Abstract  Atmospheric nitrogen (N) deposition is it recognized threat to plant diversity ill temperate and northern parts of Europe and North America. This paper assesses evidence from field experiments for N deposition effects and thresholds for terrestrial plant diversity protection across a latitudinal range of main categories of ecosystems. from arctic and boreal systems to tropical forests. Current thinking on the mechanisms of N deposition effects on plant diversity, the global distribution of G200 ecoregions, and current and future (2030) estimates of atmospheric N-deposition rates are then used to identify the risks to plant diversity in all major ecosystem types now and in the future. This synthesis paper clearly shows that N accumulation is the main driver of changes to species composition across the whole range of different ecosystem types by driving the competitive interactions that lead to composition change and/or making conditions unfavorable for some species. Other effects such its direct toxicity of nitrogen gases and aerosols long-term negative effects of increased ammonium and ammonia availability, soil-mediated effects of acidification, and secondary stress and disturbance are more ecosystem, and site-specific and often play a supporting role. N deposition effects in mediterranean ecosystems have now been identified, leading to a first estimate of an effect threshold. Importantly, ecosystems thought of as not N limited, such as tropical and subtropical systems, may be more vulnerable in the regeneration phase. in situations where heterogeneity in N availability is reduced by atmospheric N deposition, on sandy soils, or in montane areas. Critical loads are effect thresholds for N deposition. and the critical load concept has helped European governments make progress toward reducing N loads on sensitive ecosystems. More needs to be done in Europe and North America. especially for the more sensitive ecosystem types. including several ecosystems of high conservation importance. The results of this assessment Show that the Vulnerable regions outside Europe and North America which have not received enough attention are ecoregions in eastern and Southern Asia (China, India), an important part of the mediterranean ecoregion (California, southern Europe). and in the coming decades several subtropical and tropical parts of Latin America and Africa. Reductions in plant diversity by increased atmospheric N deposition may be more widespread than first thought, and more targeted Studies are required in low background areas, especially in the G200 ecoregions.

Abstract  Nitric acid (HNO3) and ozone (O3), secondary products of photochemical reactions of nitrogen oxides (NOx) and volatile organic compounds, are important pollutants in arid regions with large outputs from petrol combustion. In the Los Angeles (LA) air basin, nitrogen dry deposition rates in forests downwind of the urban areas can reach 35-40 kg ha-1 year-1, roughly equivalent to the amount of N used to fertilize agricultural fields. The marked decline in the lichen population of the LA air basin has previously been attributed to local O3 concentration gradients, which overlaid the patterns of species extirpation. Recent research in the air basin has shown that nitrate (NO3-) deposition gradients run parallel to the O3 concentration gradient, and that deposition of NO3- and HNO3 can have significant effects on forest health. Our research examines the effects of HNO3 dry deposition on the lichen Ramalina menziesii Tayl. in an effort to understand the loss of lichen species in southern California, and increase the usefulness of lichens as biomonitors of nitrogen pollutants. We transplanted healthy R. menziesii thalli from a "pristine" location into fumigation chambers and exposed them to HNO3 under humid and dry conditions, and moderate and high HNO3 fumigations. R. menziesii thalli treated with HNO3 in month-long fumigations experienced a significant decline in chlorophyll content and carbon exchange capacity compared to thalli in control chambers. Leachate conductivity, NO3- and K+ concentrations increased with HNO3 fumigation levels and time. We conclude that R. menziesii has an unequivocally negative response to HNO3 gas concentrations common to ambient summer conditions in the LA air basin.

