Atmospheric Nr Research Articles

A Decadal Change in Atmospheric Nitrogen Deposition at a Rural Site in Southern China

Elevated atmospheric reactive nitrogen (Nr) emissions and the subsequent nitrogen (N) deposition have negatively impacted the global environment, particularly in China. In order to assess the long-term trends in atmospheric N deposition in the south of China, Taojiang County in Hunan Province was selected as a representative rural area for study. We analyzed interannual variation in atmospheric Nr, including gaseous ammonia (NH3), nitrogen dioxide (NO2), nitrate acid (HNO3) vapor, particulate ammonium (NH4+), and nitrate (NO3−) in air and NH4+-N and NO3−-N in precipitation from 2011 to 2020. The 10-year average atmospheric wet-plus-dry N deposition was 41.9 kg N ha−1 yr−1, which decreased by approximately 24% after 2012, indicating that NH3 and NOx emissions were effectively reduced by emission controls introduced in 2013. Wet deposition accounted for approximately 74% of the total N deposition and was significantly influenced by annual precipitation amount. Reduced N (NH3, pNH4+, and NH4+ in rainwater) was the dominant form, comprising approximately 58% of the total N deposition, while oxidized N (pNO3−, NO2, HNO3, and NO3− in rainwater) accounted for 42% of the total N deposition. Atmospheric HNO3, NO2, and NH3 concentrations and deposition declined by 30–80% over the decade, while particulate NH4+ and NO3− concentrations and deposition remained at relatively stable levels, which suggests that ongoing research and policy should focus on rural particulate pollution. Future strategies must concentrate on the integrated control of NH3 and NOx emissions to mitigate air pollution and protect human health, particularly in rural areas because current abatement efforts are primarily directed toward urban areas and the industrial sector, whereas non-point source NH3 pollution, influenced mainly by agricultural activities, dominates in rural regions.

Interactive effects of climate-atmospheric cycling on aquatic communities and ecosystem shifts in mountain lakes of southeastern Tibetan Plateau

Recent climate warming and atmospheric reactive nitrogen (Nr) deposition are affecting a broad spectrum of physical, ecological and human systems that may be irreversible on a century time scale and have the potential to cause regime shifts in ecological systems. These changes may alter the limnological conditions with important but still unclear effects on lake ecosystems. We present changes in cladoceran with comparisons to diatom assemblages over the past ~200 years from high-resolution, well-dated sediment cores retrieved from six high mountain lakes in the southeastern (SE) margin of the Tibetan Plateau. Our findings suggest that warming and the exponential increase of atmospheric Nr deposition are the major drivers of ecological regime changes. Shifts in cladoceran and diatom communities in high alpine lakes began over a century ago and intensified since 1950 CE, indicating a regional-scale response to anthropogenic climate warming. Zooplankton in the forest lakes showed asynchronous trajectories, with increased Nr deposition as a significant explanatory factor. Forest lakes with higher dissolved organic carbon (DOC) concentrations partially buffered the impacts of Nr deposition with little structural change, while lakes with low DOC display symptoms of resilience loss related to Nr deposition. Biological community compositional turnover in subalpine lakes has shown marked shifts, equivalent to those of low-elevation lakes strongly affected by direct human impacts. This suggests that local effects override climatic forcing and that lake basin features modified by anthropogenic activity act as basin-specific filters of common forcing. Our results indicate that snow and glacial meltwaters along with nutrient enrichment related to climate warming and atmospheric Nr deposition, represent major threats for lake ecosystems, even in remote areas. We reveal that climate and atmospheric contaminants will further impact ecological conditions and alter aquatic food webs in higher altitude biomes if climate and anthropogenic forcing continue.

Contrasting potential for biological N2 fixation at three polluted central European Sphagnum peat bogs: combining the 15N2-tracer and natural-abundance isotope approaches

