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Atmospheric ammonia (NH3) and ammonium (NH4+) can substantially influence air quality, ecosystems, and climate. NH3 volatilization from fertilizers and wastes (v-NH3) has long been assumed to be the primary NH3 source, but the contribution of combustion-related NH3 (c-NH3, mainly fossil fuels and biomass burning) remains unconstrained. Here, we collated nitrogen isotopes of atmospheric NH3 and NH4+ and established a robust method to differentiate v-NH3 and c-NH3. We found that the relative contribution of the c-NH3 in the total NH3 emissions reached up to 40 ± 21% (6.6 ± 3.4 Tg N yr−1), 49 ± 16% (2.8 ± 0.9 Tg N yr−1), and 44 ± 19% (2.8 ± 1.3 Tg N yr−1) in East Asia, North America, and Europe, respectively, though its fractions and amounts in these regions generally decreased over the past decades. Given its importance, c-NH3 emission should be considered in making emission inventories, dispersion modeling, mitigation strategies, budgeting deposition fluxes, and evaluating the ecological effects of atmospheric NH3 loading.

Anthropogenic nitrogen (N) emissions and deposition have been increasing over past decades. However, spatiotemporal variations of N deposition levels and major sources remain unclear in many regions, which hinders making strategies of emission mitigation and evaluating effects of elevated N deposition. By investigating moss N contents and δ15 N values in southwestern (SW) China in 1954-1964, 1970-1994, and 2005-2015, we reconstructed fluxes and source contributions of atmospheric ammonium ( NH4+ ) and nitrate ( NO3- ) deposition and evaluated their historical changes. For urban and non-urban sites, averaged moss N contents did not differ between 1954-1964 and 1970-1994 (1.2%-1.3%) but increased distinctly in 2005-2015 (1.6%-2.3%), and averaged moss δ15 N values decreased from +0.4‰ to +3.3‰ in 1954-1964 to -1.9‰ to -0.7‰ in 1974-1990, and to -4.8‰ to -3.6‰ in 2005-2015. Based on quantitative estimations, N deposition levels from the 1950s to the 2000s did not change in the earlier 20 years but were elevated substantially in the later 30 years. Moreover, the elevation of NH4+ deposition (by 12.2 kg-N/ha/year at urban sites and 4.6 kg-N/ha/year at non-urban sties) was higher than that of NO3- deposition (by 6.0 and 2.9 kg-N/ha/year, respectively) in the later 30 years. This caused a shifted dominance from NO3- to NH4+ in N deposition. Based on isotope source apportionments, contributions of combustion-related NH3 sources (vehicle exhausts, coal combustion, and biomass burning) to the elevation of NH4+ deposition were two times higher than volatilization NH3 sources (wastes and fertilizers) in the later 30 years. Meanwhile, non-fossil fuel NOx sources (biomass burning, microbial N cycles) contributed generally more than fossil fuel NOx sources (vehicle exhausts and coal combustion) to the elevation of NO3- deposition. These results revealed significant contributions of combustion-related NH3 and non-fossil fuel NOx emissions to the historical elevation of N deposition in SW China, which is useful for emission mitigation and ecological effect evaluation of atmospheric N loading.

Aims Climate warming and increasing nitrogen (N) deposition have influenced plant nutrient status and thus plant carbon (C) fixation and vegetation composition in boreal peatlands. Phenols, which are secondary metabolites in plants for defense and adaptation, also play important roles in regulating peatland C dynamics due to their anti-decomposition properties. However, how the phenolic levels of different functional types of plants vary depending on nutrient availability remain unclear in boreal peatlands. Methods Here, we investigated total phenols contents (TPC) and total tannins contents in leaves of 11 plant species in 18 peatlands of the Great Hing’an Mountains area in northeastern China, and examined their variations with leaf N and phosphorus (P) and underlying mechanisms. Important Findings Shrubs had higher TPC than graminoids, indicating less C allocation to defense and less uptake of organic N in faster-growing and nonmycorrhizal graminoids than in slower-growing and mycorrhizal shrubs. For shrubs, leaf TPC decreased with increasing N contents but was not influenced by changing leaf phosphorus (P) contents, which suggested that shrubs would reduce the C investment for defense with increasing N availability. Differently, leaf TPC of graminoids increased with leaf N contents and decreased with leaf P contents. As graminoids are more N-limited and less P-limited, we inferred that graminoids would increase the defensive C investment under increased nutrient availability. We concluded that shrubs would invest more C in growth than in defense with increasing N availability, but it was just opposite for graminoids, which might be an important mechanism to explain the resource competition and encroachment of shrubs in boreal peatlands in the context of climate warming and ever-increasing N deposition.

