detailed

descriptions of the complex interactive cycles of NO

x

in the atmosphere can be

CH13-P374347.indd 456CH13-P374347.indd 456 5/31/2008 6:17:02 PM5/31/2008 6:17:02 PM

Exchange of Gaseous Nitrogen Compounds 457

found in many references (e.g., Ehhalt et al., 1992 ; Liu et al., 1992 ; Williams et al.,

1992 ; Holland and Lamarque, 1997 ).

As a general simple description ( Figure 7 ), when NO enters the atmosphere it

reacts rapidly with O

3

to form NO

2

. In the sunlight, part of NO

2

– in a complex set

of reactions that involve hydrocarbons, ozone, and CO – dissociates to reform NO

and O

3

. In the presence of NO

x

, CO is oxidized to produce CO

2

and O

3

. The NO

2

is

further oxidized to a mixture of products referred to as NO

y

(NO

x

 organic nitrates,

inorganic nitrates and nitrites). During the daytime this set of reactions takes place

within a few hours when the sun is bright but may require a few days when clouds

block the sun ( Williams et al., 1992 ). At night NO

2

accumulates. At very low

NO

x

concentrations (  0.08 ppbv) there is net O

3

consumption. As NO

x

concen-

trations increase from 0.08 ppbv to about 1 ppbv, OH and O

3

production increases

proportionately with increasing NO

x

. At NO

x

concentrations above 1.1 ppbv there

is a decline in OH and O

3

production ( Holland and Lamarque, 1997 ). Nitric acid

production becomes the dominant end product. Nitric acid and peroxynitric acid

(HO

2

NO

2

) are soluble and are removed from the atmosphere by precipitation.

Estimates of NO

y

deposition ( Table 3 ) indicate that a balance between emission of

NO

x

and deposition of NO

y

exists.

Figure 7 . A cartoon depiction of the terrestrial–atmosphere exchange of NO

x

and

deposition of NO

y

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

Although the lifetime of NO

x

in the atmosphere is short (hours to a few days) it

can be deposited back to earth at long distances from its point of origin. Ferm (1998)

NO

2



NH

4



NOx

NOy

OH, O

3

CO, CH

4

etc

48

6

22

7

13

CH13-P374347.indd 457CH13-P374347.indd 457 5/31/2008 6:17:02 PM5/31/2008 6:17:02 PM

458 Nitrogen in the Environment

estimated that dry NO

y

deposition is rather constant the first 800 km from the emis-

sion source then is halved approximately every 600 km ( Figure 5 ). This long-distance

transport of NO

y

serves to fertilize the global oceans and terrestrial systems. Prospero

et al. (1996) estimated NO

y

deposition to the open ocean portion of the North Atlantic

Table 3.

Total NO

y

-N deposition on the earth ’ s surface.

NO

y

-N Deposited

Deposition location Mean (Tg N) Range (Tg N)

Oceans 16.2 11.7–25.8

Land 22.9 15.4–27.6

Ice or deserts 1.3 1.2–1.4

Agricultural areas 10.6 6.6–14.1

Natural vegetation 10.9 7.7–13.8

Forests 3.1 2.2–4

Unforested land 7.8 5.4–9.8

Total global deposition 39.1 35.3–41.2

Data presented are the mean and range of deposition quantities

determined by ﬁ ve, 3-dimensional chemical transport models and

values in parenthesis are the range of the ﬁ ve estimates ( Holland et

al. 1997 ), from Mosier (1998).

Ocean to be about 5 Tg N/year, or about 25% of the NO

x

emission from North

America and Europe. Globally, about 11 Tg of NO

y

-N is returned to agricultural land

annually ( Table 3 ) according to model estimates ( Holland et al., 1997 ). This NO

y

dep-

osition was equivalent to 14% of the synthetic N fertilizer used globally in 1990 and

is not generally included in N budget balance sheets in agricultural systems.

4 . PERSPECTIVES ON SOIL–ATMOSPHERE EXCHANGE OF GASEOUS

N COMPOUNDS

Reactive N species are highly mobile through the emissions mainly of NO

x

and

NH

3

via the atmosphere and release of nitrate into ground and surface waters. These

compounds may cross national boundaries and be deposited as NO

y

and NH

x

or

nitrate far from the source of N release ( Figure 4 and Figure 7 ) ( Aber et al., 1995 ;

Matson et al., 1997 ; Vitousek et al., 1997 ). Additional production of N

2

O from N

deposition and release is part of the anthropogenic N

2

O global budget ( IPCC 1997 ).

The redistribution of N from industrial, automobile, and agricultural sources has

and is having profound impacts on the quality of terrestrial and aquatic ecosystems

on our atmosphere. The recent estimates noted herein indicate that, globally, about

CH13-P374347.indd 458CH13-P374347.indd 458 5/31/2008 6:17:04 PM5/31/2008 6:17:04 PM

Exchange of Gaseous Nitrogen Compounds 459

150 Tg of biologically and chemically active forms of N are emitted to the atmos-

phere annually ( Smil, 1999 ). The emission of these compounds have increased

exponentially since the end of World War II and can be attributed most directly to

fossil fuel combustion, agricultural production, and biomass burning.

