Excess Nitrogen Deposition Research Articles

The impact of emissions controls on atmospheric nitrogen inputs to Chinese river basins highlights the urgency of ammonia abatement.

Excessive nitrogen (N) deposition affects aquatic ecosystems worldwide, but effectiveness of emissions controls and their impact on water pollution remains uncertain. In this modeling study, we assess historical and future N deposition trends in Chinese river basins and their contributions to water pollution via direct and indirect N deposition (the latter referring to transport of N to water from N deposited on land). The control of acid gas emissions (i.e., nitrogen oxides and sulfur dioxide) has had limited effectiveness in reducing total N deposition, with notable contributions from agricultural reduced N deposition. Despite increasing controls on acid gas emissions between 2011 and 2019, N inputs to rivers increased by 3%, primarily through indirect deposition. Simultaneously controlling acid gas and ammonia emissions could reduce N deposition and water inputs by 56 and 47%, respectively, by 2050 compared to 2019. Our findings underscore the importance of agricultural ammonia mitigation in protecting water bodies.

Effects of nitrogen deposition on the rhizosphere nitrogen-fixing bacterial community structure and assembly mechanisms in Camellia oleifera plantations.

Increased nitrogen deposition is a key feature of global climate change, however, its effects on the structure and assembling mechanisms of the nitrogen-fixing bacteria present at the root surface remain to be elucidated. In this pursuit, we used NH4NO3 to simulate nitrogen deposition in a 10-year-old Camellia oleifera plantation, and set up four deposition treatments, including control N0 (0 kg N hm-2 a-1), low nitrogen N20 (20 kg N hm-2 a-1), medium nitrogen N40 (40 kg N hm-2 a-1) and high nitrogen N160 (160 kg N hm-2 a-1). The results showed that nitrogen deposition affected the soil nitrogen content and the structure of the nitrogen-fixing bacterial community. Low nitrogen deposition was conducive for nitrogen fixation in mature C. oleifera plantation. With increasing nitrogen deposition, the dominant soil nitrogen-fixing bacterial community shifted from Desulfobulbaceae to Bradyrhizobium. When nitrogen deposition was below 160 kg N hm-2 a-1, the soil organic matter content, total nitrogen content, nitrate nitrogen content, ammonium nitrogen content, urease activity, soil pH and nitrate reductase activity influenced the composition of the nitrogen-fixing bacterial community, but the stochastic process remained the dominant factor. The results indicate that the strains of Bradyrhizobium japonicum and Bradyrhizobium sp. ORS 285 can be used as indicator species for excessive nitrogen deposition.

Wet deposition of sulfur and nitrogen in the Jiuzhaigou World Heritage Site, China: Spatial variations, 2010–2022 trends, and implications for karst ecosystem conservation

Excessive sulfur and nitrogen deposition is a threat to natural ecosystems. This study investigated the spatio-temporal changes of wet sulfur and nitrogen deposition in Jiuzhaigou to understand the responses of wet deposition chemistry to local and national air pollutant emission changes and to assess relevant ecological risks. The results show that local emission sources (e.g., tourism, post-earthquake reconstruction, and vegetation damages) largely elevated the concentrations of SO42−, NO3−, NH4+, and Ca2+ at the non-background sites during 2021–2022; However, the gaps of annual sulfur and nitrogen wet deposition fluxes among the background, semi-background, and non-background sites were reduced by annual precipitation. The pH was also higher at the non-background sites due to stronger acid neutralization capacities from crustal dust there, and acid precipitation (pH < 5.6) was not detected at all the sites. During the 2010 to 2022 period, the wet deposition of SO42− and TIN (i.e. total inorganic nitrogen) and the equivalent ratio of SO42−: NO3− generally presented a declining trend at the background and semi-background sites, as a result of regional and national emission reductions. The TIN reduction from 2015 to 2016 to 2021–2022 at the background site was more from wet NH4+ deposition and the site's NO3− concentrations and fluxes were relatively stable. Compared to the critical loads (CLs) of sulfur and nitrogen for acidification and eutrophication, the annual wet and wet+dry deposition fluxes of sulfur and nitrogen were lower in almost the entire watershed, except for the CLmin of nitrogen for acidification in considerable areas. However, all the wet deposition samples were calcite-unsaturated and had annual TIN concentrations higher than the national environmental standard of total nitrogen for surface water. More field investigations are needed to better assess the ecological risks.