The elevation of nitrogen (N) deposition by urbanization profoundly impacts the structure and function of surrounding forest ecosystems. Plants are major biomass sinks of external N inputs into forests. Yet, the N-use strategies of forest plants in many areas remain unconstrained in city areas, so their responses and adapting mechanisms to the elevated N deposition are open questions. Here we investigated concentrations and N isotope (δ

To examine the perturbation of atmospheric nitrogen (N) deposition on soil N status and the biogeochemical cycle is meaningful for understanding forest function evolution with environmental changes. However, levels of soil bioavailable N and their environmental controls in forests receiving high atmospheric N deposition remain less investigated, which hinders evaluating the effects of enhanced anthropogenic N loading on forest N availability and N losses. This study analyzed concentrations of soil extractable N, microbial biomass N, net rates of N mineralization and nitrification, and their relationships with environmental factors among 26 temperate forests under the N deposition rates between 28.7 and 69.0 kg N ha

Exotic plant invasion is an urgent issue occurring in the biosphere, which can be stimulated by environmental nitrogen (N) loading. However, the allocation and assimilation of soil N sources between leaves and roots remain unclear for plants in invaded ecosystems, which hampers the understanding of mechanisms behind the expansion of invasive plants and the co-existence of native plants. This work established a new framework to use N concentrations and isotopes of soils, roots, and leaves to quantitatively decipher intra-plant N allocation and assimilation among plant species under no invasion and under the invasion of Chromolaena odorata and Ageratina adenophora in a tropical ecosystem of SW China. We found that the assimilation of N derived from both soil ammonium (NH4+) and nitrate (NO3-) were higher in leaves than in roots for invasive plants, leading to higher leaf N levels than native plants. Compared with the same species under no invasion, most native plants under invasion showed higher N concentrations and NH4+ assimilations in both leaves and roots, and increases in leaf N were higher than in root N for native plants under invasion. These results inform that preferential N allocation, dominated by NH4+-derived N, to leaves over roots as an important N-use strategy for plant invasion and co-existence in the studied tropical ecosystem.

Nitrate (NO3−) is a key component of secondary inorganic aerosols and PM2.5. However, the contributions of nitrogen oxides (NOx) emission sources to NO3− in PM2.5 remain poorly constrained. This study measured nitrogen (N) isotopes of NO3− (hereafter as δ15N-NO3-) in PM2.5 collected at Beijing in 2014. We observed that δ15N-NO3- values in PM2.5 (−2.3‰ − 19.7‰; 7.3 ± 5.4‰ annually) were significantly higher in winter (11.9 ± 4.4‰) than in summer (2.2 ± 2.5‰). The δ15N differences between source NOx and NO3− in PM2.5 (hereafter as Δ values) were estimated by a computation module as 7.8 ± 2.2‰ − 10.4 ± 1.6‰ (8.8 ± 2.4‰). Using the Δ values and δ15N values of NOx from major fossil (coal combustion, vehicle exhausts) and non-fossil sources (biomass burning, microbial N cycle), contributions of major NOx sources to NO3− in PM2.5 were further estimated by the SIAR model. We found that seasonal variations of δ15N-NO3- values in PM2.5 of Beijing were mainly caused by those of NOx contributions from coal combustion (38 ± 10% in winter, 20 ± 9% in summer). Annually, NOx from coal combustion, vehicle exhausts, biomass burning, and microbial N cycle contributed 28 ± 12%, 29 ± 17%, 27 ± 15%, and 16 ± 7% to NO3− in PM2.5, respectively, showing actually comparable contributions between non-fossil NOx (43 ± 16%) and fossil NOx (57 ± 21%). These results are useful for planning the reduction of NOx emissions in city environments and for elucidating relationships between regional NOx emissions and atmospheric NO3− pollution or deposition.

As an important factor related to the self-purification capacity (e.g. denitrification, burial rate, and downstream output) in aquatic systems, water residence time (WRT) has great impacts on the nitrogen (N) dynamics and its removal process in lakes and reservoirs. In this study, we have analysed the impacts of WRT on the change rates of total nitrogen (TN) concentrations in 50 waterbodies (including 33 lakes and 17 reservoirs) in China, with different change trends (e.g. increasing trends and decreasing trends) and TN concentrations during 2012–2016. Based on the annual ecosystem-scale N mass balance, TN input and output flux in the waterbodies are estimated. The results showed that the decreases of TN concentrations usually occur in the waterbodies with the relatively high TN concentrations in 2012, and WRT has significant impacts on the TN change rates in the waterbodies. Longer WRT could slow down the TN increasing rates in the waterbodies acting as N sinks, but could accelerate the removal from the waterbodies acting as N sources. Higher water phosphorus (P) concentrations could also be beneficial for the faster N removal from the waterbodies, which is mediated via the coupled processes regulating the N transfer from water column to anoxic sediments. China has recently issued the “lake-chief” systems, addressing the specific and flexible strategies for water pollution control in different lakes. The self-purification capacity through denitrification and burial rate, which are closely related to WRT, should be taken into consideration when making specific water management plans in the future.

