《森林生态学》课程教学资源（生态学热点）Issues in ecology - 15 环境中过量的氮 Excess Nitrogen in the U.S. Environment：Trends, Risks, and Solutions

ISSUES IN ECOLOGY Published by the Ecological Society of America Excess Nitrogen in the U.S. Environment:Trends,Risks, and Solutions Eric A.Davidson,Mark B.David,James N.Galloway,Christine L.Goodale, Richard Haeuber,John A.Harrison,Robert W.Howarth,Dan B.Jaynes, R.Richard Lowrance,B.Thomas Nolan,Jennifer L.Peel,Robert W.Pinder Ellen Porter,Clifford S.Snyder,Alan R.Townsend,and Mary H.Ward Winter 2012 Report Number 15 esa

esa Published by the Ecological Society of America esa Excess Nitrogen in the U.S. Environment: Trends, Risks, and Solutions Eric A. Davidson, Mark B. David, James N. Galloway, Christine L. Goodale, Richard Haeuber, John A. Harrison, Robert W. Howarth, Dan B. Jaynes, R. Richard Lowrance, B. Thomas Nolan, Jennifer L. Peel, Robert W. Pinder, Ellen Porter, Clifford S. Snyder, Alan R. Townsend, and Mary H. Ward Winter 2012 Report Number 15 Excess Nitrogen in the U.S. Environment: Trends, Risks, and Solutions Issues inin Ecology Ecology

ISSUES IN ECOLOGY NUMBER FIFTEEN WINTER 2012 Excess Nitrogen in the U.S.Environment: Trends,Risks,and Solutions SUMMARY 心ec地 We present ne cfmuch vable nitree the biophere.cluve.There buve been mporunt Intensive development of agriculture,industry,and transportation has profoundly altered the U.S.nitrogen cycle .Nitrogen emissions from the energy and transportation sectors are declining,but agricultural emissions are escapes its inended use and is o the environment. Impacts: gen)oncs along Air pollution continu to reduce biodiversiry.A nationwide vasive species. din well wa e infections are asso om sewage entering ecc Nmeinkt the carbon cyceand otefete cme. Mitigation Options: Regulation of nissions fron energy and tra rations are implemented. eincuet ptic Society faces profound challenges to meet demands for food.fiber.and fuel while minimizing unintended environmental and human health impacts.While our ability to quantify transfers of nitrogen acro land,water,and air has improved since the first The Ecological Society of America.esahg@esa.org esa 1