Assessment of Critical Loads of Nitrogen Deposition in Natural Ecosystems of China

Excessive nitrogen (N) deposition causes a series of environmental problems, including biodiversity loss. Therefore, assessing current N deposition thresholds of natural ecosystems is critical for regional N management and pollution control. In this study, the critical loads of N deposition in mainland China were estimated using the steady-state mass balance method, and the spatial distribution of ecosystems that exceeded the critical load was evaluated. The results showed that areas with critical loads of N deposition higher than 56, in the range of 14-56, and lower than 14 kg·(hm2·a)-1 accounted for 6%, 67%, and 27% of that in China, respectively. The areas with higher critical loads of N deposition were mainly distributed in the eastern Tibetan Plateau, northeastern Inner Mongolia, and parts of south China. Lower critical loads of N deposition were mainly distributed in the western Tibetan Plateau, northwest China, and parts of southeast China. Moreover, the areas where N deposition exceeded the critical loads accounted for 21% of that in mainland China, being mainly distributed in southeast and northeast China. The exceedances of critical loads of N deposition in northeast China, northwest China, and the Qinghai-Tibet Plateau were generally lower than 14 kg·(hm2·a)-1. Therefore, the management and control of N in these areas that exceeded the critical load of deposition is more worthy of future attention.

Response of Forest Ecosystems to Decreasing Atmospheric Nitrogen Deposition

Nitrogen deposition has serious consequences to global change. Excessive nitrogen deposition leads to nitrogen saturation in forests, resulting in soil acidification, nitrate leaching, an increase in nitrous oxide emissions, and a decrease in plant species diversity and vegetation productivity. Under the reduction of atmospheric nitrogen deposition in Europe, North America, and China, summarizing the response of forests to decreasing nitrogen deposition can not only improve the knowledge framework of the impact of nitrogen deposition on forests, but also evaluate the effects of emission abatement actions, as well as provide scientific basis for future air pollution control. This study reviewed the response of soil, surface water, nitrogen cycle, and vegetation of temperate forests in Europe and North America and subtropical forests in southwest China to the reduction in atmospheric nitrogen pollution gases and thus nitrogen deposition. The soil water nitrogen concentration responded rapidly to the nitrogen deposition reduction, although the trend was inconsistent. The soil acidification and nitrogen cycles showed a delayed response of recovery from high nitrogen deposition. The nitrogen mineralization and immobilization, soil carbon retention, and net primary production might take decades to respond to the nitrogen deposition reduction. However, the soil inorganic nitrogen pool and nitrogen leaching decreased with the decline in nitrogen deposition, although a one-or two-year lag existed. The surface water nitrogen concentration was closely related to nitrogen status in forests. After the nitrogen deposition decreased, the nitrogen leaching and thus the surface water nitrogen concentration decreased in the areas with historically high nitrogen deposition. However, the low surface water nitrogen concentration in the nitrogen-limited forests was not significantly affected by the nitrogen deposition changes. The recovery of surface water acidification was affected by soil sulfur desorption/mineralization and nitrification/denitrification. The foliar nitrogen concentration decreased with the decline in nitrogen deposition. The nitrogen-saturated forests and regional surface water in southwest China showed a recovery trend from high nitrogen deposition, as a consequence of the implementation of the Total Emissions Control of Air Pollutants and later the Action Plan of Air Pollution Prevention and Control.

Nitrogen deposition from aviation emissions

Excess nitrogen deposition from anthropogenic sources of atmospheric emissions, such as agriculture and transportation, can have negative effects on natural environments. Designing effective conservation efforts requires knowledge of the contribution of individual sectors. This study utilizes a global atmospheric chemistry-transport model to quantify, for the first time, the contribution of global aviation NOx emissions to nitrogen deposition for 2005 and 2019. We find that aviation led to an additional 1.39 Tg of nitrogen deposited globally in 2019, up 72 % from 2005, with 67 % of each year's total occurring through wet deposition. In 2019, aviation was responsible for an average of 0.66 %, 1.13 %, and 1.61 % of modeled nitrogen deposition from all sources over Asia, Europe, and North America, respectively. These impacts are spatially widespread, with 56 % of deposition occurring over water. Emissions during the landing, taxi and takeoff (LTO) phases of flight are responsible for 8 % of aviation's nitrogen deposition impacts on average globally, and between 16 and 32 % over most land in regions with high aviation activity. Despite currently representing less than 1.2 % of nitrogen deposition globally, further growth of aviation emissions would result in increases in aviation's contribution to nitrogen deposition and associated critical loads.