Abstract  The possibility that different species assemblages may represent persistent alternative community states remains largely unexplored by experimental ecologists because of a variety of conceptual and experimental problems. We discuss some of the conceptual roadblocks to experimentation and propose several avenues for attacking the problem experimentally. We address the conceptual issues involved in (1) the blurring of the distinction between the processes that initiate the switch among alternative states and the positive-feedback processes that maintain those states, and (2) the role of spatial scale in initiating the switch. We suggest that the switch between alternative states requires, first, a disturbance that removes species involved in the positive feedbacks needed for maintenance and, second, the arrival of other individuals that initiate the switch to the alternative assemblage. The removal of the species that maintain the system must be large enough and over a long enough time to allow the arrival and establishment of members of the alternative assemblage, and so we hypothesize that the switch among alternative states is scale dependent. This scenario suggests that the switch among alternative states can be investigated experimentally through the manipulation of the scale of the disturbance and of the arrival of members of the alternative state. Small-scale disturbances should consistently fail to initiate a switch, while larger-scale events should initiate a switch at least part of the time. We also note that in some cases the scale of disturbance and/or the arrival of recruits cannot be manipulated or controlled and suggest that several approaches other than factorial experiments with ANOVA, such as spatial autocorrelation methods, may be useful. We illustrate the potential and the difficulties of various approaches by discussing two systems in eastern North America that may contain alternative states. Mosaics of mussel beds and algal beds occupy rocky coasts from New England northward, and patchworks of forests and heathlands occur in eastern Canada and in the Appalachian highlands. While the study of alternative states in the marine system can be approached experimentally, the scale of disturbance required to switch forests to heathlands is too large for experimentation and must rely on the use of other approaches.

Abstract  Discussions and applications of the policies and practices of the U.S. Environmental Protection Agency (USEPA) in ecological risk assessment will benefit from continued clarification of the concepts of assessment endpoints and of levels of biological organization. First, assessment endpoint entities and attributes can be defined at different levels of organization. Hence, an organism-level attribute, such as growth or survival, can be applied collectively to a population-level entity such as the brook trout in a stream. Second, assessment endpoints for ecological risk assessment are often mistakenly described as “individual level,” which leads to the idea that such assessments are intended to protect individuals. Finally, populations play a more important role in risk assessments than is generally recognized. Organism-level attributes are used primarily for population-level assessments. In addition, the USEPA and other agencies already are basing management decisions on population or community entities and attributes such as production of fisheries, abundance of migratory bird populations, and aquatic community composition.

Abstract  This article describes the governing equations, computational algorithms, and other components entering into the Community Multiscale Air Quality (CMAQ) modeling system. This system has been designed to approach air quality as a whole by including state-of-the-science capabilities for modeling multiple air quality issues, including tropospheric ozone, fine particles, acid deposition, and visibility degradation. CMAQ was also designed to have multiscale capabilities so that separate models were not needed for urban and regional scale air quality modeling. By making CMAQ a modeling system that addresses multiple pollutants and different spatial scales, it has a "one-atmosphere" perspective that combines the efforts of the scientific community. To implement multiscale capabilities in CMAQ, several issues (such as scalable atmospheric dynamics and generalized coordinates), which depend on the desired model resolution, are addressed. A set of governing equations for compressible nonhydrostatic atmospheres is available to better resolve atmospheric dynamics at smaller scales. Because CMAQ is designed to handle scale-dependent meteorological formulations and a large amount of flexibility, its governing equations are expressed in a generalized coordinate system. This approach ensures consistency between CMAQ and the meteorological modeling system. The generalized coordinate system determines the necessary grid and coordinate transformations, and it can accommodate various vertical coordinates and map projections. The CMAQ modeling system simulates various chemical and physical processes that are thought to be important for understanding atmospheric trace gas transformations and distributions. The modeling system contains three types of modeling components (Models-3): a meteorological modeling system for the description of atmospheric states and motions, emission models for man-made and natural emissions that are injected into the atmosphere, and a chemistry-transport modeling system for simulation of the chemical transformation and fate. The chemical transport model includes the following process modules: horizontal advection, vertical advection, mass conservation adjustments for advection processes, horizontal diffusion, vertical diffusion, gas-phase chemical reactions and solvers, photolytic rate computation, aqueous-phase reactions and cloud mixing, aerosol dynamics, size distributions and chemistry, plume chemistry effects, and gas and aerosol deposition velocity estimation. This paper describes the Models-3 CMAQ system, its governing equations, important science algorithms, and a few application examples.

