Issues in Ecology Number Spring 1997 year of nitrogen:the draining of wetlands.which sets all of the more than 20 Tg per year of fixed nitroge the idation of so organic matter that released in bile exha in emission nitrogen:and land from foss burning is emitted to o the atmosphere as clearing for crops,which could mobilize 20 Tg per year nitric oxide.Other activities indirectly enhance emissions from soils. to the atmosphere.Intensive fertilization of agricultural There are substantial scientific uncertainties about soils can increase the rates at which nitrogen in the form both the quantity and the fate of nitrogen mobilized by of ammonia is volatilized and lost to the air.It can also such activities.Taken together.however.they could con changes in the enha ncin the Ever cycle. runmanaged lands downwind of agri industrial areas,rain or windborne deposition of human Human Versus Natural Nitrogen Fixation generated nitrogen can spur increased emissions of ni Overall,fertilizer production.legume crops.and trogen gases from the soils. fossil fuel burning deposit approximately 140 Tg of new aogcnintolhndbasedecosystemseacthyear'afigure Nitrous Oxide that quals the pe rog n fixed na Nitrous xide isa very heat-trapping rally by organisms se ecosystem Other huma gas in the nosphere.in part because it absorbs activities liberate and make available half again that much ing radiant heat from the Earth in infrared wavelengths nitrogen.From this evidence,it is fair to conclude that that are not captured by the other major greenhouse human activities have at least doubled the transfer of gases,water vapor and carbon dioxide.By absorbing nitrogen from the atmosphere into the land-based bio and reradiating this heat back toward the farth.nitrous oxide contributes a few percent to overall greenhouse trogen is spread unev nly across the warmin Earth's surface: me area n as northe ern Europe are Although nitro oxide is unread tive and long lived in the lower atmosphere,when it rises into the strato gions in the Southern Hemisphere receive little direct in- sphere it can trigger reactions that deplete and thin the put of human-generated nitrogen.Yet no region remains stratospheric ozone layer that shields the Earth from unaffected.The increase in fixed nitrogen circulating damaging ultraviolet radiation. The concentration of nitrous oxide in the atmo ,even in cores drilled sphere e is curre ntly i ncr reasing at the rate of to th the glacia ice nd tenths of a percent per yea that d,the sources of the increase remain unre IMPACTS ON THE ATMOSPHERE solved.Both fossil fuel burning and the direct impacts of agricultural fertilization have been considered and re One major consequence of human-driven alter. jected as the major source.Rather.there is a developins ations in the nitrogen cycle has been regionl and global consensus that a wide array of human-driven sources change in the che of the atmos ntribute ically to ich the te strial nitr specifically.increased ons gen cycle. “disper rsed sources incl de gases such as nitrous oxide,nitric oxide,and ammonia nitrogen-enriched ground ater,nitrogen-saturated for (NH,).Although such releases have received less atten ests,forest burning.land clearing.and even the manu tion than increased emissions of carbon dioxide and vari- facture of nylon.nitric acid.and other industrial prod ous sulfur compounds.the trace nitrogen gases cause ucts. environmental effects both while airbome and after they The net effect is increased global concentrations ited on theground.For instan of a poter that also contributes to the long-lived in t e atmosp ere and ozone layer human-driven enhancement of the greenhouse effect that likely warms the Earth's climate.Nitric oxide is an im- Nitric Oxide and Ammonia portant precursor of acid rain and photochemical smog. Unlike nitrous oxide.which is unreactive in the Some of the human activities discussed above lower atmosphere,both nitric oxide and ammonia are affect the atmosphere directly.For instance,essentially highly reactive and therefore much shorter lived.Thus
year of nitrogen; the draining of wetlands, which sets the stage for oxidation of soil organic matter that could mobilize 10 Tg per year or more of nitrogen; and land clearing for crops, which could mobilize 20 Tg per year from soils. There are substantial scientific uncertainties about both the quantity and the fate of nitrogen mobilized by such activities. Taken together, however, they could contribute significantly to changes in the global nitrogen cycle. Human Versus Natural Nitrogen Fixation Overall, fertilizer production, legume crops, and fossil fuel burning deposit approximately 140 Tg of new nitrogen into land-based ecosystems each year, a figure that equals the upper estimates for nitrogen fixed naturally by organisms in these ecosystems. Other human activities liberate and make available half again that much nitrogen. From this evidence, it is fair to conclude that human activities have at least doubled the transfer of nitrogen from the atmosphere into the land-based biological nitrogen cycle. This extra nitrogen is spread unevenly across the Earths surface: Some areas such as northern Europe are being altered profoundly while others such as remote regions in the Southern Hemisphere receive little direct input of human-generated nitrogen. Yet no region remains unaffected. The