Deposition of N can stimulate productivity in N-limited grasslands, forests, and

aquatic systems ( Galloway et al., 1995 ; Holland et al., 1997 ). Excessive N deposi-

tion, however, can and has led to acidification and eutrophication of aquatic systems

( Aber et al., 1995 ) and to forest decline a decrease in plant species diversity and soil

acidification ( Ferm, 1998 ; Pitcairn et al., 1998 ). The NO

x

emissions also contribute

to the increase of tropospheric O

3

which is damaging crop production in some areas

of the globe ( Holland and Lamarque, 1997 ). Ozone in the troposphere is also a

greenhouse gas. Nitrogen deposition onto formerly pristine areas also contribute to

increased radiative forcing through increased N

2

O emissions ( Mosier et al., 1998 )

and decreased soil consumption of atmospheric CH

4

( Steudler et al., 1989 ). A large

portion of this N deposition occurs on agricultural lands (10–20 Tg annually) that

needs to be accounted for in fertilizer recommendations, an aspect of the agricul-

tural N cycle that has been neglected to date.

Because of the above characteristics of gaseous N emissions, we continue to be

faced with the challenge of limiting their emissions from food production systems

while sustaining the systems themselves. Technical advances are allowing acces-

sibility of field scale analytical devices to a larger number and variety of research

groups to permit larger scale assessment of NH

3

, NO

x

, and N

2

O emissions. A host

of such studies are needed to assess the impact of changes in management in crop

production such as tillage, crop varieties, fertilization timing, and livestock produc-

tion. Particularly interesting is the role of plants in the balance of NH

3

and NO

x

in

the atmosphere. Schjorring and Mattsson (2000) conclude that crops can represent

a significant input of NH

3

. Davidson and Kingerlee (1997) note that NO

x

emissions

from fertilized fields can be large but that the resorption of NO

2

by plant foliage

prevents an unknown and likely variable amount of NO

x

from exiting the plant can-

opy. All of the problems, environmental and economic, associated with the loss of

N from the soil and its redistribution, reinforces the need to more tightly couple the

crop and livestock parts of food production.

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463

Chapter 14. The Impacts of Nitrogen Deposition on

Forest Ecosystems

K.J. Nadelhoffer

Department of Ecology and Evolutionary Biology, University of Michigan,

Ann Arbor, MI 48109-1048, USA

Forest flora rely mainly on ammonium (NH

4



) released from decomposing lit-

ter and humus, and often on nitrate (NO

3



) produced by ammonium oxidizing bac-

teria ( “ nitrifiers ” ), to meet their N uptake demands ( Waring and Running, 1998 ).

Although some forest species are capable of taking up amino acids from soils

( Näsholm et al., 1998 ), most N assimilated by tree roots is inorganic. Forest lit-

ter and soil N pools are maintained, in turn, by organic inputs originating as plant

materials. The total amount of N in forest floor and soil pools typically exceeds the

amount in vegetation and is at least an order of magnitude greater than the amount

of N taken up annually by forest vegetation ( Johnson and Curtis, 2001 ). Low min-

eralization and turnover rates of soil and litter N pools combine with the pivotal

role of N as a component of photosynthetic and respiratory enzymes, to limit annual

primary productivity (plant biomass production per unit area) in forests. This is par-

ticularly true in temperate and boreal forests, but is less so in tropical forests where

primary production is often limited by other elements, often phosphorus ( Matson

et al., 1999 ). Therefore, primary production in most temperate and boreal forests is

considered “ N-limited ” at yearly time scales ( Vitousek and Howarth, 1991 ).

Forest litter and soils typically interact with vegetation to form “ closed ”

N cycles in which annual rates of plant N uptake per unit area approximately bal-

ance annual N returns to the forest floors and soil as litter and root material. This

closed pattern of N cycling contrasts with the more “ open ” cycles of agricultural

systems subjected to fertilizer N inputs ( Frink et al., 1999 ). It also differs from

the open cycles of many wetland, freshwater, and marine ecosystems in which

inputs from surrounding landscapes, biological N-fixation, or upwelling bot-

tom waters function to supply much of the N used to support primary production

( Vollenweider, 1968 ; Mitsch and Gosselink, 1986 ; Jahnke, 1990 ). Increased N dep-

osition on forests and other ecosystems with relatively closed cycles has the poten-

tial to drive forest N cycles to the more open states by increasing the importance of

N inputs and outputs relative to rates of internal N cycling.

The increases in N emission and deposition that have occurred in industrialized

regions of the Northern Hemisphere, and which are projected to continue into the

future across more of Earth ’ s surface ( Galloway et al., 1994, 1995 ; Holland et al.,

1999a ), could serve to diminish the degree of N limitation of forest growth, to alter

Nitrogen in the Environment: Sources, Problems, and Management

J.L. Hatfield and R.F. Follett (Eds)

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forest community composition, to change plant–soil interactions, and to open forest

N cycles. These responses to N deposition have implications not only for forests,

but also for the atmosphere and for ecosystems that receive nutrient outputs from

forests. This chapter summarizes research addressing the influences of N deposition

on forest N cycles and on forest ecosystem structure and function.