Anthropogenic nitrogen (N) emissions and deposition have been increasing over past decades. However, spatiotemporal variations of N deposition levels and major sources remain unclear in many regions, which hinders making strategies of emission mitigation and evaluating effects of elevated N deposition. By investigating moss N contents and δ

The ecosystem carbon (C) balance in permafrost regions, which has a global significance in understanding the terrestrial C-climate feedback, is significantly regulated by nitrogen (N) dynamics. However, our knowledge on temporal changes in vegetation N limitation (i.e., the supply of N relative to plant N demand) in permafrost ecosystems is still limited. Based on the combination of isotopic observations derived from a re-sampling campaign along a ~3000 km transect and simulations obtained from a process-based biogeochemical model, here we detect changes in ecosystem N cycle across the Tibetan alpine permafrost region over the past decade. We find that vegetation N limitation becomes stronger despite the increased available N production. The enhanced N limitation on vegetation growth is driven by the joint effects of elevated plant N demand and gaseous N loss. These findings suggest that N would constrain the future trajectory of ecosystem C cycle in this alpine permafrost region., Massive stores of carbon and nutrients in permafrost could be released by global warming. Here the authors show that though warming across the Tibetan alpine permafrost region accelerates nitrogen liberation, contrary to expectations the elevated nutrients do not alleviate plant nitrogen limitation.

Conifers are considered to prefer to take up ammonium (NH4 + ) over nitrate (NO3 - ). However, this conclusion is mainly based on hydroponic experiments that separate roots from soils. It remains unclear to what extent mature conifers can use nitrate compared to ammonium under field conditions where both roots and soil microbes compete for nitrogen (N). We conducted an in situ whole mature tree nitrogen-15 (15 N) labeling experiment (15 NH4 + vs 15 NO3 - ) over 15 d to quantify ammonium and nitrate uptake and assimilation rates in four 40-yr-old monoculture coniferous plantations (Pinus koraiensis, Pinus sylvestris, Picea koraiensis and Larix olgensis, respectively). For the whole tree, 15 NO3 - contributed 39% to 90% to total 15 N tracer uptake among four plantations during the study period. At day 3, the 15 NO3 - accounted for 77%, 64%, 62% and 59% by Larix olgensis, Pinus koraiensis, Pinus sylvestris and Picea koraiensis, respectively. Our study indicates that mature coniferous trees assimilated nitrate as efficiently as ammonium from soils even at low soil nitrate concentration, in contrast to the results from hydroponic experiments showing that ammonium uptake dominated over nitrate. This implies that mature conifers can adapt to increasing availability of nitrate in soil, for example, under the context of globalization of N deposition and global warming.

Large-scale and rapid improvement in wastewater treatment is common practice in developing countries, yet this influence on nutrient regimes in receiving waterbodies is rarely examined at broad spatial and temporal scales. Here, we present a study linking decadal nutrient monitoring data in lakes with the corresponding estimates of five major anthropogenic nutrient discharges in their surrounding watersheds over time. Within a continuous monitoring dataset covering the period 2008 to 2017, we find that due to different rates of change in TN and TP concentrations, 24 of 46 lakes, mostly located in China's populated regions, showed increasing TN/TP mass ratios; only 3 lakes showed a decrease. Quantitative relationships between in-lake nutrient concentrations (and their ratios) and anthropogenic nutrient discharges in the surrounding watersheds indicate that increase of lake TN/TP ratios is associated with the rapid improvement in municipal wastewater treatment. Due to the higher removal efficiency of TP compared with TN, TN/TP mass ratios in total municipal wastewater discharge have continued to increase from a median of 10.7 (95% confidence interval, 7.6 to 15.1) in 2008 to 17.7 (95% confidence interval, 13.2 to 27.2) in 2017. Improving municipal wastewater collection and treatment worldwide is an important target within the 17 sustainable development goals set by the United Nations. Given potential ecological impacts on biodiversity and ecosystem function of altered nutrient ratios in wastewater discharge, our results suggest that long-term strategies for domestic wastewater management should not merely focus on total reductions of nutrient discharges but also consider their stoichiometric balance.

Plant nitrogen (N) use is a key component of the N cycle in terrestrial ecosystems. The supply of N to plants affects community species composition and ecosystem processes such as photosynthesis and carbon (C) accumulation. However, the availabilities and relative importance of different N forms to plants are not well understood. While nitrate (NO3−) is a major N form used by plants worldwide, it is discounted as a N source for Arctic tundra plants because of extremely low NO3− concentrations in Arctic tundra soils, undetectable soil nitrification, and plant-tissue NO3− that is typically below detection limits. Here we reexamine NO3− use by tundra plants using a sensitive denitrifier method to analyze plant-tissue NO3−. Soil-derived NO3− was detected in tundra plant tissues, and tundra plants took up soil NO3− at comparable rates to plants from relatively NO3−-rich ecosystems in other biomes. Nitrate assimilation determined by 15N enrichments of leaf NO3− relative to soil NO3− accounted for 4 to 52% (as estimated by a Bayesian isotope-mixing model) of species-specific total leaf N of Alaskan tundra plants. Our finding that in situ soil NO3− availability for tundra plants is high has important implications for Arctic ecosystems, not only in determining species compositions, but also in determining the loss of N from soils via leaching and denitrification. Plant N uptake and soil N losses can strongly influence C uptake and accumulation in tundra soils. Accordingly, this evidence of NO3− availability in tundra soils is crucial for predicting C storage in tundra., ツンドラの生態系でも硝酸イオンは大切な窒素源だった --最先端の測定技術で「見えない」硝酸イオンの重要性を検証--. 京都大学プレスリリース. 2018-03-14.