Abstract. Availability of reactive nitrogen (Nr) is a key control on carbon (C) sequestration in wetlands. To complement the metabolic demands of Sphagnum in pristine rain-fed bogs, diazotrophs supply additional Nr via biological nitrogen fixation (BNF). As breaking the triple bond of atmospheric N2 is energy-intensive, it is reasonable to assume that increasing inputs of pollutant Nr will lead to BNF downregulation. However, recent studies have also documented measurable BNF rates in Sphagnum-dominated bogs in polluted regions, indicating the adaptation of N2 fixers to changing N deposition. Our aim was to quantify BNF in high-elevation peatlands located in industrialized central Europe. A 15N2-tracer experiment was combined with a natural-abundance N-isotope study at three Sphagnum-dominated peat bogs in the northern Czech Republic in an attempt to assess the roles of individual BNF drivers. High short-term BNF rates (8.2 ± 4.6 g N m2 d−1) were observed at Malé mechové jezírko, which receives ∼ 17 kg Nr ha−1 yr−1. The remaining two peat bogs, whose recent atmospheric Nr inputs differed from Malé mechové jezírko by only 1–2 kg ha−1 yr−1 (Uhlír̆ská and Brumiště), showed zero BNF. The following parameters were investigated to elucidate the BNF difference: the NH4+-N / NO3--N ratio, temperature, wetness, Sphagnum species, organic-N availability, possible P limitation, possible molybdenum (Mo) limitation, SO42- deposition, and pH. At Malé mechové jezírko and Uhlír̆ská, the same moss species (S. girgensohnii) was used for the 15N2 experiment; therefore, the host identity could not explain the difference in BNF at these sites. Temperature and moisture were also identical in all incubations and could not explain the between-site differences in BNF. The N : P stoichiometry in peat and bog water indicated that Brumiště may have lacked BNF due to P limitation, whereas non-detectable BNF at Uhlír̆ská may have been related to the 70-fold higher SO42- concentration in bog water. Across the sites, the mean natural-abundance δ15N values increased in the following order: atmospheric deposition (−5.3 ± 0.3 ‰) < Sphagnum (−4.3 ± 0.1 ‰) < bog water (−3.9 ± 0.4 ‰) < atmospheric N2 (0.0 ‰). Only at Brumiště was N in Sphagnum significantly isotopically heavier than in atmospheric deposition, possibly indicating a longer-term BNF effect. Collectively, our data highlight spatial heterogeneity in BNF rates under high Nr inputs as well as the importance of environmental parameters other than atmospheric Nr pollution in regulating BNF.

Contrasting change trends in dry and wet nitrogen depositions during 2011 to 2020: Evidence from an agricultural catchment in subtropical Central China

Over the past decade, China has experienced a decline in atmospheric reactive nitrogen (Nr) emissions. Given that China's subtropical region is a significant nitrogen (N) deposition hotspot, it is essential to accurately quantify the ten-year variations in dry and wet N depositions in the context of reductions in atmospheric Nr emissions. Here, we evaluated the spatiotemporal variation in N deposition on forest, paddy field and tea field ecosystems in a typical subtropical agricultural catchment from 2011 to 2020. Our findings indicated a significant decrease in total N deposition in both the tea field ecosystem (41.5–30.5 kg N ha−1) and the forest ecosystem (40.8–25.7 kg N ha−1) (P < 0.05), but no significant change in the paddy field ecosystem (29.3–32.9 kg N ha−1). Specifically, dry N deposition exhibited significant declines except in the paddy field ecosystem, whereas wet N deposition had no significant change. The reduction in total oxidized and reduced N depositions in forest and tea field ecosystems is attributed to the decrease in NOx and NH3 emissions. Additionally, The ratio of NHx deposition to total N deposition all exceeded 0.5 in three ecosystems and the NHx/NOy ratio had an increasing trend (P < 0.05) in the paddy field, indicating that reactive N emissions from agricultural sources were the primary contributor to overall N deposition. Our study emphasizes that despite the decreasing trend in N deposition, it still exceeds the critical loads of natural ecosystems and requires stringent N emissions control, particularly from agricultural sources, in the future.

Marmots do not drink coffee: Human urine contributions to the nitrogen budget of a popular national park destination

AbstractReactive nitrogen (Nr) concentrations are higher than expected for mountain lakes in Rocky Mountain National Park, and for many years, high Nr concentrations have been attributed to atmospheric Nr deposition from regional and more distant emission sources, including combustion of fossil fuels and agricultural activities. Here, we estimated the contribution from a very local source, that of human urine, related to intensive use by visitors in Loch Vale Watershed (LVWS). Not only does urine convey hormones, pharmaceuticals, antibiotic‐resistant bacteria, and antibiotic‐resistant genes to the environment, but it also contributes Nr, which contributes to loss of biodiversity and eutrophication. Using caffeine as a specific marker for human urine, we compared the calculated maximum potential input of urine with that from wet atmospheric Nr deposition. The maximum potential input is a worst‐case scenario. Nearly 30,000 and 45,000 people hiked the 4.0 km to the Loch, the lowest lake in LVWS, in June–September 2019 and 2020, respectively. Informal trails and informal latrine sites were mapped, and the contribution of human urine was calculated based on several assumptions, including that each visitor voided their bladder on the ground once per visit somewhere in Loch Vale. The resulting Nr input from urine in Loch Vale for the summer months of June through September was 0.02 kg Nr ha−1, and prorated to a full year, the 2019 potential contribution of human waste was 0.06 kg ha−1 year−1. These values are compared with June–September 1.2 kg Nr ha−1 from wet atmospheric deposition or annual measured 2019 deposition of 2.5 kg Nr ha−1 year−1, to indicate a contribution of 2% Nr to the waters of Loch Vale from local human urine. Most Nr in this alpine and subalpine watershed is still attributable to emissions and subsequent wet atmospheric deposition, but a 2% contribution from human waste is not insignificant. In the very broadest sense, our results document an ecological disturbance from an unprecedented level of human activity in a protected and designated wilderness area. Local solutions to this local problem could include greater outreach to visitors of public lands about the consequences of their activities and installation of latrines.