The Habitats Directive and Heath: The Strain of Climate Change and N Deposition

The Habitats Directive is the cornerstone of EU nature conservation law charged with both the conservation and restoration of a wide degree of heathlands. However, heathlands in western-Europe are on a steady decline with grim prospects for the future, which can be contributed to the combined effect of nitrogen deposition and climate change. In turn, this article researches the capacity of the Habitats Directive to regulate aforementioned threats. It finds that the Directive is not sufficient with regard to the reintroduction of keystone species, connectivity restoration, climate change adaptation and/or mitigation measures – due to a lack of specific restoration norms. This article also assesses whether a Restoration Directive can serve as a supplementary means of achieving restoration in European heathlands. From an academic perspective, this article contributes to the debate on the functioning of the Habitats Directive and EU environmental law. From a societal perspective, there is a significant concern regarding excessive nitrogen deposition in combination with climate change in various EU countries. Whilst this problem should be studied from a multitude of disciplines, effective legal norms geared towards restoration play a distinctive role in solving this complex puzzle. Habitats Directive – Nitrogen Deposition – Climate Change – EU Nature conservation law – Restoration – Conservation – Heathlands – Restoration Directive – Biodiversity

Divergent responses of soil microbial functional groups to long-term high nitrogen presence in the tropical forests

A massive rise in atmospheric nitrogen deposition (ND) has threatened ecosystem health through accelerating soil nitrogen (N) cycling rates. While soil microbes serve a crucial function in soil N transformation, it remains poorly understood on how excess ND affects microbial functional populations regulating soil N transformation in tropical forests. To address this gap, we conducted 13-year N (as NH4NO3) addition experiments in one N-rich tropical primary forest (PF) and two N-poor tropical reforested forests (rehabilitated and disturbed) in South China. Based on our data, 13-year N introduction markedly enhanced soil N2O generation in all forests, regardless of soil N status, but microbial functional groups showed divergent responses to excess N addition among the studied forests. In the PF, long-term N introduction markedly decreased presence of bacterial 16S rRNA gene, nitrifier (amoA) and denitrifier genes (nirK, nirS and nosZ) and bacteria/fungi ratio, which could be attributed to the decreases in soil pH, dissolved organic carbon to N ratio and understory plant richness. In the two reforested forests, however, long-term N introduction generally did neither alter soil properties nor the abundance of most microbial groups. We further found that the elevated N2O generation was related to the increased soil N availability and decreased nosZ abundance, and the PF has the highest N2O generation than the other two forests. Overall, our data indicates that the baseline soil N status may dominate response of microbial functional groups to ND in tropical forests, and N-rich forests are more responsive to excess N inputs, compared to those with low-N status. Forests with high soil N status can produce more N2O than those with low-N status. With the spread of elevated ND from temperate to tropical zones, tropical forests should merit more attention because ecosystem N saturation may be common and high N2O emission will occur.

High atmospheric wet nitrogen deposition and major sources in two cities of Yangtze River Delta: Combustion-related NH3 and non-fossil fuel NOx