Abstract  The US Acid Rain Program (Title IV of the 1990 Clean Air Act Amendments) has achieved substantial reductions in emissions of sulfur dioxide (SO2) and nitrogen oxides (NOx) from power plants in the United States. We compare new estimates of the benefits and costs of Title IV to those made in 1990. Important changes in our understanding of and ability to quantify the benefits of Title IV have occurred. Benefits to human health now take a much higher profile because the contribution of SO2 and NOx emissions to the formation of fine particulate (PM2.5) is substantial, and evidence of the harmful human health effects of PM2.5 has emerged in the last 15 years. New estimates of the health benefits of PM2.5 reductions are the largest category of quantified health and environmental benefits and total over US$100 billion annually for 2010 when the program is expected to be fully implemented. Although important uncertainties exist in any specific estimate of the benefits, even if the estimates were calculated using more limiting assumptions and interpretations of the literature they would still substantially exceed the costs. Estimates of annualized costs for 2010 are about US$3 billion, which is less than half of what was estimated in 1990. Research since 1990 also suggests that environmental problems associated with acid deposition and nitrogen deposition are more challenging to resolve than originally thought and will require larger reductions in emissions to reverse. The greater than expected benefits to human health, the greater vulnerability of natural resources and ecosystems, and the lower than expected costs all point to the conclusion that further reductions in SO2 and NOx emissions from power plants beyond those currently required by Title IV are warranted.

Abstract  Global warming is predicted to be most pronounced at high latitudes, and observational evidence over the past 25 years suggests that this warming is already under way. One-third of the global soil carbon pool is stored in northern latitudes, so there is considerable interest in understanding how the carbon balance of northern ecosystems will respond to climate warming. Observations of controls over plant productivity in tundra and boreal ecosystems have been used to build a conceptual model of response to warming, where warmer soils and increased decomposition of plant litter increase nutrient availability, which, in turn, stimulates plant production and increases ecosystem carbon storage. Here we present the results of a long-term fertilization experiment in Alaskan tundra, in which increased nutrient availability caused a net ecosystem loss of almost 2,000 grams of carbon per square meter over 20 years. We found that annual aboveground plant production doubled during the experiment. Losses of carbon and nitrogen from deep soil layers, however, were substantial and more than offset the increased carbon and nitrogen storage in plant biomass and litter. Our study suggests that projected release of soil nutrients associated with high-latitude warming may further amplify carbon release from soils, causing a net loss of ecosystem carbon and a positive feedback to climate warming.

Abstract  We compiled chemical data and phytoplankton biomass (PB) data (chlorophyll a) from unproductive lakes in 42 different regions in Europe and North America, and compared these data to inorganic nitrogen (N) deposition over these regions. We demonstrate that increased deposition of inorganic N over large areas of Europe and North America has caused elevated concentrations of inorganic N in lakes. In addition, the unproductive lakes in high N deposition areas had clearly higher PB relative to the total phosphorus (P) concentrations illustrating that the elevated inorganic N concentrations has resulted in eutrophication and increased biomass of phytoplankton. The eutrophication caused by inorganic N deposition indicates that PB yield in a majority of lakes in the northern hemisphere is (was) limited by N in their natural state. We, therefore, suggest that P limitation largely concerns lakes where the balance between N and P has been changed because of increased anthropogenic input of N.

Abstract  [ 1] We employed a fast response thermal dissociation-chemical ionization mass spectrometer (TD-CIMS) system to measure eddy covariance fluxes of peroxyacetyl nitrate ( PAN), peroxypropionyl nitrate (PPN) and peroxymethacryloyl nitrate (MPAN). Fluxes were measured for eight consecutive days in July 2003 at a Loblolly pine forest in North Carolina along with eddy covariance NOy fluxes. Covariances between PAN concentration and vertical wind velocity indicated consistent deposition fluxes that ranged up to approximately - 14 ng N m(-2) s(-1). The average daytime flux peaked at - 6.0 ng N m(-2) s(-1) and accounted for similar to 20% of the daytime NOy flux. Calculations suggest minimum daytime surface resistances for PAN in the range of 70 - 130 s m(-1). It was estimated that approximately half of daytime uptake was through plant stomates. Average PAN deposition velocities, V-d(PAN), showed a daytime maximum of similar to 10.0 mm s(-1); however, deposition did not cease during nighttime periods. V-d(PAN) was highly variable at night and increased when canopy elements were wet from either precipitation or dew formation. Diel patterns of deposition velocity of MPAN and PPN were similar to that of PAN. These results suggest that deposition of PAN, at least to coniferous forest canopies, is much faster than predicted with current deposition algorithms. Although deposition of PAN is unlikely to compete with thermal dissociation during warm summer periods, it will likely play an important role in removing PAN from the atmosphere in colder regions or during winter. The fate of PAN at the surface and within the plants remains unknown, but may present a previously ignored source of nitrogen to ecosystems.