Evaluation of global EMEP MSC-W (rv4.34) WRF (v3.9.1.1) model surface concentrations and wet deposition of reactive N and S with measurements

Abstract. Atmospheric pollution has many profound effects on human health, ecosystems, and the climate. Of concern are high concentrations and deposition of reactive nitrogen (Nr) species, especially of reduced N (gaseous NH3, particulate NH4+). Atmospheric chemistry and transport models (ACTMs) are crucial to understanding sources and impacts of Nr chemistry and its potential mitigation. Here we undertake the first evaluation of the global version of the EMEP MSC-W ACTM driven by WRF meteorology (1∘×1∘ resolution), with a focus on surface concentrations and wet deposition of N and S species relevant to investigation of atmospheric Nr and secondary inorganic aerosol (SIA). The model–measurement comparison is conducted both spatially and temporally, covering 10 monitoring networks worldwide. Model simulations for 2010 compared use of both HTAP and ECLIPSEE (ECLIPSE annual total with EDGAR monthly profile) emissions inventories; those for 2015 used ECLIPSEE only. Simulations of primary pollutants are somewhat sensitive to the choice of inventory in places where regional differences in primary emissions between the two inventories are apparent (e.g. China) but are much less sensitive for secondary components. For example, the difference in modelled global annual mean surface NH3 concentration using the two 2010 inventories is 18 % (HTAP: 0.26 µg m−3; ECLIPSEE: 0.31 µg m−3) but is only 3.5 % for NH4+ (HTAP: 0.316 µg m−3; ECLIPSEE: 0.305 µg m−3). Comparisons of 2010 and 2015 surface concentrations between the model and measurements demonstrate that the model captures the overall spatial and seasonal variations well for the major inorganic pollutants NH3, NO2, SO2, HNO3, NH4+, NO3-, and SO42- and their wet deposition in East Asia, Southeast Asia, Europe, and North America. The model shows better correlations with annual average measurements for networks in Southeast Asia (mean R for seven species: R7‾=0.73), Europe (R7‾=0.67), and North America (R7‾=0.63) than in East Asia (R5‾=0.35) (data for 2015), which suggests potential issues with the measurements in the latter network. Temporally, both model and measurements agree on higher NH3 concentrations in spring and summer and lower concentrations in winter. The model slightly underestimates annual total precipitation measurements (by 13 %–45 %) but agrees well with the spatial variations in precipitation in all four world regions (0.65–0.94 R range). High correlations between measured and modelled NH4+ precipitation concentrations are also observed in all regions except East Asia. For annual total wet deposition of reduced N, the greatest consistency is in North America (0.75–0.82 R range), followed by Southeast Asia (R=0.68) and Europe (R=0.61). Model–measurement bias varies between species in different networks; for example, bias for NH4+ and NO3- is largest in Europe and North America and smallest in East Asia and Southeast Asia. The greater uniformity in spatial correlations than in biases suggests that the major driver of model–measurement discrepancies (aside from differing spatial representativeness and uncertainties and biases in measurements) are shortcomings in absolute emissions rather than in modelling the atmospheric processes. The comprehensive evaluations presented in this study support the application of this model framework for global analysis of current and potential future budgets and deposition of Nr and SIA.

Nitrogen emission and deposition budget in an agricultural catchment in subtropical central China

The study of emissions and depositions of atmospheric reactive nitrogen species (Nrs) in a region is important to uncover the sources and sinks of atmospheric Nrs in the region. In this study, atmospheric total Nrs depositions including both wet-only and dry deposition were monitored simultaneously across major land-use types in a 105 km2 catchment called Jinjing River Catchment (JRC) in subtropical central China from 2015 to 2016. Based on activity data and emission factors for the main Nrs emission sources, ammonia (NH3) and nitrogen oxides (NOx) emission inventories for the catchment were also compiled. The estimated total Nrs deposition in JRC was 35.9 kg N ha−1 yr−1, with approximately 49.7 % attributed to reduced compounds (NHx), and 40.5 % attributed to oxidized (NOy). The total Nrs emission rate in JRC was 80.4 kg N ha−1 yr−1, with 61.5 and 18.9 kg N ha−1 yr−1 from NH3 and NOx emissions, respectively. Livestock excretion and fertilization were the two main contributing emission sources for NH3, while vehicle sources contributed the bulk of NOx emissions. The net atmospheric budgets of Nrs in paddy field, forest, and tea field were +3.7, −36.1, and +23.8 kg N ha−1 yr−1, respectively. At the catchment scale, the net atmospheric budget of Nrs was +47.7 kg N ha−1 yr−1, with +43.7 kg N ha−1 yr−1 of NHx and +4.0 kg N ha−1 yr−1 of NOy, indicating that the subtropical catchment was net sources of atmospheric Nrs. Considering that excessive atmospheric Nr emissions and deposition may cause adverse effects on the environment, effects should be conducted to mitigate the Nrs emissions from agriculture and transportation, and increasing the area of forest is good for reducing the net positive budget of atmospheric Nrs in the subtropical catchments in China.