Elevated ozone (O3) decreases nitrogen derived from rhizodeposition (NdfR). However, the changes in the partitioning of NdfR in soil N pools due to O3 remain unclear. The aims of this study were to investigate the contribution of NdfR to different soil N pools and its response to elevated O3 conditions. Spring wheat was labeled with 15N–urea using a split-root technique under ambient (~40 ppb) and elevated O3 treatments (60 ± 5 ppb and 110 ± 5 ppb) in open-top chambers. Mineral-N, microbial biomass (MB)-N and fixed ammonium (FA)-N in rhizospheric soils were analyzed. N rhizodeposition contributed 12–33% of mineral N, 10–14% of FA-N and 6–16% of MB-N under ambient O3. Elevated O3 significantly decreased mineral-15NdfR and increased FA-15NdfR but had no significant influence on MB-15NdfR. The decrease in mineral-NdfR was likely due to the decrease in rhizodeposition inputs and the increase in FA-NdfR. Our results showed that elevated O3 altered the contribution of NdfR to soil N pools. The present study increases our understanding of the dynamics of NdfR and the changes in soil N cycling induced by projected future O3 levels.

Anthropogenic nitrogen (N) and sulfur (S) emissions triggered acid deposition and affected the environmental quality and ecosystem functions. Lichen N and S concentrations have been found increasing with N and S deposition at relatively low levels, respectively. However, it remains unclear whether lichen N and S concentrations can respond to corresponding deposition fluxes under high pollution environments and how to use lichen N isotopes (δ15N) to evaluate source contributions of N deposition quantitatively. Along an urban-to-rural transect surrounding a polluted area in northern China, we investigated lichen N and S to examine their sensitivity and applicability in estimating N and S deposition. Moreover, we established a new method to reconstruct site-based δ15N values of ammonium N (NHx) and oxidized N (NOy) deposition using lichen δ15N and then to differentiate relatively contributions of major N emission sources. Lichen N (1.0–3.9%, 2.5 ± 0.6%) and S (0.09–0.33%, 0.21 ± 0.06%) decreased linearly with distances from the polluted center area. Wet inorganic N and sulfate deposition (29.0 ± 6.1 kg-N/ha/yr and 25.8 ± 7.9 kg-S/ha/yr, respectively) estimated by integrating relationships from previous literature data were comparable with levels based on direct deposition observations in the study area. Lichen δ15N varied between −12.1 to −4.1‰ and averaged −7.1 ± 2.0‰ among our study sites, and reconstructed δ15N values of NHx and NOy deposition averaged −12.8 ± 1.0‰ and −3.2 ± 0.5‰, respectively. Source contribution analyses revealed significant contributions of volatilization ammonia (66 ± 3%) from wastes and fertilizers to NHx deposition and non-fossil fuel N oxides (62 ± 7%) from biomass burnings and microbial N cycles to NOy deposition. These results indicate that lichen N and S are sensitive to deposition fluxes under high pollution and applicable to estimating deposition levels. This work improves the methodology of atmospheric deposition biomonitoring based on lichen element and isotope records.

To constrain sources of anthropogenic nitrogen (N) deposition is critical for effective reduction of reactive N emissions and better evaluation of N deposition effects. This study measured δ15N signatures of nitrate (NO3−), ammonium (NH4+) and total dissolved N (TDN) in precipitation at Guiyang, southwestern China and estimated contributions of dominant N sources using a Bayesian isotope mixing model. For NO3−, the contribution of non-fossil N oxides (NOx, mainly from biomass burning (24 ± 12%) and microbial N cycle (26 ± 5%)) equals that of fossil NOx, to which vehicle exhausts (31 ± 19%) contributed more than coal combustion (19 ± 9%). For NH4+, ammonia (NH3) from volatilization sources (mainly animal wastes (22 ± 12%) and fertilizers (22 ± 10%)) contributed less than NH3 from combustion sources (mainly biomass burning (17 ± 8%), vehicle exhausts (19 ± 11%) and coal combustions (19 ± 12%)). Dissolved organic N (DON) accounted for 41% in precipitation TDN deposition during the study period. Precipitation DON had higher δ15N values in cooler months (13.1‰) than in warmer months (−7.0‰), indicating the dominance of primary and secondary ON sources, respectively. These results newly underscored the importance of non-fossil NOx, fossil NH3 and organic N in precipitation N inputs of urban environments.