increase in fixed nitrogen circulating around the globe and falling to the ground as wet or dry deposition is readily detectable, even in cores drilled from the glacial ice of Greenland. IMPACTS ON THE ATMOSPHERE One major consequence of human-driven alterations in the nitrogen cycle has been regional and global change in the chemistry of the atmosphere (Figure 4) specifically, increased emissions of nitrogen-based trace gases such as nitrous oxide, nitric oxide, and ammonia (NH3). Although such releases have received less attention than increased emissions of carbon dioxide and various sulfur compounds, the trace nitrogen gases cause environmental effects both while airborne and after they are deposited on the ground. For instance, nitrous oxide is long-lived in the atmosphere and contributes to the human-driven enhancement of the greenhouse effect that likely warms the Earths climate. Nitric oxide is an important precursor of acid rain and photochemical smog. Some of the human activities discussed above affect the atmosphere directly. For instance, essentially all of the more than 20 Tg per year of fixed nitrogen released in automobile exhausts and in other emissions from fossil fuel burning is emitted to the atmosphere as nitric oxide. Other activities indirectly enhance emissions to the atmosphere. Intensive fertilization of agricultural soils can increase the rates at which nitrogen in the form of ammonia is volatilized and lost to the air. It can also speed the microbial breakdown of ammonium and nitrates in the soil, enhancing the release of nitrous oxide. Even in wild or unmanaged lands downwind of agricultural or industrial areas, rain or windborne deposition of humangenerated nitrogen can spur increased emissions of nitrogen gases from the soils. Nitrous Oxide Nitrous oxide is a very effective heat-trapping gas in the atmosphere, in part because it absorbs outgoing radiant heat from the Earth in infrared wavelengths that are not captured by the other major greenhouse gases, water vapor and carbon dioxide. By absorbing and reradiating this heat back toward the Earth, nitrous oxide contributes a few percent to overall greenhouse warming. Although nitrous oxide is unreactive and longlived in the lower atmosphere, when it rises into the stratosphere it can trigger reactions that deplete and thin the stratospheric ozone layer that shields the Earth from damaging ultraviolet radiation. The concentration of nitrous oxide in the atmosphere is currently increasing at the rate of two- to threetenths of a percent per year. While that rise is clearly documented, the sources of the increase remain unresolved. Both fossil fuel burning and the direct impacts of agricultural fertilization have been considered and rejected as the major source. Rather, there is a developing consensus that a wide array of human-driven sources contribute systematically to enrich the terrestrial nitrogen cycle. These dispersed sources include fertilizers, nitrogen-enriched groundwater, nitrogen-saturated forests, forest burning, land clearing, and even the manufacture of nylon, nitric acid, and other industrial products. The net effect is increased global concentrations of a potent greenhouse gas that also contributes to the thinning of the stratospheric ozone layer. Nitric Oxide and Ammonia Unlike nitrous oxide, which is unreactive in the lower atmosphere, both nitric oxide and ammonia are highly reactive and therefore much shorter lived. Thus 6 Issues in Ecology Number 1 Spring 1997
Issues in Ecology Number Spring 1997 Human-Caused Global Nitrogen Emissions 100% Figure 4-Human for large f trogen-containing trace gases,including 40% of the nitrous oxide,80%or more of nitric ox- ide.and 70%of ammonia releases.the result is increasing atmospheric concentrations of the og,andofbiologicalyavailabte from the atm sphere to fertil ize ecosystems Ammonia data are from Schlesinger and Hartley (1992).nitric oxide from Delmas et al.(in press).and nitrous oxide from Prather et al.(1995). (NH) (NO) (N.O) changes in their atmospheric concentrations can be de- EFFECTS ON THE CARBON CYCLE tected only at local or regional scales. Nitric oxide plays several critical roles in atmo- Increased emissions of airborne nitrogen have led spheric chemistry,including catalyzing the formation of to enhanced deposition of nitrogen on land and in the photochemical (or brown)smog.In the presence of sun oceans.Thanks to the fertilizer effects of nitrogen in light,nitric oxide and react with hydrocarb may be e.the most atmospherei dir ty by altering the g bal carbon cycle. serious detrimental effects on human health as well as Over much of the Earth's surface,the lushness the health and productivity of crops and forests. of plant growth and the accumulation of standing stocks Nitric oxide.along with other oxides of nitrogen of plant material historically have been limited by scanty and sulfur,can be transformed in the atmosphere into nitrogen supplies,particularly in temperate and boreal aio compo regions.Hur an activity has substa tially ased the depositio a.whichraisc Although a number of sources contribute toni mroHow mro tric oxide emissions,combustion is the dominant one been caused by human-generated nitrogen additions?As Fossil fuel burning emits more than 20 Tg per year of a result.how much extra carbon has been stored in ter nitric oxide.Human burning of forests and other plant restrial ecosystems rather than contributing to the rising material may add about 10 Tg,and global emissions of concentrations of carbon dioxide in the atmosphere? nitric oxide from soils.a substantial fraction of which are Answers to these questions could help explain ,year al,80 the imbalance i in the car。 