Chronic atmospheric reactive N deposition has breached the N sink capacity of a northern ombrotrophic peatbog increasing the gaseous and fluvial N losses

Peatlands play an important role in modulating the climate, mainly through sequestration of carbon dioxide into peat carbon, which depends on the availability of reactive nitrogen (Nr) to mosses. Atmospheric Nr deposition in the UK has been above the critical load for functional and structural changes to peatland mosses, thus threatening to accelerate their succession by vascular plants and increasing the possibility of Nr export to downstream ecosystems. The N balance of peatlands has received comparatively little attention, mainly due to the difficulty in measuring gaseous N losses as well as the Nr inputs due to biological nitrogen fixation (BNF). In this study we have estimated the mean annual N balance of an ombrotrophic bog (Migneint, North Wales) by measuring in situ N2 + N2O gaseous fluxes and also BNF in peat and mosses. Fluvial N export was monitored through a continuous record of DON flux, while atmospheric N deposition was modelled on a 5 × 5 km grid. The mean annual N mass balance was slightly positive (0.7 ± 4.1 kg N ha−1 y−1) and varied interannually indicating the fragile status of this bog ecosystem that has reached N saturation and is prone to becoming a net N source. Gaseous N losses were a major N output term accounting for 70% of the N inputs, mainly in the form of the inert N2 gas, thus providing partial mitigation to the adverse effects of chronic Nr enrichment. BNF was suppressed by 69%, compared to rates in pristine bogs, but was still active, contributing ~2% of the N inputs. The long-term peat N storage rate (8.4 ± 0.8 kg N ha−1 y−1) cannot be met by the measured N mass balance, showing that the bog catchment is losing more N than it can store due its saturated status.

Chronic Atmospheric Reactive Nitrogen Deposition Suppresses Biological Nitrogen Fixation in Peatlands.

Biological nitrogen fixation (BNF) represents the natural pathway by which mosses meet their demands for bioavailable/reactive nitrogen (Nr) in peatlands. However, following intensification of nitrogen fertilizer and fossil fuel use, atmospheric Nr deposition has increased exposing peatlands to Nr loading often above the ecological threshold. As BNF is energy intensive, therefore, it is unclear whether BNF shuts down when Nr availability is no longer a rarity. We studied the response of BNF under a gradient of Nr deposition extending over decades in three peatlands in the U.K., and at a background deposition peatland in Sweden. Experimental nitrogen fertilization plots in the Swedish site were also evaluated for BNF activity. In situ BNF activity of peatlands receiving Nr deposition of 6, 17, and 27 kg N ha-1 yr-1 was not shut down but rather suppressed by 54, 69, and 74%, respectively, compared to the rates under background Nr deposition of ∼2 kg N ha-1 yr-1. These findings were corroborated by similar BNF suppression at the fertilization plots in Sweden. Therefore, contribution of BNF in peatlands exposed to chronic Nr deposition needs accounting when modeling peatland's nitrogen pools, given that nitrogen availability exerts a key control on the carbon capture of peatlands, globally.

Atmospheric Nitrogen Deposition to a Southeast Tibetan Forest Ecosystem

With atmospheric reactive nitrogen (Nr) emissions increasing globally, research into Nr deposition has attracted increasing attention, especially in remote environments. These ecosystems are very sensitive to global change, especially enhanced Nr deposition. Forest environments, in particular, are highlighted because of their important ecological function. We quantified atmospheric Nr concentrations and deposition over four years of continuous monitoring in a southeast Tibetan boreal forest ecosystem, an ecosystem in which forest biomass and carbon density are high around the world. Average annual bulk Nr deposition was 3.00 kg N ha−1 y−1, with those of reduced and oxidized species estimated at 1.60 and 1.40 kg N ha−1 y−1, respectively. Bulk deposition of both NH4+ and NO3− were controlled by precipitation amount: both Nr deposition and precipitation were highest in summer and lowest in winter. Dry deposition of NH3 and NO2 were 1.18 and 0.05 kg N ha−1 y−1, respectively. Atmospheric NH3 concentrations were in the range 1.15–3.53 mg N L−1, highest in summer and lowest in winter. In contrast, no clear trend in seasonal NO2 concentrations was observed. Monthly NO2 concentrations were 0.79–1.13 mg N L−1. Total Nr deposition (bulk plus dry) was 4.23 (3.00 + 1.23) kg N ha−1 y−1 in the forest. Reduced nitrogen was the dominant species. In conclusion, Nr deposition was in the range at which forest net productivity and carbon sequestration are sensitive to any variation in nitrogen input, so quantification of Nr deposition should continue and with greater detail.