© The Ecological Society of America • esahq@esa.org esa 1 ISSUES IN ECOLOGY NUMBER FIFTEEN WINTER 2012 Excess Nitrogen in the U.S. Environment: Trends, Risks, and Solutions SUMMARY I t is not surprising that humans have profoundly altered the global nitrogen (N) cycle in an effort to feed 7 billion people, because nitrogen is an essential plant and animal nutrient. Food and energy production from agriculture, combined with indus￾trial and energy sources, have more than doubled the amount of reactive nitrogen circulating annually on land. Humanity has dis￾rupted the nitrogen cycle even more than the carbon (C) cycle. We present new research results showing widespread effects on ecosystems, biodiversity, human health, and climate, suggesting that in spite of decades of research quantifying the negative con￾sequences of too much available nitrogen in the biosphere, solutions remain elusive. There have been important successes in reducing nitrogen emissions to the atmosphere and this has improved air quality. Effective solutions for reducing nitrogen losses from agriculture have also been identified, although political and economic impediments to their adoption remain. Here, we focus on the major sources of reactive nitrogen for the United States (U.S.), their impacts, and potential mitigation options: Sources: • Intensive development of agriculture, industry, and transportation has profoundly altered the U.S. nitrogen cycle. • Nitrogen emissions from the energy and transportation sectors are declining, but agricultural emissions are increasing. • Approximately half of all nitrogen applied to boost agricultural production escapes its intended use and is lost to the environment. Impacts: •Two-thirds of U.S. coastal systems are moderately to severely impaired due to nutrient loading; there are now approximately 300 hypoxic (low oxygen) zones along the U.S. coastline and the number is growing. One third of U.S. streams and two fifths of U.S. lakes are impaired by high nitrogen concentrations. • Air pollution continues to reduce biodiversity. A nation-wide assessment has documented losses of nitrogen￾sensitive native species in favor of exotic, invasive species. • More than 1.5 million Americans drink well water contaminated with too much (or close to too much) nitrate (a regulated drinking water pollutant), potentially placing them at increased risk of birth defects and cancer. More research is needed to deepen understanding of these health risks. • Several pathogenic infections, including coral diseases, bird die-offs, fish diseases, and human diarrheal diseases and vector-borne infections are associated with nutrient losses from agriculture and from sewage entering eco￾systems. • Nitrogen is intimately linked with the carbon cycle and has both warming and cooling effects on the climate. Mitigation Options: •Regulation of nitrogen oxide (NOX) emissions from energy and transportation sectors has greatly improved air quality, especially in the eastern U.S. Nitrogen oxide is expected to decline further as stronger regulations take effect, but ammonia remains mostly unregulated and is expected to increase unless better controls on ammonia emissions from livestock operations are implemented. • Nitrogen loss from farm and livestock operations can be reduced 30-50% using current practices and technologies and up to 70-90% with innovative applications of existing methods. Current U.S. agricultural policies and sup￾port systems, as well as declining investments in agricultural extension, impede the adoption of these practices. Society faces profound challenges to meet demands for food, fiber, and fuel while minimizing unintended environmental and human health impacts. While our ability to quantify transfers of nitrogen across land, water, and air has improved since the first publication of this series in 1997, an even bigger challenge remains: using the science for effective management policies that reduce climate change, improve water quality, and protect human and environmental health. Cover photo credit: Nitrogen deposition at the Joshua Tree National Park in California has increased the abundance of exotic grasses, which are more prone to fire than native vegetation. The upper photo shows a site dominated by exotic annual grasses five years after a burn, and the lower shows a site immediately post-burn. Photos courtesy of Edith Allen

ISSUES IN ECOLOGY NUMBER FIFTEEN WINTER 2012 Excess Nitrogen in the U.S.Environment: Trends,Risks,and Solutions Eric A.Davidson,Mark B.David,James N.Galloway,Christine L.Goodale,Richard Haeuber, John A.Harrison,Robert W.Howarth,Dan B.Jaynes,R.Richard Lowrance,B.Thomas Nolan, Jennifer L.Peel,Robert W.Pinder,Ellen Porter,Clifford S.Snyder,Alan R.Townsend,and Mary H.Ward Introduction liCsgtrtoatrolutionhavecontnue bated Thanks largely to the early 20th century by unanticipared new demands for biofuel invention of synthetically manufactured nitro crops,which created further demand for agri- gen (N)fer human popula nd inputs.Yet ever before in human history.About o Significant air quality imp ovements are the dnitrcgcnem in agricultural productivity and human nutri oped countries.The amount of nitrogen inar izer applied to cropland-often over halfis mated.Progress has ben made on impro manag ame time,energy,transportation,and indus. Evidence of the links between excess reactive rial secors also emit nitrogen pollut nitrogen n ne environment and specifi ibed the magnitude.causes.and con quences of these he nitrogen cycle rogress in reducing nit ogen pollution is tha ation in the restria se tre apply ther :L ving human the mpacts are often felt locally.and the o I use of fert major sources of reactive nitro nitrous oxide,and increa uatic and terrestrial habitats. Fifteer cen losses and impacts we now a The maior anthr css in finding solutions "no For the the com criencing eutrophication and hypoxia (low rimes larger than natural sources of inputs from oxygen waters)has grown,and biodiversity biological nitrogen fixation(see Glossary for 2 esa The Ecological Society of America esahq@esa.org