Abstract  Twenty-two high-elevation lakes (>3000 in) in Rocky Mountain National Park and Indian Peaks Wilderness Area, Colorado, were surveyed during summer 1998 to explore relationships among benthic invertebrates, water chemistry (particularly nitrate concentrations), and other environmental variables. Water samples were collected from the deepest portion of each lake and analyzed for ions and other water chemistry parameters. Benthic invertebrates were collected front the littoral zone using both a sweep net and Hess sampler. Physical and geographical measurements were derived from maps. Relationships among benthic invertebrate assemblages and environmental variables were examined using canonical correspondence analysis, and the importance of sampling methodology and taxonomic resolution on these relationships was evaluated.

Choice of sampling methodology strongly influenced the outcome of statistical analyses, whereas taxonomic resolution did not. Presence/absence of benthic invertebrate taxa among the study lakes was best explained by elevation and presence of fish. Relative abundance and density of benthic invertebrate taxa were more strongly influenced by sampling date and water chemistry. Nitrate (NO3-) concentration, potentially on the rise due to regional nitrogen deposition, was unrelated to benthic invertebrate distribution regardless of sampling method or taxonomic resolution.

Abstract  Watershed budget Studies at the Hubbard Brook Experimental Forest (HBEF), New Hampshire, USA, have demonstrated high calcium depletion of soil during the 20th century due, in part, to acid deposition. Over the past 25 years, tree growth (especially for Sugar maple) has declined on the experimental watersheds at the HBEF. In October 1999, 0.85 Mg Ca/ha was added to Watershed I (WI) at the HBEF in the form of wollastonitc (CaSiO3) a treatment that, by summer 2002, had raised the pH in the O-ie horizon from 3.8 to 5.0 and, in the O-a horizon, from 3.9 to 4.2. We measured the response of sugar maple to the calcium fertilization treatment on WI.

Foliar calcium concentration of canopy sugar maples in WI increased markedly beginning the second year after treatment, and foliar manganese declined in years four and five. By 2005, the crown condition of sugar maple was much healthier in the treated watershed as compared with the untreated reference watershed (W6). Following high seed production in 2000 and 2002, the density of sugar maple seedlings increased significantly oil WI in comparison with W6 in 2001 and 2003. Survivorship of the 2003 cohort through July 2005 was much higher oil WI (36.6%) than W6 (10.2%). In 2003, sugar maple germinants oil WI were similar to 50% larger than those in reference plots, and foliar chlorophyll concentrations were significantly greater (0.27 g/m(2) vs:0.23 g/m(2) leaf area). Foliage and fine-root calcium concentrations were roughly twice as high, and manganese concentrations twice as low in the treated than the reference seedlings in 2003 and 2004. Mycorrhizal colonization of seedlings was also much greater in the treated (22.4% of root length) than the reference sites (4.4%). A similar, though less dramatic, difference was observed for mycorrhizal colonization of mature sugar maples (56% vs. 35%). These results reinforce and extend other regional observations that sugar maple decline in the northeastern United States and southern Canada is caused in part by anthropogenic effects on soil calcium status, but the causal interactions among inorganic nutrition, physiological stress, mycorrhizal colonization, and seedling growth and health remain to be established.

Abstract  Our meta-analysis of 126 nitrogen addition experiments evaluated nitrogen (N) limitation of net primary production (NPP) in terrestrial ecosystems. We tested the hypothesis that N limitation is widespread among biomes and influenced by geography and climate. We used the response ratio (R approximately equal ANPP(N)/ANPP(ctrl)) of aboveground plant growth in fertilized to control plots and found that most ecosystems are nitrogen limited with an average 29% growth response to nitrogen (i.e., R = 1.29). The response ratio was significant within temperate forests (R = 1.19), tropical forests (R = 1.60), temperate grasslands (R = 1.53), tropical grasslands (R = 1.26), wetlands (R = 1.16), and tundra (R = 1.35), but not deserts. Eight tropical forest studies had been conducted on very young volcanic soils in Hawaii, and this subgroup was strongly N limited (R = 2.13), which resulted in a negative correlation between forest R and latitude. The degree of N limitation in the remainder of the tropical forest studies (R = 1.20) was comparable to that of temperate forests, and when the young Hawaiian subgroup was excluded, forest R did not vary with latitude. Grassland response increased with latitude, but was independent of temperature and precipitation. These results suggest that the global N and C cycles interact strongly and that geography can mediate ecosystem response to N within certain biome types.