High ammonia (NH3) and nitrogen oxide (NOx) emissions are related to serious air pollution in urban areas and the negative impacts of excessive reactive nitrogen (N) deposition on many ecosystems. However, whether there is a relationship between N deposition rates and their sources with urbanization or not remains unclear in many areas. Here, we investigated the deposition rates of ammonium (NH4+), nitrate (NO3−), dissolved organic N, and water-insoluble particular N from July 2017 to June 2018 at two urban and two suburban sites in the Yangtze River Delta (YRD). The δ15N values of precipitation NH4+ and NO3− were measured, and major sources were analyzed using a Bayesian isotope mixing model. Wet N deposition rates were higher in Yangzhou (developing city, 20.3–22.7 kg N ha−1 yr−1) than those in Nanjing (developed city, 19.4–20.5 kg N ha−1 yr−1), and were higher at urban sites (20.4–22.5 kg N ha−1 yr−1) than those at suburban sites (18.7–20.3 kg N ha−1 yr−1). δ15N values of precipitation NH4+ increased with an increase in precipitation pH because ambient acidity affects the equilibrium isotope fractionation between NH3 and NH4+ and wet scavenging coefficients of NH3 and particulate NH4+. For NH4+, combustion-related NH3 sources (62%–65% with 5.5–6.4 kg N ha−1 yr−1, including coal combustion, vehicle exhaust, and biomass burning) contributed more than volatilization NH3 sources (35%–38% with 2.9–3.9 kg N ha−1 yr−1, including fertilizer application and waste volatilization). For NO3−, non-fossil fuel NOx sources (50%–63% with 3.4–4.1 kg N ha−1 yr−1, including biomass burning and microbial N cycle) were comparable to fossil fuel NOx sources (37%–50% with 2.4–3.4 kg N ha−1 yr−1, including coal combustion and vehicle exhaust). This study evidenced high N deposition rates and the importance of combustion-related NH3 emissions and non-fossil fuel NOx emissions in city areas of the YRD.

Nitrogen discourages legumes

Ecology Anthropogenic nutrient enrichment of the global environment has been proceeding for many decades. Nitrogen deposition in particular leads to higher ecosystem productivity at the expense of biodiversity. In a global experiment spanning 45 sites, Tognetti et al. investigated how the addition of nitrogen affected the leguminous plants in grassland. Legumes under natural conditions compete successfully in low-nutrient conditions because of their ability to fix atmospheric nitrogen using their rhizobial symbionts. However, excess nitrogen deposition places them at a competitive disadvantage, leading to declines in their biomass and abundance. In turn, these declines may have adverse consequences for grassland biodiversity and food webs more generally as nitrogen deposition continues. Proc. Natl. Acad. Sci. U.S.A. 118 , e2023718118 (2021).

Current situation of nitrogen application and its effects on yield and quality of Chinese materia medica

Nitrogen fertilizer has been the long-lasting crucial component in cultivation of Chinese materia medica(CMM) and crops for its profound effects on enhancing the productivity. In consideration of its role in better production, intensive and excessive application of N fertilizer is often found in CMM cultivation. Therefore, firstly, this review summarized various concentrations of N application with regards to different CMM and districts from the literatures published in the last two decades. The recommended concentration of nitrogen application of forty seven CMM species were covered in this review. We found that the optimum rates of nitrogen fertili-zer for different medicinal plants species were varied in the range between 0-1 035.55 kg·hm~(-2). Most of the optimum rates of nitrogen fertilizer for CMM in published researches fell between 100-199 kg·hm~(-2). The optimum rate of nitrogen fertilizer is not only related to amount of nitrogen required for different medicinal plants but also to soil fertilities of different fields. In addition, we outlined the diffe-rent effects of proper and excessive nitrogen deposition on yield of CMM. Proper nitrogen deposition benefits the yield of CMM, howe-ver, excessive nitrogen use accounts for a decrease in CMM yield. We elucidated that nutritional content, water use efficiencies, and photosynthesis capacity were major influencing factors. Researches showed that proper nitrogen fertilizer could promote the water use efficiencies of plants and boost photosynthesis. Consequently, the yield of CMM can be enhanced after nitrogen deposition. However, negative effects of nitrogen fertilizer were also found on plant including producing toxic substances to the soil and causing severe pest damages. Lastly we analyzed the impact of N fertilizer application on secondary metabolites which accounts for a large part of active pharmaceutical ingredients of CMM. It usually caused an increase in nitrogen-containing secondary metabolites content and a decrease in non-nitrogen-containing secondary metabolites content respectively. The potential underlying mechanisms are the different synthetic pathways of these metabolites and the plant nutritional status. Synthesis of non-nitrogen-containing secondary metabolites like phenols can be inhibited after nitrogen application because of the competition of the same precursor substances between metabolites synthesis and plant growth. To sum up, impacts and mechanisms of nitrogen fertilizer on yield and quality enhancement of CMM were discussed in this review. Negative effects of excessive nitrogen application on CMM should be paid special attention in CMM cultivation and prescription fertilization based on the field soil quality is strongly recommended. Overall, this review aims to provides insights on improving the proper application of N fertilizer in the cultivation of CMM.