Reviewing global estimates of surface reactive nitrogen concentration and deposition using satellite retrievals

Abstract. Since the industrial revolution, human activities have dramatically changed the nitrogen (N) cycle in natural systems. Anthropogenic emissions of reactive nitrogen (Nr) can return to the earth's surface through atmospheric Nr deposition. Increased Nr deposition may improve ecosystem productivity. However, excessive Nr deposition can cause a series of negative effects on ecosystem health, biodiversity, soil, and water. Thus, accurate estimations of Nr deposition are necessary for evaluating its environmental impacts. The United States, Canada and Europe have successively launched a number of satellites with sensors that allow retrieval of atmospheric NO2 and NH3 column density and therefore estimation of surface Nr concentration and deposition at an unprecedented spatiotemporal scale. Atmosphere NH3 column can be retrieved from atmospheric infra-red emission, while atmospheric NO2 column can be retrieved from reflected solar radiation. In recent years, scientists attempted to estimate surface Nr concentration and deposition using satellite retrieval of atmospheric NO2 and NH3 columns. In this study, we give a thorough review of recent advances of estimating surface Nr concentration and deposition using the satellite retrievals of NO2 and NH3, present a framework of using satellite data to estimate surface Nr concentration and deposition based on recent works, and summarize the existing challenges for estimating surface Nr concentration and deposition using the satellite-based methods. We believe that exploiting satellite data to estimate Nr deposition has a broad and promising prospect.

Satellite-derived leaf area index and roughness length information for surface–atmosphere exchange modelling: a case study for reactive nitrogen deposition in north-western Europe using LOTOS-EUROS v2.0

Abstract. The nitrogen cycle has been continuously disrupted by human activity over the past century, resulting in almost a tripling of the total reactive nitrogen fixation in Europe. Consequently, excessive amounts of reactive nitrogen (Nr) have manifested in the environment, leading to a cascade of adverse effects, such as acidification and eutrophication of terrestrial and aquatic ecosystems, and particulate matter formation. Chemistry transport models (CTMs) are frequently used as tools to simulate the complex chain of processes that determine atmospheric Nr flows. In these models, the parameterization of the atmosphere–biosphere exchange of Nr is largely based on few surface exchange measurement and is therefore known to be highly uncertain. In addition to this, the input parameters that are used here are often fixed values, only linked to specific land use classes. In an attempt to improve this, a combination of multiple satellite products is used to derive updated, time-variant leaf area index (LAI) and roughness length (z0) input maps. As LAI, we use the Moderate Resolution Imaging Spectroradiometer (MODIS) MCD15A2H product. The monthly z0 input maps presented in this paper are a function of satellite-derived normalized difference vegetation index (NDVI) values (MYD13A3 product) for short vegetation types (such as grass and arable land) and a combination of satellite-derived forest canopy height and LAI for forests. The use of these growth-dependent satellite products allows us to represent the growing season more realistically. For urban areas, the z0 values are updated, too, and linked to a population density map. The approach to derive these dynamic z0 estimates can be linked to any land use map and is as such transferable to other models. We evaluated the sensitivity of the modelled Nr deposition fields in LOng Term Ozone Simulation – EURopean Operational Smog (LOTOS-EUROS) v2.0 to the abovementioned changes in LAI and z0 inputs, focusing on Germany, the Netherlands and Belgium. We computed z0 values from FLUXNET sites and compared these to the default and updated z0 values in LOTOS-EUROS. The root mean square difference (RMSD) for both short vegetation and forest sites improved. Comparing all sites, the RMSD decreased from 0.76 (default z0) to 0.60 (updated z0). The implementation of these updated LAI and z0 input maps led to local changes in the total Nr deposition of up to ∼30 % and a general shift from wet to dry deposition. The most distinct changes are observed in land-use-specific deposition fluxes. These fluxes may show relatively large deviations, locally affecting estimated critical load exceedances for specific natural ecosystems.