that has co e to be percent o more of nitric ide emr known as the ing sink. know generated by human activities.and in many regions the carbon dioxide from human activities such as fossil fue result is increased smog and acid rain. burning and deforestation exceed by more than 1,00C In contrast to nitric oxide,ammonia acts as the Tg the amount of carbon dioxide known to be accumu primary acid-neutralizing agent in the atmosphere.hav lating in the atmosphere each year.Could increased ing an opposite influence on the acidity of aerosols growth rates in terrestrial vegetation be thesinktha and rainfall.Nearly 70 per accounts for the fate of much oftha mis ammonia emissions are human-caused Experiments in Europe and America indicate tha tilized from fertilized fields contributes an estimated 1 a large portion of the extra nitrogen retained by forest Tg per year:ammonia released from domestic animal wetland,and tundra ecosystems stimulates carbon up wastes about 32 Tg:and forest burning some 5 Tg. take and storage.On the other hand,this nitrogen can
EFFECTS ON THE CARBON CYCLE Increased emissions of airborne nitrogen have led to enhanced deposition of nitrogen on land and in the oceans. Thanks to the fertilizer effects of nitrogen in stimulating plant growth, this deposition may be acting to influence the atmosphere indirectly by altering the global carbon cycle. Over much of the Earths surface, the lushness of plant growth and the accumulation of standing stocks of plant material historically have been limited by scanty nitrogen supplies, particularly in temperate and boreal regions. Human activity has substantially increased the deposition of nitrogen over much of this area, which raises important questions: How much extra plant growth has been caused by human-generated nitrogen additions? As a result, how much extra carbon has been stored in terrestrial ecosystems rather than contributing to the rising concentrations of carbon dioxide in the atmosphere? Answers to these questions could help explain the imbalance in the carbon cycle that has come to be known as the missing sink. The known emissions of carbon dioxide from human activities such as fossil fuel burning and deforestation exceed by more than 1,000 Tg the amount of carbon dioxide known to be accumulating in the atmosphere each year. Could increased growth rates in terrestrial vegetation be the sink that accounts for the fate of much of that missing carbon? Experiments in Europe and America indicate that a large portion of the extra nitrogen retained by forest, wetland, and tundra ecosystems stimulates carbon uptake and storage. On the other hand, this nitrogen can changes in their atmospheric concentrations can be detected only at local or regional scales. Nitric oxide plays several critical roles in atmospheric chemistry, including catalyzing the formation of photochemical (or brown) smog. In the presence of sunlight, nitric oxide and oxygen react with hydrocarbons emitted by automobile exhausts to form ozone, the most dangerous component of smog. Ground-level ozone has serious detrimental effects on human health as well as the health and productivity of crops and forests. Nitric oxide, along with other oxides of nitrogen and sulfur, can be transformed in the atmosphere into nitric acid and sulfuric acid, which are the major components of acid rain. Although a number of sources contribute to nitric oxide emissions, combustion is the dominant one. Fossil fuel burning emits more than 20 Tg per year of nitric oxide. Human burning of forests and other plant material may add about 10 Tg, and global emissions of nitric oxide from soils, a substantial fraction of which are human-caused, total 5 to 20 Tg per year. Overall, 80 percent or more of nitric oxide emissions worldwide are generated by human activities, and in many regions the result is increased smog and acid rain. In contrast to nitric oxide, ammonia acts as the primary acid-neutralizing agent in the atmosphere, having an opposite influence on the acidity of aerosols, cloudwater, and rainfall. Nearly 70 percent of global ammonia emissions are human-caused. Ammonia volatilized from fertilized fields contributes an estimated 10 Tg per year; ammonia released from domestic animal wastes about 32 Tg; and forest burning some 5 Tg. 7 Figure 4-Human activities are responsible for a large proportion of the global emissions of nitrogen-containing trace gases, including 40% of the nitrous oxide, 80% or more of nitric oxide, and 70% of ammonia releases. The result is increasing atmospheric concentrations of the greenhouse gas nitrous oxide, of the nitrogen precursors of smog, and of biologically available nitrogen that falls from the atmosphere to fertilize ecosystems. Ammonia data are from Schlesinger and Hartley (1992), nitric oxide from Delmas et al. (in press), and nitrous oxide from Prather et al. (1995). Issues in Ecology Number 1 Spring 1997 Ammonia (NH )3 Nitric oxide (NO) Nitrous oxide (N O) 2 20% 40% 60% 80% 100% Human-Caused Global Nitrogen Emissions