Nitrogen deposition is positively correlated to foliar nitrogen content in Vaccinium myrtillus and other understory species in temperate forests on acidic soil

Excessive atmospheric nitrogen deposition is known to alter nutrient cycles and species composition in temperate forest ecosystems. Foliar N concentrations reflect the nutrient status of plants and can be used to assess the impact of atmospheric N deposition on ecosystems. Our objective was to test whether the foliar N content reflects increasing N deposition in the understory vegetation of temperate forests on acidic soil. Therefore, we analyzed the foliar N content of seven plant taxa representing different functional groups of the understory vegetation and correlated them to modeled deposition datasets. Linear mixed models revealed no relationship of N deposition with foliar N concentrations across all species examined but decreasing foliar concentrations with increasing soil C/N ratio. Nevertheless, we found positive correlations of foliar N concentrations with increasing deposition in conifer saplings (Picea abies and Abies alba), the dwarf shrub Vaccinium myrtillus and the graminoid Luzula luzuloides. Such an increase was, however, not found in saplings of broad-leaved trees (Fagus sylvatica and Quercus petraea/robur) and the moss Polytrichum formosum. This suggests marked species-specific differences in nitrogen uptake and allocation. Foliar N concentrations were more strongly correlated to NOy than NHx and to wet than dry deposition. Since foliar N concentrations were correlated to deposition while the species’ cover was not, they may provide a valuable indicator of N deposition.

Long- and Short-Term Inorganic Nitrogen Runoff from a Karst Catchment in Austria

Excess nitrogen (N) deposition and gaseous N emissions from industrial, domestic, and agricultural sources have led to increased nitrate leaching, the loss of biological diversity, and has affected carbon (C) sequestration in forest ecosystems. Nitrate leaching affects the purity of karst water resources, which contribute around 50% to Austria’s drinking water supply. Here we present an evaluation of the drivers of dissolved inorganic N (DIN) concentrations and fluxes from a karst catchment in the Austrian Alps (LTER Zöbelboden) from 27 years of records. In addition, a hydrological model was used together with climatic scenario data to predict expected future runoff dynamics. The study area was exposed to increasing N deposition during the 20th century (up to 30 to 35 kg N ha−1 y−1), which are still at levels of 25.5 ± 3.6 and 19.9 ± 4.2 kg N ha−1 y−1 in the spruce and the mixed deciduous forests, respectively. Albeit N deposition was close to or exceeded critical loads for several decades, 70–83% of the inorganic N retained in the catchment from 2000 to 2018, and NO3- concentrations in the runoff stayed &lt;10 mg L−1 unless high-flow events occurred or forest stand-replacing disturbances. We identified tree growth as the main sink for inorganic N, which might together with lower runoff, increase retention of only weakly decreasing N deposition in the future. However, since recurring forest stand-replacement is predicted in the future as a result of a combination of climatically driven disturbance agents, pulses of elevated nitrate concentrations in the catchment runoff will likely add to groundwater pollution.

Ammonia should be considered in field experiments mimicking nitrogen deposition

Excess nitrogen deposition has significant impacts on water eutrophication, soil acidification, eleveted greenhouse gas emissions, and biodiversity loss. These impacts mostly derive from convention...

Interspecific Differences in Canopy-Derived Water, Carbon, and Nitrogen in Upland Oak-Hickory Forest

Oaks (Quercus) are a dominant forest species throughout much of the eastern United States. However, oak regeneration failure due to a myriad of issues (e.g., suppression of natural fire, excess nitrogen deposition, pressure from herbivore activity) is leading to a decline in oak dominance. This change may alter forest hydrology and nutrients through variation in species characteristics. Throughfall (TF) and stemflow (SF) quantity and chemistry were sampled during storm events under oak and non-oak (hickory, Carya) species to quantify differences in canopy-derived water and nutrients from an upland oak-hickory forest in Mississippi. Stemflow partitioning was 86% higher in hickory species compared to oak species (394.50 L m−2; p &lt; 0.001). Across all species, dissolved organic carbon (DOC) was 1.5 times greater in throughfall (p = 0.024) and 8.7 times greater in stemflow (p &lt; 0.001) compared to rainfall. White oak DOC concentrations (TF: 22.8 ± 5.5 mg L−1; SF: 75.1 ± 9.5 mg L−1) were greater compared to hickory species (TF: 21.0 ± 18.3 mg L−1; SF: 34.5 ± 21.0 mg L−1) (p = 0.048). Results show that while smoother-barked hickory species generate more stemflow volume, rougher-barked oak species generate stemflow that is more enriched in nutrients, which is a function of the canopy characteristics of each species. Within a single stand, this study demonstrates how variable water and nutrient fluxes may be and provide insights into species-level variability in oak-hickory forest types that may be undergoing compositional changes.