1 . NITROGEN INPUTS

Although N mineralized from decomposing litter and humus supplies most

N taken up by forest plants, forest N cycles are not completely closed. In mid-

successional and mature forests that are not subject to elevated atmospheric N dep-

osition or to recent physical disturbance, N inputs are small (  2–4 kg N/ha/year)

relative to the ⬃ 40 to 130 kg N/ha/year that cycles between vegetation and soils in

temperate and boreal forests ( Aber et al., 1985 ; Bonan and Van Cleve, 1991 ; Reich

et al., 1997 ). Longer growing seasons in tropical and subtropical regions can lead

to higher annual rates of N exchange between forest vegetation and soils ( Vitousek

and Matson, 1988 ) with little or no increase in N inputs (except where N-fixers

are active, below). Biological N-fixation (the enzymatic reduction of inert N

2

gas

to NH

4



) and atmospheric N deposition, although usually of minor importance

to plant N uptake on an annual basis under pristine conditions (no anthropogenic

N deposition), are the primary processes responsible for N accretion in forests.

1.1 . Biological N-Fixation

The presence of certain plant species (e.g., Robinia, Acacia, Alnus, Myrica )

capable of hosting N-fixing microbial symbionts ( Rhizobia, Frankia ) in root nod-

ules can increase N-fixation and the reliance of vegetation on this process to meet

N demands. Symbiotic N-fixation is of greater importance when host plant species

are abundant and when other elements (e.g., phosphorus, calcium, molybdenum)

required by N-fixers are in sufficient supply ( Liu and Deng, 1991 ; Binkley et al.,

1992 ; Högberg and Alexander, 1994 ). Nitrogen-fixation by free-living microbes in

soils and litter can also function to add N to forests, but generally at much lower

rates than are required for plant growth ( Johnson and Curtis, 2001 ). Nitrogen-

fixation, both by symbiotic and free-living organisms, is typically more important

early in primary succession than in mid- or late-succession, on N-deficient than more

N-rich soils, and in tropical and subtropical regions than in temperate and boreal

regions soils ( Vitousek and Sanford, 1986 ; Binkley et al., 1992 ; Zou et al., 1995 ).

1.2 . Nitrogen Deposition

Before the industrial age, rates of atmospheric N deposition on forests were

exceedingly low and contributed insignificantly to N uptake by vegetation ( Galloway

et al., 1982 ; Likens et al., 1987 ; Hedin et al., 1995 ). Most of these inputs were as

NO

3



(generated by lighting-driven N

2

oxidation) and particulate organic N that is

not immediately available to plants. Increases in fertilizer use, animal husbandry

and fossil fuel combustion, especially during the latter half of the 20th century,

464 Nitrogen in the Environment

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The Impacts of Nitrogen Deposition on Forest Ecosystems 465

have served increased emissions of NH

x

(  NH

3

and NH

4



) and NO

x

(  NO and

NO

2

). Transport and transformations of NH

x

and NO

x

in the atmosphere have led

to elevated deposition of biologically active N (mostly NH

x

and NO

3



) on forests,

particularly in populated temperate areas in eastern North America, central and

western Europe, and far eastern Asia ( Galloway et al., 1995 ; Holland et al., 1999a ).

Atmospheric N-deposition rates on forests vary widely across industrialized

regions in the North Temperate Zone. Total inorganic N deposition (wet plus dry)

ranges from about 3 to 32 kg N/ha/year in North American forests ( Ollinger et al.,

1993 ; Fenn et al., 1998 ) and from about 1 to 75 kg N/ha/year in Europe ( Dise and

Wright, 1995 ). The highest deposition rates typically occur near agricultural “ hot

spots ” of intensive fertilization or dense animal concentrations where ammonia vol-

atilization occurs, near point sources of NO

x

emission, and at high elevations sub-

ject to fog and cloud droplet formation ( van Breemen and van Dijk, 1988 ; Glatzel,

1990 ; Lovett and Kinsman, 1990 ; Bytnerowicz and Fenn, 1996 ). Interpolations of

N-deposition measurements across regions, such as by Ollinger et al. (1993) , illus-

trate the influences of elevation and distances from emission sources on the spatial

pattern of N deposition ( Figure 1 ).

Total N deposition

(kg/ha /year)

N

3.32 – 4.02

4.03 – 4.74

4.75 – 5.46

5.47 – 6.18

6.19 – 6.90

6.91 –7.62

7.63 – 8.34

8.35 – 9.06

9.07 – 9.78

9.79 – 10.50

10.51– 11.22

11.23 – 11.94

11.95 – 12.66

Figure 1 . Patterns of N deposition in the northeastern United States. Higher rates

occur near to sources of N emission in the Midwest and Mid-Atlantic regions

(from the southwest) and at higher elevations. Modiﬁ ed from Ollinger et al. (1993) .

Courtesy of S. Ollinger.

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

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