Atmospheric reactive nitrogen concentration and deposition trends from 2011 to 2018 at an urban site in north China

Atmospheric reactive nitrogen (Nr) emissions and deposition have caused much damage to the global environment and also impacted human health. Determining long-term trends in atmospheric Nr concentrations and deposition is important for evaluating air quality and ecosystem effects and formulating control measures to reduce negative impacts. We measured monthly concentrations of NH3, pNH4+, HNO3, pNO3-, and NO2 and calculated their dry deposition rates using an inferential method from 2011 to 2018 at an urban site in Zhengzhou in the North China Plain. Inorganic N (NH4+ and NO3−) concentrations in precipitation and wet/bulk N deposition were also quantified in 2011, 2016, 2017 and 2018. The results showed high concentrations of atmospheric Nr species, with considerable variation in some species over the monitoring period. Annual mean NH3 concentrations significantly increased from 2011 to 2018 (p < 0.001) with that in 2018 being twice that in 2011. However, pNO3- and pNH4+ concentrations decreased after 2013. The annual mean NO2 concentration in 2018 had decreased by 33.4% compared to the peak in 2011. Highest NH3 concentrations were measured in spring or summer, and lowest concentrations in winter. pNH4+ and pNO3- concentrations peaked mainly in winter. Total annual dry N deposition ranged from 21.9 to 41.2 kg N ha−1. Bulk NH4+ and NO3− deposition was lower in 2016–2018 than in 2011. However, atmospheric Nr concentrations and N deposition remained high in this urban ecosystem. NH3 emissions urgently need control measures, and Nr impacts on the surrounding ecosystems and human health should be considered. Continuous long-term measurements of atmospheric Nr concentrations and deposition at a regional scale are necessary to help formulate emission control measures and protect human and ecosystem health.

δ15N systematics in two minerotrophic peatlands in the eastern U.S.: Insights into nitrogen cycling under moderate pollution

Availability of reactive nitrogen (Nr; mainly ammonium, NH4+, and nitrate, NO3−) via precipitation and upwelling groundwater affects carbon (C) accumulation in peat deposits and biological greenhouse gas emissions. Isotope composition of Nr was studied at Wolf Swamp (WS) and The Glades (GL), two peatlands located in western Maryland (U.S.), to provide a more robust understanding of N cycling at the ecosystem level. In recent years, both sites experienced a moderate atmospheric pollution (3–9 kg of deposited Nr ha−1 yr−1), and possible additional Nr inputs via groundwater contaminated by fertilizers/livestock. Sampling of living Sphagnum, vertical peat profiles, atmospheric deposition, and surface bog water was carried out on October 22–24, 2017. The δ15Nr signatures of vertical precipitation were negative (mass-weighted means of −7.2 and −8.1‰ at WS and GL, respectively) despite reports that NOx emitted by coal-burning power plants, a precursor of deposited nitrate, had been extremely positive (up to +26‰). Mean δ15N values at both sites increased in the order: vertical precipitation < living Sphagnum < atmospheric N2 < surface bog water < maturating peat. Intermediate δ15N values of living Sphagnum (−1.5‰ at both sites) suggested simultaneous incorporation of isotopically light atmospheric Nr and isotopically heavy Nr from bog water in the newly formed biomass. The δ15N values of bulk peat increased downcore by 4.5 and 4.9‰ at WS and GL, respectively. Gradual mineralization of organic N, mainly by nitrification and denitrification, is isotopically selective, leaving behind higher δ15N values of bulk peat at greater peat depths. Nitrogen concentration gradients and δ15N trends in vertical peat profiles at these locations therefore cannot be used as an archive of past changes in pollution levels and N isotope signatures of incoming Nr.

Spatial–temporal patterns of inorganic nitrogen air concentrations and deposition in eastern China

Abstract. Five-year (2011–2015) measurements of gaseous NH3, NO2, and HNO3 and particulate NH4+ and NO3− in air and/or precipitation were conducted at 27 sites in the Nationwide Nitrogen Deposition Monitoring Network (NNDMN) to better understand spatial and temporal (seasonal and annual) characteristics of reactive nitrogen (Nr) concentrations and deposition in eastern China. Our observations reveal annual average concentrations (16.4–32.6 µg N m−3), dry deposition fluxes (15.8–31.7 kg N ha−1 yr−1), and wet/bulk deposition fluxes (18.4–28.0 kg N ha−1 yr−1) based on land use, ranked as urban &gt; rural &gt; background sites. Annual concentrations and dry deposition fluxes of each Nr species in air were comparable at urban and background sites in northern and southern regions, but were significantly higher at northern rural sites. These results, together with good agreement between spatial distributions of NH3 and NO2 concentrations determined from ground measurements and satellite observations, demonstrate that atmospheric Nr pollution is heavier in the northern region than in the southern region. No significant inter-annual trends were found in the annual Nr dry and wet/bulk N deposition at almost all of the selected sites. A lack of significant changes in annual averages between the 2013–2015 and 2011–2012 periods for all land use types suggests that any effects of current emission controls are not yet apparent in Nr pollution and deposition in the region. Ambient concentrations of total Nr exhibited non-significant seasonal variation at all land use types, although significant seasonal variations were found for individual Nr species (e.g. NH3, NO2, and pNO3−) in most cases. In contrast, dry deposition of total Nr exhibited a consistent and significant seasonal variation at all land use types, with the highest fluxes in summer and the lowest in winter. Based on sensitivity tests by the GEOS-Chem model, we found that NH3 emissions from fertilizer use (including chemical and organic fertilizers) were the largest contributor (36 %) to total inorganic Nr deposition over eastern China. Our results not only improve the understanding of spatial–temporal variations of Nr concentrations and deposition in this pollution hotspot, but also provide useful information for policy-makers that mitigation of NH3 emissions should be a priority to tackle serious N deposition in eastern China.