The impact of nitrogen deposition on photobiont-mycobiont balance of epiphytic lichens in subtropical forests of central China.

Excessive nitrogen (N) deposition can impact lichen diversity in forest ecosystems, and this is a particular situation in China. Here, we examined the N uptake, assimilation, and the impact of excessive N deposition on the symbiotic balance of dominant epiphytic lichens in the subtropical forests in the Mts. Shennongjia of central China. The results show that lichen species took up, assimilated and utilized more ammonium than nitrate in a species‐specific way, following the increase of N availability. The photobiont of the lichens decreased with the increase of N concentration following an initial increase, while the mycobiont response to the N addition was not apparent. Considerable variation in response to excessive N deposition exists among the lichen species. Usnea longissima could regulate its N uptake, resulting in a stable photobiont‐mycobiont ratio among N treatments. In contrast, the photobiont‐mycobiont ratio of other four lichens increased initially but decreased when N concentration exceeded a certain level, and N stress may have broken the balance between photobiont and mycobiont of these lichens. Our results suggest that most epiphytic lichens in subtropical forest of central China could uptake and assimilate more ammonium than nitrate and that the balance between photobiont and mycobiont of many epiphytic lichens might change with the increasing N deposition load, which could impact the lichen diversity of this forest ecosystem.

Long-term trends of wet inorganic nitrogen deposition in Rocky Mountain National Park: Influence of missing data imputation methods and associated uncertainty

Excess reactive nitrogen (Nr) deposition is occurring in Rocky Mountain National Park and impacting sensitive ecosystems. In 2006, the National Park Service, State of Colorado, and Environmental Protection Agency established the goal to reduce Nr deposition to below the ecosystem critical load by 2032. Progress is tracked using 5-year averages of annual wet inorganic nitrogen (IN) deposition measured at Loch Vale, Colorado, by the National Atmospheric Deposition Program (NADP). This remote high alpine site is challenging to operate, and large fractions of the annual precipitation, at times >40%, had invalid IN concentrations. Annual wet IN deposition is calculated using the NADP protocol, which replaces missing concentrations with the annual precipitation-weighted mean (PWM) concentration of valid samples. This protocol does not account for seasonal variations in IN concentrations and the inverse relationship between concentration and precipitation amounts. Invalid samples occurred more frequently in the winter and at high and low precipitation amounts, and the NADP protocol generally overestimated annual deposition rates, by as much as 20%. Here, a new method for imputing missing weekly IN concentrations that accounts for their seasonal and precipitation dependence is introduced. Using a bootstrapping analysis shows that the new method reduced the errors in the annual deposition rates by about 30% compared to the NADP protocol and the biases were near zero. The overall trend in the wet IN deposition rates was found to be flat from 1990 to 2017, but the nitrate contribution decreased about 33%, which was offset by a nearly equal increase in ammonium wet deposition. These trends are consistent with known changes in nitrate and ammonium precursor emissions. The long-term trends in the annual IN deposition rates were similar using both data imputation methods, but the 2013–2017 average was about 10% smaller using the new method.

Ozone effects on plants in natural ecosystems.

Tropospheric ozone (O3 ) is an important stressor in natural ecosystems, with well-documented impacts on soils, biota and ecological processes. The effects of O3 on individual plants and processes scale up through the ecosystem through effects on carbon, nutrient and hydrologic dynamics. Ozone effects on individual species and their associated microflora and fauna cascade through the ecosystem to the landscape level. Systematic injury surveys demonstrate that foliar injury occurs on sensitive species throughout the globe. However, deleterious impacts on plant carbon, water and nutrient balance can also occur without visible injury. Because sensitivity to O3 may follow coarse physiognomic plant classes (in general, herbaceous crops are more sensitive than deciduous woody plants, grasses and conifers), the task still remains to use stomatal O3 uptake to assess class and species' sensitivity. Investigations of the radial growth of mature trees, in combination with data from many controlled studies with seedlings, suggest that ambient O3 reduces growth of mature trees in some locations. Models based on tree physiology and forest stand dynamics suggest that modest effects of O3 on growth may accumulate over time, other stresses (prolonged drought, excess nitrogen deposition) may exacerbate the direct effects of O3 on tree growth, and competitive interactions among species may be altered. Ozone exposure over decades may be altering the species composition of forests currently, and as fossil fuel combustion products generate more O3 than deteriorates in the atmosphere, into the future as well.