Human activities have distinctly enhanced the deposition levels of atmospheric nitrogen (N) pollutants into terrestrial ecosystems, but whether and to what extents soil carbon (C) and N status have been influenced by elevated N inputs remain poorly understood in the ‘real’ world given related knowledge has largely based on N-addition experiments. Here we reported soil organic C (OC) and total N (TN) for twenty-seven forests along a gradient of N deposition (22.4–112.9 kg N/ha/yr) in the Beijing-Tianjin-Hebei (BTH) region of northern China, a global hotspot of high N pollution. Levels of soil TN in forests of the BTH region have been elevated compared with investigations in past decades, suggesting that long-term N deposition might cause soil TN increases. Combining with major geographical and environmental factors among the study forests, we found unexpectedly that soil moisture and pH values rather than N deposition levels were major regulators of the observed spatial variations of soil OC and TN contents. As soil moisture and pH values increased with mean annual precipitation and temperature, respectively, soil C and N status in forests of the BTH region might be more responsive to climate change than to N pollution. These evidence suggests that both N deposition and climate differences should be considered into managing ecosystem functions of forest resources in regions with high N pollution.

Knowledge of the characteristics of chemical components transported in the xylem sap of trunks remains deficient and limited because no appropriate method exists to extract the xylem sap from this part of the tree. We thus explored the differences in xylem sap components extracted by means of centrifugation and water displacement methods and depicted the level and behavior of chemical components in the xylem sap of trunks and branches of different aged trees from a pine forest in northern China. There were no significant differences between the two methods with respect to nitrogen (N) compounds and inorganic ions in the xylem sap. Potassium concentrations obtained by the methods were similar and consistent with the values obtained from earlier publications on woody species. This suggests that contamination of the xylem sap by the centrifugation method is negligible, and this method would be a reliable and robust tool for collection of the trunk xylem sap. Dissolved organic N was the dominant component of total N followed by nitrate (NO3−) and ammonium (NH4+). Potassium and chloride were the predominant cation and anion, respectively, of the xylem sap. The NO3− concentration basically did not change, whereas the NH4+ concentration was larger transported from the trunk to branches for the large tree class during foliage senescence. More inorganic N components (mainly NO3−) were found in young trees than in old trees. Our study contributes to improve the diagnostic assessments of tree physiological processes and growth in mature forest trees under environmental changes.

Anthropogenic nitrogen (N) emission has been the major cause of rapidly increasing atmospheric N deposition in many regions of the world. However, fluxes, distributions, and physi-chemical compositions of regional N deposition and the response of N deposition levels to N emissions remain poorly characterized, particularly in high N-emission regions. To explore these issues, this study collated observations on N deposition fluxes in the Beijing-Tianjin-Hebei (BTH) region of northern China. We found that total inorganic N (TIN) deposition in the study region averaged 61.3 ± 13 kg-N/ha/yr based on observations from 1998 to 2017. Respectively, about 97%, 73%, 20%, 32%, and 59% of farmland, constructed land, forest, grassland, and water areas had TIN deposition above 50 kg-N/ha/yr. These results indicated substantial ecological risks of atmospheric N loading to these ecosystems. Dry deposition accounted for 59% of TIN deposition and gaseous N accounted for 73% in dry IN deposition. In gaseous N deposition, ammonia, N dioxides, and nitric acid accounted for 42%, 47%, and 11%, respectively. Ratios of NH3-N (N in ammonia) to (NO2-N + HNO3-N) (N in N dioxides, nitric acid) or NH4-N (N in ammonium) to NO3-N (N in nitrate) averaged 0.8, 2.0, 1.0, 1.9, and 1.3 in gaseous, particulate, dry, wet, and total deposition fluxes, respectively. These indicated different transformation and deposition mechanisms between NH4-N and NO3-N under high emissions. Moreover, we found a non-linear relationship between TIN deposition and anthropogenic IN emissions. All these results provide a systematic assessment of the N deposition and its responses to human activities in a high N-emission region, which is valuable for emission mitigation and effect evaluation of regional N pollution.

Moss nitrogen (N) concentrations and natural 15N abundance (δ15N values) have been widely employed to evaluate annual levels and major sources of atmospheric N deposition. However, different moss species and one-off sampling were often used among extant studies, it remains unclear whether moss N parameters differ with species and different samplings, which prevented more accurate assessment of N deposition via moss survey. Here concentrations, isotopic ratios of bulk carbon (C) and bulk N in natural epilithic mosses (Bryum argenteum, Eurohypnum leptothallum, Haplocladium microphyllum and Hypnum plumaeforme) were measured monthly from August 2006 to August 2007 at Guiyang, SW China. The H. plumaeforme had significantly (P < 0.05) lower bulk N concentrations and higher δ13C values than other species. Moss N concentrations were significantly (P < 0.05) lower in warmer months than in cooler months, while moss δ13C values exhibited an opposite pattern. The variance component analyses showed that different species contributed more variations of moss N concentrations and δ13C values than different samplings. Differently, δ15N values did not differ significantly between moss species, and its variance mainly reflected variations of assimilated N sources, with ammonium as the dominant contributor. These results unambiguously reveal the influence of inter-species and intra-annual variations of moss N utilization on N deposition assessment.