Abstract  The influence of N pulses in the form of experimental additions of HNO3 at two times ambient and (NH4)(2)SO4 at two and four times ambient on the herbaceous and woody understory plants in three Adirondack Mountain hardwood forests was evaluated. Addition of (NH4)(2)SO4 decreased cover of dominant herbs and woody seedlings at Woods Lake, a site in the western Adirondacks. Tissue N concentrations of combined herbs and ferns at Woods Lake increased with addition of NH4+ at both levels (9% at two times ambient; 13% at four times ambient) and increased with all three N treatments at Huntington Forest, a central Adirondack site (14% with NO3- and NH4+ at two times ambient; 22% with NH4+ at four times ambient). Seedlings of American beech (Fagus grandifolia Ehrh.) increased foliar N concentration 7-8% with addition of NH4+ treatments at Huntington Forest, but did nor respond to N addition at Woods Lake and Pancake Hall Creek, a western Adirondack site. In general, greatest plant nutrient response to N addition occurred at Huntington Forest, where atmospheric inputs of N are lower than at Woods Lake and Pancake Hall Creek.

Abstract  At the global scale, among all N (nitrogen) species in the atmosphere and their deposition on to terrestrial vegetation and other receptors, NH3 (ammonia) is considered to be the foremost. The major sources for atmospheric NH3 are agricultural activities and animal feedlot operations, followed by biomass burning (including forest fires) and to a lesser extent fossil fuel combustion. Close to its sources, acute exposures to NH3 can result in visible foliar injury on vegetation. NH3 is deposited rapidly within the first 4û5 km from its source. However, NH3 is also converted in the atmosphere to fine particle NH4+ (ammonium) aerosols that are a regional scale problem. Much of our current knowledge of the effects of NH3 on higher plants is predominantly derived from studies conducted in Europe. Adverse effects on vegetation occur when the rate of foliar uptake of NH3 is greater than the rate and capacity for in vivo detoxification by the plants. Most to least sensitive plant species to NH3 are native vegetation > forests > agricultural crops. There are also a number of studies on N deposition and lichens, mosses and green algae. Direct cause and effect relationships in most of those cases (exceptions being those locations very close to point sources) are confounded by other environmental factors, particularly changes in the ambient SO2 (sulfur dioxide) concentrations. In addition to direct foliar injury, adverse effects of NH3 on higher plants include alterations in: growth and productivity, tissue content of nutrients and toxic elements, drought and frost tolerance, responses to insect pests and disease causing microorganisms (pathogens), development of beneficial root symbiotic or mycorrhizal associations and inter species competition or biodiversity. In all these cases, the joint effects of NH3 with other air pollutants such as all-pervasive O3 or increasing CO2 concentrations are poorly understood. While NH3 uptake in higher plants occurs through the shoots, NH4+ uptake occurs through the shoots, roots and through both pathways. However, NH4+ is immobile in the soil and is converted to NO3- (nitrate). In agricultural systems, additions of NO3- to the soil (initially as NH3 or NH4+) and the consequent increases in the emissions of N2O (nitrous oxide, a greenhouse gas) and leaching of NO3- into the ground and surface waters are of major environmental concern. At the ecosystem level NH3 deposition cannot be viewed alone, but in the context of total N deposition. There are a number of forest ecosystems in North America that have been subjected to N saturation and the consequent negative effects. There are also heathlands and other plant communities in Europe that have been subjected to N-induced alterations. Regulatory mitigative approaches to these problems include the use of N saturation data or the concept of critical loads. Current information suggests that a critical load of 5û10 kg ha-1 year-1 of total N deposition (both dry and wet deposition combined of all atmospheric N species) would protect the most vulnerable terrestrial ecosystems (heaths, bogs, cryptogams) and values of 10û20 kg ha-1 year-1 would protect forests, depending on soil conditions. However, to derive the best analysis, the critical load concept should be coupled to the results and consequences of N saturation.