Atmospheric deposition of inorganic nitrogen in a semi-arid grassland of Inner Mongolia, China

Due to increasing global demand for crop production and energy use, more and more reactive nitrogen (Nr) has been generated and emitted to the environment. As a result, global atmospheric nitrogen (N) deposition has tripled since the industrial revolution and the ecological environment and human health have been harmed. In this study, we measured dry and wet/bulk N deposition from July 2013 to December 2015 in a semi-arid grassland of Duolun County, Inner Mongolia, China. The samples of dry and wet/bulk N deposition were collected monthly with a DELTA (DEnuder for Long Term Atmospheric sampling) system and with Gradko passive samplers and a precipitation gauge. The measured results show that the annual mean concentrations of NH3, NO2, HNO3, particulate NH4+ (pNH4+) and particulate NO3− (pNO3−) in atmosphere were 2.33, 1.90, 0.18, 1.42 and 0.42 μg N/m3, respectively, and that the annual mean volume-weighted concentrations ofNH4+-N and NO3−-N in precipitation were 2.71 and 1.99 mg N/L, respectively. The concentrations of Nr components (including NH3, NO2, HNO3, pNH4+, pNO3−,NH4+-N and NO3−-N) exhibited different seasonal variations. Specifically, NO2 and HNO3 exhibited higher concentrations in autumn than in summer, while the other Nr components (NH3, pNH4+, pNO3−,NH4+-N and NO3−-N) showed the highest values in summer. Based on measured concentrations of Nr components and their deposition velocities estimated using the GEOS-Chem global atmospheric chemical transport model, the calculated annual mean dry deposition fluxes were 3.17, 1.13, 0.63, 0.91 and 0.36 kg N/(hm2•a) for NH3, NO2, HNO3, pNH4+ and pNO3−, respectively, and the calculated annual mean wet/bulk deposition fluxes were 5.37 and 3.15 kg N/(hm2•a) forNH4+-N and NO3−-N, respectively. The estimated annual N deposition (including dry N deposition and wet/bulk N deposition) reached 14.7 kg N/(hm2•a) in grassland of Duolun County, approaching to the upper limit of the N critical load (10–15 kg N/(hm2•a)). Dry and wet/bulk deposition fluxes of all Nr components (with an exception of HNO3) showed similar seasonal variations with the maximum deposition flux in summer and the minimum in winter. Reduced Nr components (e.g., gaseous NH3 and pNH4+ in atmosphere andNH4+-N in precipitation) dominated the total N deposition at the sampling site (accounted for 64% of the total N deposition), suggesting that the deposited atmospheric Nr mainly originated from agricultural activities. Considering the projected future increases in crop and livestock production in Inner Mongolia, the ecological and human risks to the negative effects of increased N deposition could be increased if no mitigation measures are taken.

Using nitrogen concentration and isotopic composition in lichens to spatially assess the relative contribution of atmospheric nitrogen sources in complex landscapes

Reactive nitrogen (Nr) is an important driver of global change, causing alterations in ecosystem biodiversity and functionality. Environmental assessments require monitoring the emission and deposition of both the amount and types of Nr. This is especially important in heterogeneous landscapes, as different land-cover types emit particular forms of Nr to the atmosphere, which can impact ecosystems distinctively. Such assessments require high spatial resolution maps that also integrate temporal variations, and can only be feasibly achieved by using ecological indicators. Our aim was to rank land-cover types according to the amount and form of emitted atmospheric Nr in a complex landscape with multiple sources of N. To do so, we measured and mapped nitrogen concentration and isotopic composition in lichen thalli, which we then related to land-cover data. Results suggested that, at the landscape scale, intensive agriculture and urban areas were the most important sources of Nr to the atmosphere. Additionally, the ocean greatly influences Nr in land, by providing air with low Nr concentration and a unique isotopic composition. These results have important consequences for managing air pollution at the regional level, as they provide critical information for modeling Nr emission and deposition across regional as well as continental scales.