Mosses, among all types of terrestrial vegetation, are excellent scavengers of anthropogenic nitrogen (N), but their utilization of dissolved organic N (DON) and their reliance on atmospheric N remain uncharacterized in natural environments, which obscures their roles in N cycles. Natural (15) N abundance of N sources (nitrate (NO(3)(-)), ammonium (NH(4)(+)) and DON in deposition and soil) for epilithic and terricolous mosses was analyzed at sites with different N depositions at Guiyang, China. Moss NO(3)(-) assimilation was inhibited substantially by the high supply of NH(4)(+) and DON. Therefore, contributions of NH(4)(+) and DON to moss N were partitioned using isotopic mass-balance methods. The N contributions averaged 56% and 46% from atmospheric NH(4)(+), and 44% and 17% from atmospheric DON in epilithic and terricolous mosses, respectively. In terricolous mosses, soil NH(4)(+) and soil DON accounted for 16% and 21% of bulk N, which are higher than current estimations obtained using (15) N-labeling methods. Moreover, anthropogenic NH(4)(+) deposition suppressed utilization of DON and soil N because of the preference of moss for NH(4)(+) under elevated NH(4)(+) deposition. These results underscore the dominance of, and preference for, atmospheric NH(4)(+) in moss N utilization, and highlight the importance of considering DON and soil N sources when estimating moss N sequestration and the impacts of N deposition on mosses.

The continuing increase in human activities is causing global changes such as increased deposition of atmospheric nitrogen. There is considerable interest in understanding the effects of increasing atmospheric nitrogen deposition on soil enzyme activities, specifically in terms of global nitrogen cycling and its potential future contribution to global climate change. This paper summarizes the ecological effects of atmospheric nitrogen deposition on soil enzyme activities, including size-effects, stage-effects, site-effects, and the effects of different levels and forms of atmospheric nitrogen deposition. We discuss needs for further research on the relationship between atmospheric nitrogen deposition and soil enzymes.

Using isotope mixing model (IsoSource) and natural δ15N method, this study evaluated contributions of major sources to N of PM2.5 at Beijing (collected during a severe haze episode of January 22nd–30th, 2013) and a background site (Menyuan, Qinghai province; collected from September to October of 2013) of China. At Beijing, δ15N values of PM2.5 (−4.1 – +13.5 ‰; mean = +2.8 ± 6.4 ‰) distributed within the range reported for major anthropogenic sources (including NH3 and NO2 from coal combustion, vehicle exhausts and domestic wastes/sewage). However, δ15N values of PM2.5 at the background site (+8.0 – +27.9 ‰; mean = +18.5 ± 5.8 ‰) were significantly higher than that of potential sources (including NH3 and NO2 from biomass burning, animal wastes, soil N cycle, fertilizer application, and organic N of soil dust). Evidences from molecular ratios of NH4+ to NO3− and/or SO42− in PM2.5, NH3 to NO2 and/or SO2 in ambient atmosphere suggested that the equilibrium of NH3 ↔ NH4 + caused apparent 15N enrichment only in NH4 + of PM2.5 at the background site due to more abundant NH3 than SO2 and NO2. Therefore, a net 15N enrichment (33 ‰) was assumed for NH3 sources of background PM2.5 when fractional contributions were estimated by IsoSource model. Results showed that 41 %, 30 % and 14 % of N in PM2.5 of Beijing originated from coal combustion, vehicle exhausts and domestic wastes/sewage, respectively. Background PM2.5 derived N mainly from biomass burning (58 %), animal wastes (15 %) and fertilizer application (9 %). These results revealed the regulation of the stoichiometry between ammonia and acidic gases on δ15N signals in PM2.5. Emissions of NO2 from coal combustion and NH3 from urban transportation should be strictly controlled to advert the risk of haze episodes in Beijing.

Nitrate (NO3−) is an important form of nitrogen (N) available to plants. The measurements of NO3− concentration [NO3−] and isotopes (δ15N and δ18O) in plants provide unique insights into ecosystem NO3− availability and plant NO3− dynamics. This work investigated the variability of these parameters in individuals of a broadleaved (Aucuba japonica) plant and a coniferous (Platycladus orientalis) plant, and explored the applicability of tissue NO3− isotopes for deciphering plant NO3− utilization mechanisms. The NO3− in washed leaves showed concentration and isotopic ratios that were much lower than that in unwashed leaves, indicating a low contribution of atmospheric NO3− to NO3− in leaves. Current leaves showed higher [NO3−] and isotopic ratios than mature leaves. Moreover, higher leaf [NO3−] and isotopic enrichments (relative to soil NO3−) were found under higher soil NO3− availability for A. japonica. In contrast, leaves of P. orientalis showed low [NO3−] and negligible isotopic enrichments despite high soil NO3−. Higher [NO3−] was found in both fine and coarse roots of the P. orientalis plant, but significant isotopic enrichment was found only in coarse roots. These results reflect that the NO3− accumulation and isotopic effects decreased with leaf age, but increased with soil NO3− supply. Leaves are therefore identified as a location of NO3− reduction for A. japonica, while P. orientalis did not assimilate NO3− in leaves but in coarse roots. This work provided the first organ-specific information on NO3− isotopes in plant individuals, which will stimulate further studies of NO3− dynamics in a broader spectrum of plant ecosystems.