2 esa © The Ecological Society of America • esahq@esa.org Introduction Thanks largely to the early 20th century invention of synthetically manufactured nitro￾gen (N) fertilizers, the growing human popula￾tion is, on average, better nourished now than ever before in human history. About 40 to 60% of the current human population depends upon crops grown with synthetic nitrogen fer￾tilizer. Unfortunately, this impressive advance in agricultural productivity and human nutri￾tion has come at a high price of environmen￾tal degradation and human health risks from pollution. A large fraction of nitrogen fertil￾izer applied to cropland – often over half – is not used by the crops and is lost to air, water, and downstream and downwind habitats, pol￾luting landscapes and waterscapes. At the same time, energy, transportation, and indus￾trial sectors also emit nitrogen pollution into the air through increasing use of fossil fuels. In 1997, the first Issue in Ecology described the magnitude, causes, and consequences of these human alterations of the nitrogen cycle, docu￾menting how humans have more than doubled the amount of reactive nitrogen (see Glossary for definitions) annually in circulation in the terrestrial biosphere. Several of these trends have continued along with increasing num￾bers of people, including improving human diets in the developing world, increasing global use of fertilizers, increasing atmospheric concentrations of the potent greenhouse gas nitrous oxide, and increasing eutrophication of aquatic and terrestrial habitats. Fifteen years later, we now ask: “Has scientific aware￾ness of the growing problems of nitrogen pol￾lution fostered progress in finding solutions?” In some respects, the answer is a disappoint￾ing “no.” Atmospheric nitrous oxide is still increasing, the number of aquatic ecosystems experiencing eutrophication and hypoxia (low oxygen waters) has grown, and biodiversity losses due to air pollution have continued. Indeed, these problems have been exacerbated by unanticipated new demands for biofuel crops, which created further demand for agri￾cultural expansion and fertilizer inputs. Yet there have been important success stories. Significant air quality improvements are the result of regulations and technological innova￾tions that have reduced nitrogen emissions from industry and automobiles in many devel￾oped countries. The amount of nitrogen in air pollution that some ecosystems can sustain (the “critical load”) without significant loss in diversity or ecosystem function has been esti￾mated. Progress has also been made on improv￾ing the efficiency of fertilizer use and on iden￾tifying effective management options to reduce nitrogen losses from agricultural lands. Evidence of the links between excess reactive nitrogen in the environment and specific human health outcomes is growing, providing compelling motivation for pollution abate￾ment. Perhaps the most encouraging aspect of progress in reducing nitrogen pollution is that technological solutions do exist. Research is needed to reduce costs of these solutions, and better communication is needed to foster the cultural and political will to apply them. While the nitrogen cycle disruption is global, the impacts are often felt locally, and the solu￾tions are region-specific. Here, we focus on the major sources of reactive nitrogen for the U.S., their impacts on ecosystems, climate, and human health, and options to minimize nitro￾gen losses and impacts. The Major Anthropogenic Sources Of Reactive Nitrogen In The U.S. For the U.S., the combined anthropogenic sources of reactive nitrogen are about four times larger than natural sources of inputs from biological nitrogen fixation (see Glossary for Excess Nitrogen in the U.S. Environment: Trends, Risks, and Solutions Eric A. Davidson, Mark B. David, James N. Galloway, Christine L. Goodale, Richard Haeuber, John A. Harrison, Robert W. Howarth, Dan B. Jaynes, R. Richard Lowrance, B. Thomas Nolan, Jennifer L. Peel, Robert W. Pinder, Ellen Porter, Clifford S. Snyder, Alan R. Townsend, and Mary H. Ward ISSUES IN ECOLOGY NUMBER FIFTEEN WINTER 2012