Concentrations and isotopic characteristics of atmospheric reactive nitrogen around typical sources in Beijing, China

With rapid economic growth in China, anthropogenic reactive nitrogen (Nr) emissions have more than doubled over the last two or three decades. Atmospheric Nr pollution is an environmental concern in China especially in megacities such as Beijing. In order to identify the impact of emission sources on atmospheric Nr pollution, we measured atmospheric Nr concentrations and their isotopic composition (δ15N) dynamics at three typical sites: landfill, pig farm and road traffic sites in Beijing from April 2010 to March 2011. Passive samplers were used for monitoring ammonia (NH3) and nitrogen dioxide (NO2), two major Nr species, while their δ15N values were measured by a diffusion method combined with mass spectrometer approach. The raw water pool of the landfill and fattening house of the pig farm were important NH3 sources with mean NH3 concentrations being 2,829 and 2,369 µg/m3, respectively, while the road traffic site was a minor NH3 source (10.6 µg/m3). NH3 concentrations at sites besides the landfill and roads were high in summer and low in winter due to the annual variation of temperature and the change of emission source intensity. In contrast, the NH3 concentrations inside the pig farm house were high in winter and low in summer, for the barn windows were open in summer and closed in winter. The mean NO2 concentrations were 89.8, 32.9 and 23.0 µg/m3 at the road traffic, the landfill and pig farm sites, respectively. Due to vehicle fuel combustion, NO2 concentration at the road traffic was the highest among the three sources, and the road traffic was a main NO2 emission source. PM10, pNH4 + and pNO3–concentrations in particulate matter were higher in summer than in winter (except PM10 for the pig farm). The δ15NH3 values ranged from–19.14‰ to 7.82‰, with an average of–0.05‰ for the landfill site, and the lowest values were observed in June and July. The δ15NH3 values for the pig farm site ranged from–29.78‰ to–14.05‰ with an average of–24.51‰, and the δ15NH3 values were more negative in summer than in the other seasons. The δ15NO2 values were–9.63‰ to 7.04‰ with an average of–3.72‰ for the road traffic site. The δ15NO2 values were more negative in summer than those in the other seasons. The different δ15N values for the various Nr species in different sources may serve as important indicators for identifying atmospheric Nr sources in megacities. The results may also provide the theoretical basis for research on the atmospheric N deposition and its sources.

Different responses of sedimentary δ15N to climatic changes and anthropogenic impacts in lakes across the Eastern margin of the Tibetan Plateau

Knowledge of historical variations in sedimentary δ15N values and their relation to modern ones in lakes is critical in understanding N cycling in inland aquatic systems. Here we present late Holocene sedimentary δ15N variations in four lakes across the Eastern margin of the Tibetan Plateau (ETP), namely Lake Qinghai and Lake Chaonaqiu over north ETP (N-ETP) area, and Lake Yihai and Lake Lugu over south ETP (S-ETP) area. The results show that: (1) during historical periods when anthropogenic forcing was weak, the centennial/multi-decadal changes in δ15N values were dominated by climatic changes. Higher precipitation brought greater nutrients supply and led to higher Lacustrine Primary Productivity (LPP) and higher δ15N values in lake sediments, and vice versa. (2) During the recent/modern epoch with enhanced anthropogenic impacts, sedimentary δ15N values in Lake Qinghai and Lake Chaonaqiu (mid-high latitude lakes with low LPP) showed decreasing trends, possibly related to the increased global atmospheric reactive nitrogen (Nr) deposition. On the contrary, sedimentary δ15N values in Lake Lugu and Lake Yihai (subtropical lakes with high LPP) showed increasing trends, most likely because more dissolved inorganic nitrogen in lake water was used during LPP processes. This study highlights that LPP plays an important role in modifying sedimentary δ15N values both for historical and modern times, whereas atmospheric Nr deposition seems to influence the modern/recent sedimentary δ15N trends in lakes with low LPP.

Fewer new species colonize at low frequency N addition in a temperate grassland

Summary Biologically reactive nitrogen (Nr) enrichment threatens biodiversity in diverse ecosystems. Previous controlled N addition experiments may overestimate the effects of atmospheric Nr deposition on the rate of species loss, as it has been found that low frequency Nr additions, as used in traditional studies, lead to more rapid biodiversity loss. It remains unclear, however, whether the colonization of new species (gain) or extinction of old species (loss) is the cause of this difference. By independently manipulating the frequency (twice vs. monthly additions year−1) and the rate (from 0 to 50 g N m−2 year−1) of NH4NO3 inputs for six years in a temperate grassland of northern China, we aimed to examine the contribution of gain and loss of species to the reduction in species richness under different regimes of Nr inputs. Results showed that the gain of new species was higher at a high frequency of N addition than that at a low addition frequency, while loss of existing species was similar between the two frequencies of N addition. The number of new species gained decreased and old species lost increased with the increasing rate of Nr addition at both annual and five‐year intervals. Cumulative gain of new species was negatively correlated with soil acidification, ammonium concentration and community biomass accumulation, whereas cumulative loss of old species was positively correlated with these variables. Our results revealed lower new species colonization results in lower species richness at low frequency of Nr addition. Findings from this study highlight the important role of N addition frequency in regulating the effects of Nr addition on community dynamics. To assess the effects of atmospheric Nr deposition on ecosystem structure and functioning, it is necessary to assess not only the dose but also the frequency of N addition.