Nitric acid (HNO3) or nitrate (NO3−) is the dominant sink for reactive nitrogen oxides (NOx = NO + NO2) in the atmosphere. In many Chinese cities, HNO3 is becoming a significant contributor to acid deposition. In the present study, we measured nitrogen (N) and oxygen (O) isotopic composition of NO3− in 113 precipitation samples collected from Guangzhou City in southern China over a two-year period (2008 and 2009). We attempted to better understand the spatial and seasonal variability of atmospheric NOx sources and the NO3− formation pathways in this N-polluted city in the Pearl River Delta region. The δ15N values of NO3− (versus air N2) ranged from −4.9 to +10.1‰, and averaged +3.9‰ in 2008 and +3.3‰ in 2009. Positive δ15N values were observed throughout the year, indicating the anthropogenic contribution of NOx emissions, particularly from coal combustion. Different seasonal patterns of δ15N-NO3− were observed between 2008 and 2009, which might reflect different human activities associated with the global financial crisis and the intensive preparations for the 16th Asian Games. Nitrate δ18O values (versus Vienna Standard Mean Ocean Water) varied from +33.4 to +86.5‰ (average +65.0‰ and +67.0‰ in 2008 and 2009, respectively), a range being lower than those reported for high latitude and polar areas. Sixteen percent of δ18O values was observed lower than the expected minimum of +55‰ at our study site. This was likely caused by the reaction of NO with peroxy radicals; peroxy radicals can compete with O3 to convert NO to NO2, thereby donate O atoms with much lower δ18O value than that of O3 to atmospheric NO3−. Our results highlight that the influence of human activities on atmospheric chemistry can be recorded by the N and O isotopic composition of atmospheric NO3− in a N-polluted city.

Nitrogen concentration and delta15N in 175 epilithic moss samples were investigated along four directions from urban to rural sites in Guiyang, SW China. The spatial variations of moss N concentration and delta15N revealed that atmospheric N deposition is dominated by NHx-N from two major sources (urban sewage NH3 and agricultural NH3), the deposition of urban-derived NHx followed a point source pattern characterized by an exponential decline with distance from the urban center, while the agricultural-derived NHx was shown to be a non-point source. The relationship between moss N concentration and distance (y=1.5e(-0.13x)+1.26) indicated that the maximum transporting distance of urban-derived NHx averaged 41 km from the urban center, and it could be determined from the relationship between moss delta(15)N and distance [y=2.54ln(x)-12.227] that urban-derived NHx was proportionally lower than agricultural-derived NHx in N deposition at sites beyond 17.2 km from the urban center. Consequently, the variation of urban-derived NHx with distance from the urban center could be modeled as y=56.272e(-0.116x)-0.481 in the Guiyang area.

Nitrogen isotope (δ15N) has been employed to differentiate major sources of atmospheric N. However, it remains a challenge to quantify contributions of multiple sources based on δ15N values of the ...

Several studies have shown that soil microorganisms play a key role in the success of plant invasion. Thus, ecologists have become increasingly interested in understanding the ecological effects of biological invasion on soil microbial communities given continuing increase in the effects of invasive plants on native ecosystems. This paper aims to provide a relatively complete depiction of the characteristics of soil microbial communities under different degrees of plant invasion. Rhizospheric soils of the notorious invasive plant Wedelia trilobata with different degrees of invasion (uninvaded, low-degree, and high-degree using its coverage in the invaded ecosystems) were collected from five discrete areas in Hainan Province, P. R. China. Soil physicochemical properties and community structure of soil microorganisms were assessed. Low degrees of W. trilobata invasion significantly increased soil pH values whereas high degrees of invasion did not significantly affected soil pH values. Moreover, the degree of W. trilobata invasion exerted significant effects on soil Ca concentration but did not significantly change other indices of soil physicochemical properties. Low and high degrees of W. trilobata invasion increased the richness of the soil fungal community but did not pose obvious effects on the soil bacterial community. W. trilobata invasion also exerted obvious effects on the community structure of soil microorganisms that take part in soil nitrogen cycling. These changes in soil physicochemical properties and community structure of soil microbial communities mediated by different degrees of W. trilobata invasion may present significant functions in further facilitating the invasion process.