ISSUES IN ECOLOGY NUMBER FIFTEEN WINTER 2012 definitions)in native ecosystems and from Movement and redistribution of reactive nitrogen in land,water,and air. the global average.While nitrogen fertilizer use no which osphere poN further NH.NO. fixed reactive nitrogen shown in Table 1,the ure 1.The most important transfers and redistributions of reactive nitrogen among human health.A 2011 EPA report (sce sugge Table 1 Estimates of the maio os of naturel and anthr IMPACTS ON ECOSYSTEMS pogenic N inputs to the United States in 1990 and 2008 and iections for 2014 Research during the last few decades has led to nimproved of the US Nitrogen Sources 1990 2008 2014 Projection Millions of metric tons N per year Natural sources Lightning Biological Nfixation 8 84 0.1 6.4 Agriculture lances Many native e hard Synthetic N fertilizer Crop biological N fixation Food imports 02 0.2 0.2 severa combustion Conifer r Industrial NO. 15 tation No 3.7 26 2.0 gen n NO 1.8 8 0.6 ndustrial uses 4.2 4.2 4.2 ceptible to changes in species composition due TOTAL 34.0 35.2 35.8 and the eason is short,for example in the we ne is nimrion The Ecological Society of Americaesahq@esa.org esa 3

© The Ecological Society of America • esahq@esa.org esa 3 ISSUES IN ECOLOGY NUMBER FIFTEEN WINTER 2012 definitions) in native ecosystems and from lightning (Table 1). Because of intensive agri￾cultural and industrial development, the alter￾ation of the U.S. nitrogen cycle is greater than the global average. While nitrogen fertilizer use is growing in emerging-market regions such as Asia, it has nearly leveled off in the U.S. Soybean production has been increasing, which increases biological nitrogen fixation in crop￾lands. Nitrogen oxide (NOX) emissions have declined and are expected to decline further. In addition to the annual inputs of newly fixed reactive nitrogen shown in Table 1, the redistributions and transfers of nitrogen across landscape components are also important (Figure 1). These include ammonia, nitrogen oxides and nitrous oxide emissions from soils to the atmosphere, leaching of nitrate and dis￾solved organic nitrogen from land to water, food harvests, and sewage disposal. This movement of reactive nitrogen into air, water, and non￾agricultural land leads to unintended, mostly undesirable consequences for ecosystem and human health. A 2011 EPA report (see sugges￾tions for further reading) describes these esti￾mates in more detail. Our emphasis here will be in describing and quantifying impacts on human and ecosystem health and potential solutions. IMPACTS ON ECOSYSTEMS Research during the last few decades has led to an improved understanding of the relationship between nitrogen inputs and nitrogen demand by plant communities. Nitrogen generally enhances the growth of plants, but plant species differ in their ability to respond to increased nitrogen, due to variation in their inherent growth rates and their responses to other associ￾ated changes, such as acidification and nutrient imbalances. Many native hardwood tree species in the eastern U.S., such as red and sugar maple, white ash, black cherry, tulip poplar, and red oak, respond positively to nitrogen deposition from air pollution, whereas beech and several birch and oak species show no growth response. Conifer responses are mixed, with several species, particularly red pine, showing reduced growth with increasing nitrogen deposition (Figure 2). In addition to effects on trees, the understory vegetation is often particularly sus￾ceptible to changes in species composition due to increasing nitrogen deposition. Trees grow slowly where soils are thin and the growing season is short, for example in the Rocky Mountains. Field research has shown that much less nitrogen deposition is needed to satu￾Figure 1. The most important transfers and redistributions of reactive nitrogen among landscapes and waterscapes (see Glossary for abbreviations). Biological nitrogen fixation, denitrification, and a few minor transfers are omitted for visual simplicity. Table 1. Estimates of the major sources of natural and anthro￾pogenic N inputs to the United States in 1990 and 2008 and projections for 2014. U.S. Nitrogen Sources 1990 2008 2014 Projection Millions of metric tons N per year Natural sources Lightning 0.1 0.1 0.1 Biological N fixation 6.4 6.4 6.4 Agriculture Synthetic N fertilizer 9.7 11.4 11.9 Crop biological N fixation 5.4 8.3 9.1 Food imports 0.2 0.2 0.2 Combustion Industrial NOX 1.5 1.1 1.1 Transportation NOX 3.7 2.6 2.0 Electric generation NOX 1.8 0.8 0.6 Industrial uses* 4.2 4.2 4.2 TOTAL 34.0 35.2 35.8 *Industrial uses of synthetic reactive N include nylon production and munitions. The only estimate available is for 2002, which we assume is constant for this time period for lack of better data. Sources include EPA reports and datasets (EPA-SAB-11-013, EPA-HQ-OAR-2009-0491, http://www.epa.gov/ttnchie1/trends/) and the International Fertilizer Industry Association