Abstract  This Summary sets out the key policy-relevant findings of the Fourth Assessment of Working Group II of the Intergovernmental Panel on Climate Change (IPCC). The Assessment is of current scientific understanding of the impacts of climate change on natural, managed and human systems, the capacity of these systems to adapt and their vulnerability.1 It builds upon past IPCC assessments and incorporates new knowledge gained since the Third Assessment. Statements in this Summary are based on chapters in the Assessment and principal sources are given at the end of each paragraph.

Abstract  N/A.

Abstract  This criteria document focuses on a review and assessment of the effects on human health and welfare of the nitrogen oxides, nitric oxide (NO) and nitrogen dioxide (NO2), and the related compounds, nitrites, nitrates, nitrogenous acids, and nitrosamines. Although the emphasis is on presentation of health and welfare effects data, other scientific data are presented in order to provide a better understanding of these pollutants in the environment. To this end, separate chapters are included which discuss the nitrogen cycle, sources and emissions, atmospheric chemical processes which transform emissions of nitrogen oxides into related airborne compounds, transport and removal processes, measurement methods, and atmospheric concentrations of nitrogenous pollutants.

Abstract  The majority of this book was written in 1983-84 while the senior author was a Visiting Scientist at Oak Ridge National Laboratory (ORNL) in Oak Ridge, Tennessee. We believe that the approach to the problem of acid deposition effects on soils and waters developed during this collaboration contains ele­ ments that are significantly different from most prior work in this area. Some of the material and the software used in the development of these concepts stem from earlier individual efforts of the authors. However, what we believe to be the more significant concepts concerning the processes by which alkalinity may be developed in acid soil solutions, and by which acid deposition may contrib­ ute to the loss of this alkalinity, were the result of this collaboration. The ultimate usefulness of these concepts in understanding and dealing with various aspects of the problems associated with acid deposition cannot be adequately gauged at the present time. They must first withstand tests of con­ sistency with available observation, and of direct experimentation. It is our hope that dissemination through this book will facilitate this process within the scientific community. The authors wish to thank the administration of the Environmental Science Division at ORNL, and the College of Agricultural Sciences at Colorado State University for their support in arranging this collaboration. We also wish to express our appreciation for the financial support provided by EPA. Personal thanks are due to Dr.

Abstract  Four years of severe drought from 1999 through 2003 led to unprecedented bark beetle activity in ponderosa and Jeffrey pine in the San Bernardino and San Jacinto Mountains of southern California. Pines in the San Bernardino Mountains also were heavily impacted by ozone and nitrogenous pollutants originating from urban and agricultural areas in the Los Angeles basin. We studied bark beetle activity and bark beetle associated tree mortality in pines at two drought-impacted sites in the San Bernardino Mountains, one receiving high levels of atmospheric pollutants, and one with more moderate atmospheric input. We also investigated the effects of nitrogen addition treatments of 0, 50 and 150 kg N ha-1 year-1 at each site. Tree mortality and beetle activity were significantly higher at the high pollution site. Differences in beetle activity between sites were significantly associated with ozone injury to pines, while differences in tree mortality between sites were significantly associated with both ozone injury and fertilization level. Tree mortality was 9% higher and beetle activity 50% higher for unfertilized trees at the high pollution site compared to the low pollution site. Tree mortality increased 8% and beetle activity increased 20% under the highest rates of nitrogen additions at the low pollution site. The strong response in beetle activity to nitrogen additions at the low pollution site suggests that atmospheric nitrogen deposition increased tree susceptibility to beetle attack at the high deposition site. While drought conditions throughout the region were a major factor in decreased tree resistance, it appears that both ozone exposure and atmospheric nitrogen deposition further increased pine susceptibility to beetle attack.