Source regions contributing to excess reactive nitrogen deposition in the Greater Yellowstone Area (GYA) of the United States

Abstract. Research has shown that excess reactive nitrogen (Nr) deposition in the Greater Yellowstone Area (GYA) of the United States has passed critical load (CL) thresholds and is adversely affecting sensitive ecosystems in this area. To better understand the sources causing excess Nr deposition, the Comprehensive Air Quality Model with Extensions (CAMx), using Western Air Quality Study (WAQS) emission and meteorology inputs, was used to simulate Nr deposition in the GYA. CAMx's Particulate Source Apportionment Technology (PSAT) was employed to estimate contributions from agriculture (AG), oil and gas (OG), fire (Fire), and other (Other) source sectors from 27 regions, including the model boundary conditions (BCs) to the simulated Nr for 2011. The BCs were outside the conterminous United States and thought to represent international anthropogenic and natural contributions. Emissions from the AG and Other source sectors are predominantly from reduced N and oxidized N compounds, respectively. The model evaluation revealed a systematic underestimation in ammonia (NH3) concentrations by 65 % and overestimation in nitric acid concentrations by 108 %. The measured inorganic N wet deposition at National Trends Network sites in the GYA was overestimated by 31 %–49 %, due at least partially to an overestimation of precipitation. These uncertainties appear to result in an overestimation of distant source regions including California and BCs and an underestimation of closer agricultural source regions including the Snake River valley. Due to these large uncertainties, the relative contributions from the modeled sources and their general patterns are the most reliable results. Source apportionment results showed that the AG sector was the single largest contributor to the GYA total Nr deposition, contributing 34 % on an annual basis. A total of 74 % of the AG contributions originated from the Idaho Snake River valley, with Wyoming, California, and northern Utah contributing another 7 %, 5 %, and 4 %, respectively. Contributions from the OG sector were small at about 1 % over the GYA, except in the southern Wind River Mountain Range during winter where they accounted for more than 10 %, with 46 % of these contributions coming from OG activities in Wyoming. Wild and prescribed fires contributed 18 % of the total Nr deposition, with fires within the GYA having the highest impact. The Other source category was the largest winter contributor (44 %) with high contributions from California, Wyoming, and northern Utah.

Wet deposition fluxes of atmospheric inorganic reactive nitrogen at an urban and rural site in the Indo-Gangetic Plain

Excess nitrogen deposition is a matter of concern for sensitive ecosystems. However, understanding the sources and transport of Nr species has been a challenge due to limited observations of atmospheric deposition of the key Nr species across India. In this study, wet deposition of atmospheric inorganic Nr species was investigated during the year 2013 at two regionally representative sites: Delhi (an urban site) and Jaunpur (a rural site). These sites are located in the Indo-Gangetic Plain (IGP) region, which is one of the most populated and fertile regions of India. The average NH4+ concentrations in rain water were found to be 25.4 μeql−1 and 98.5 μeql−1 at the rural and urban sites, respectively, whereas average NO3− concentrations were 12.4 μeql−1 and 28.7 μeql−1 at the rural and urban sites (respectively). The annual average wet deposition fluxes of NH4+ and NO3− at Delhi were calculated as 10.45 and 3.05 kgN ha−1 yr−1 respectively, whereas at Jaunpur the fluxes were 3.19 and 1.56 kgN ha−1 yr−1 respectively. In order to assess the Nr deposition, our estimates showed 486% increase in NO3− (from 0.52 to 3.05 kgN ha−1 yr−1) while 283% NH4+ (2.72–10.44 kgN ha−1 yr−1) between 1994 and 2013 at Delhi, clearly indicating the effect of urbanization and Land Use Land Cover (LULC) change. Reduced versus oxidized N deposition contribution was also estimated. This study provides key quantitative information to support regional nitrogen budget estimates in south Asia.