ISSUES IN ECOLOGY NUMBER FIFTEEN WINTER 2012 品e d pine(right),s ons al 2070.Nature Geo 13-7 rate the plant demand in high elevation forests and to many fish and other fauna in streams Much of and lakesAc stressing plant.Air polluted with nitro n i often accompanied by oone pollution,which uppresses p hesis ger rolerate these deposition calulated for each type sses from air pollution.Species-specific (ee Box 1). n a pperties.Both nitrogen and sulfur from air ies,many of which are non-native,respond and alters the availability of pho horus.Soil ke alu- cal popula Critical load exceedance maps of mixed conifer forests in California nitrog s of lichen is indicat al Mar b kg Nha'yr'■N<31■31sN<52■Na52 kgNha'yr■N<17■N7 can be found 4 esa The Ecological Society of America esahq@esa.org

ISSUES IN ECOLOGY NUMBER FIFTEEN WINTER 2012 rate the plant demand in high elevation forests compared to low elevation forests. Much of the nitrogen that is not taken up by the plants then enters streams, groundwater, and lakes, where it affects algal productivity and aquatic food webs. This type of research throughout the country is leading to estimates of “critical loads” of nitrogen deposition calculated for each ecosystem type and location (see Box 1). In addition to supplying an essential plant nutrient, nitrogen deposition also affects soil properties. Both nitrogen and sulfur from air pollution contribute to the acidification of soils, which leads to the loss of essential plant nutrients, such as calcium and magnesium, and alters the availability of phosphorus. Soil acidification mobilizes elements like alu￾minum, which is toxic to many plants on land and to many fish and other fauna in streams and lakes. Acidification of soils can increase forest susceptibility to disease and drought by stressing plants. Air polluted with nitrogen is often accompanied by ozone pollution, which suppresses plant photosynthesis. Species vary in their ability to tolerate these stresses from air pollution. Species-specific responses to elevated nitrogen deposition have reduced the diversity of terrestrial and aquatic ecosystems. In general, fast growing, “weedy” species, many of which are non-native, respond quickly and positively to increased nitrogen deposition, whereas slow-growing native species that are adapted to naturally low levels of nitro￾gen are less able to use the additional nitrogen. The differing responses can drive local popula￾tions of rare, slow-growing, native plant species 4 esa © The Ecological Society of America • esahq@esa.org Figure 2. Nitrogen deposition from air pollution increases the growth of many hardwood tree species, such as the red maple (left). Some conifers, such as red pine (right), show a decreased growth rate. Responses of the 24 most common species in the eastern U.S. can be found in Thomas et al. 2010. Nature Geoscience, 3:13-17. Figure 3. In California, airborne nitrogen is impacting one third of the state’s natural land areas. Lichens and stream nitrate concentrations have been used as effective indicators of undesirable changes in ecosystems. Areas shaded in red indicate conifer forests at risk because inputs from nitrogen in air pollution are exceeding the estimated critical load, either because (a) the species of lichen is expected to change or (b) nitrate in stream water is expected to exceed an established threshold value. Green shading indicates areas where pollution inputs are less than the critical loads. Redrawn from Fenn et al. (2010. Journal of Environmental Management 91:2404-2423), where critical load exceedance maps for additional California ecosystems can be found