Abstract  Acidic atmospheric deposition, popularly referred to as acid rain, is the transfer of strong acids and acid forming substances from the atmosphere to the Earth’s surface. Acidic deposition is comprised of sulfuric and nitric acids, and ammonium derived from atmospheric emissions of sulfur dioxide, nitrogen oxides, and ammonia respectively. These compounds are emitted by the burning of fossil fuels and by agricultural activities. Once such compounds enter an ecosystem, they can acidify soil and surface waters and bring about a series of ecological changes. The term acidic deposition encompasses all forms in which these compounds are deposited to the Earth, including gases, particles, rain, snow, clouds, and fog (see Box 3.1). Acidic deposition was first reported in the United Kingdom in the later half of the 19th Century (Gorham 1992). Ecological effects were first documented in Scandinavia in the 1960s with the link between acidic deposition, surface water acidification and loss of fisheries (Gorham 1992). Atmospheric deposition of sulfate, nitrate and ammonium are elevated in eastern North America, Europe and large portions of Asia (Rodhe et al. 1995).

Abstract  Previous inventories of ammonia emissions for the United States have not characterized the seasonal and geographic variations that are necessary for accurately predicting ambient concentrations of ammonium nitrate and ammonium sulfate aerosol. This research calculates the seasonal and geographic variation in ammonia emissions from dairy cows in the United States. Monthly, county-level emission factors are calculated with a process- based model of dairy farm emissions, the national distribution of farming practices, seasonal climate conditions, and animal populations. Annual, county-level emission factors are estimated to range between 13.1 and 55.5, with a national average of 23.9 kg NH3 cow(-1) yr(-1). The seasonal variation of the emission factor is estimated to be as high as a factor of seven in some counties. Emissions are predicted to be the highest in the spring and fall, because of high manure application rates during the spring planting and after the fall harvest. Summer emissions are higher than winter, resulting from the temperature dependence of housing and storage emissions. In the summer and winter, the majority of emissions are from animal housing. In the spring and fall, the majority of emissions are from field applied manure. The 5% and 95% confidence interval about the national annual average emission factor is between 18 and 36 kg NH3 COW I yr(-1). Uncertainties in farming practices contribute most to the total uncertainty, yet uncertainty in the timing of manure application, the quantity of manure and nitrogen excreted by cows, and the physical processes of volatilization affecting applied manure are also significant. (C) 2004 Elsevier Ltd. All rights reserved.

Abstract  Rates of atmospheric deposition of biologically active nitrogen (N) are two to seven times the pre-industrial rates in many developed nations because of combustion of fossil fuels and agricultural fertilization. They are expected to increase similarly over the next 50 years in industrializing nations of Asia and South America. Although the environmental impacts of high rates of nitrogen addition have been well studied, this is not so for the lower, chronic rates that characterize much of the globe. Here we present results of the first multi-decadal experiment to examine the impacts of chronic, experimental nitrogen addition as low as 10 kg N ha(-1) yr(-1) above ambient atmospheric nitrogen deposition (6 kg N ha(-1) yr(-1) at our site). This total input rate is comparable to terrestrial nitrogen deposition in many industrialized nations. We found that this chronic low-level nitrogen addition rate reduced plant species numbers by 17% relative to controls receiving ambient N deposition. Moreover, species numbers were reduced more per unit of added nitrogen at lower addition rates, suggesting that chronic but low-level nitrogen deposition may have a greater impact on diversity than previously thought. A second experiment showed that a decade after cessation of nitrogen addition, relative plant species number, although not species abundances, had recovered, demonstrating that some effects of nitrogen addition are reversible.

Abstract  Global energy use and food production have increased nitrogen inputs to ecosystems worldwide, impacting plant community diversity, composition, and function. Previous studies show considerable variation across terrestrial herbaceous ecosystems in the magnitude of species loss following nitrogen (N) enrichment. What controls this variation remains unknown. We present results from 23 N-addition experiments across North America, representing a range of climatic, soil and plant community properties, to determine conditions that lead to greater diversity decline. Species loss in these communities ranged from 0 to 65% of control richness. Using hierarchical structural equation modelling, we found greater species loss in communities with a lower soil cation exchange capacity, colder regional temperature, and larger production increase following N addition, independent of initial species richness, plant productivity, and the relative abundance of most plant functional groups. Our results indicate sensitivity to N addition is co-determined by environmental conditions and production responsiveness, which overwhelm the effects of initial community structure and composition.