Mosses are effective accumulators and indicators of N deposition, but the mechanisms of moss N utilisation remain unclear. This study monitored nitrate concentrations ([NO3–]) in solutions supplied to Hypnum plumaeforme Wils. to characterise NO3– uptake from rain events. Concentrations and isotopic ratios (δ15N and δ18O) of residual NO3– in moss tissues were measured to interpret induced NO3– reduction. Noninduced NO3– reduction was inferred from endogenous [NO3–] and isotopic variations that occurred during 65 days of N deprivation. H. plumaeforme scavenges NO3– effectively from supplied solutions. The uptake rate increased with substrate [NO3–] (0.4−3.9 mg N L–1) and generally obeyed saturation (Michaelis–Menten) kinetics. The uptake rate was maximised within 60 min after receiving NO3–, irrespective of the initial substrate [NO3–]. Lower tissue [NO3–] and greater isotopic enrichment verified the inducibility of nitrate reductase activity (NRA) by NO3– availability, but short-term darkness did not markedly influence moss NO3– uptake or reduction. Significant reduction and isotopic enrichment were detected in moss NO3– reserves during N deprivation, showing 15ε of 12.1‰ and 18ε of 14.4‰. The Δδ15N : Δδ18O ratios of ~1 : 1 implied that NRA is the single process driving 15N and 18O fractionations. These results provide new isotopic insights into the nitrate reductase dynamics of the moss.

[1] Characterizing the level and sources of atmospheric N deposition in a large-scale area is not easy when using physical monitoring. In this study, we attempted to use epilithic mosses (Haplocladium microphyllum (Hedw.)) as a bioindicator. A gradient of atmospheric N deposition from 13.8 kg N ha−1 yr−1 to 47.7 kg N ha−1 yr−1 was estimated on the basis of moss tissue N concentrations and the linear equation between them. The estimated results are reliable because the highest atmospheric N deposition occurred in the middle parts of the Yangtze River, where the highest TN concentrations were also observed. Moss δ15N values in cities and forests were found in distinctly different ranges of approximately −10‰ to −6‰ and approximately −2‰ to 2‰, respectively, indicating that the main N sources in most of these cities were excretory wastes and those in forests were soil emissions. A negative correlation between moss δ15N values and the ratios of NH4-N/NO3-N in deposition (y = −1.53 x + 1.78) has been established when the ratio increased from 1.6 to 6.5. On the basis of the source information, the negative moss δ15N values in this study strongly indicate that NHy-N is the dominant N form in N deposition in the whole drainage basin. These findings are supported by the existing data of chemical composition of local N deposition.

We investigated the carbon (C) and nitrogen (N) concentrations and isotopic signatures (delta 13C and delta15 N) in epilithic mosses collected from urban sites to rural sites along four directions at Guiyang area. Mosses C (34.47%-52.76%) decreased significantly with distance from urban to rural area and strongly correlated with tissue N (0.85%-2.97%), showing atmospheric N deposition has positive effect on C assimilation of epilithic mosses, higher atmospheric N/NHx deposition at urban area has improved the photosynthesis and C fixation of mosses near urban, which also caused greater 13C discrimination for urban mosses. Mosses delta 13C signatures (-30.69% per hundred - -26.96% per hundred) got less negative with distance from urban to rural area, which was also related to the anthropogenic CO2 emissions in the city, and these 12C-enriched CO2 sources would lead to more negative mosses delta 13C through enhancing the atmospheric CO2 concentration in urban area. Moreover, according to the characteristics of mosses C and delta 13C variations with distance, it is estimated that the influences of urban anthropogenic CO2 sources on plants was mainly within 20 km from city center. This study mainly focused on the factors regulating tissue C and delta 13C of mosses in city area and the interaction between C and N in mosses, the responses of mosses C and delta 13C to urban CO2 emission and atmospheric N deposition have been revealed, which could provide new geochemical evidences for the control of city atmospheric pollution and the protection of ecosystems around city.

Tissue N concentration and delta15 N signature in epilithic mosses were investigated along 4 directions from urban area to rural area at Guiyang city. Mosses N concentration ranged from 0.85% to 2.97% and showed significant decrease from urban area (2.24% +/- 0.32%) to suburb (1.27% +/- 0.13%), reflecting the level of atmospheric N deposition decreased away from urban. While slightly higher tissue N re-emerged at rural area beyond 25km (mean = 1.33%-1.75%), suggesting increased N deposition occurred in the rural area. Mosses delta15 N values varied from - 12.50 per thousand to - 1.39 per thousand characterized with getting less negative from urban to rural area. More negative delta15 N signatures of urban mosses (mean= - 8.87 per thousand(-) - 8.59 per thousand) mainly indicated the extensive NH3 sources released from excretory wastes and sewage, while mosses growing at farther suburb or rural area had less depleted delta15 N values (mean = - 3.83 per thousand(-) - 2.48 per thousand), indicating the large sources of agricultural NH3 emission connected with fertilizer application. Moreover, the pattern of mosses delta15 N variation in this study was opposite to areas where N deposition was dominated by oxidized form N (NO(x)) with generally positive mosses delta15 N. Accordingly, we concluded that reduced NH, was the dominant N form in atmospheric deposition at Guiyang area, which would be of great value for understanding the spatial variation and sources of atmospheric N deposition.