Inorganic Nitrogen Deposition Research Articles

Regime shift in secondary inorganic aerosol formation and nitrogen deposition in the rural United States

Secondary inorganic aerosols play an important role in air pollution and climate change, and their formation modulates the atmospheric deposition of reactive nitrogen (including oxidized and reduced nitrogen), thus impacting the nitrogen cycle. Large-scale and long-term analyses of secondary inorganic aerosol formation based on model simulations have substantial uncertainties. Here we improve constraints on secondary inorganic aerosol formation using decade-long in situ observations of aerosol composition and gaseous precursors from multiple monitoring networks across the United States. We reveal a shift in the secondary inorganic aerosol formation regime in the rural United States between 2011 and 2020, making rural areas less sensitive to changes in ammonia concentrations and shortening the effective atmospheric lifetime of reduced forms of reactive nitrogen. This leads to potential increases in reactive nitrogen deposition near ammonia emission hotspots, with ecosystem impacts warranting further investigation. Ammonia (NH3), a critical but not directly regulated precursor of fine particulate matter in the United States, has been increasingly scrutinized to improve air quality. Our findings, however, show that controlling NH3 became significantly less effective for mitigating fine particulate matter in the rural United States. We highlight the need for more collocated aerosol and precursor observations for better characterization of secondary inorganic aerosols formation in urban areas.

Increase in Agricultural-Derived NHx and Decrease in Coal Combustion-Derived NOx Result in Atmospheric Particulate N-NH4+ Surpassing N-NO3- in the South China Sea.

The atmospheric deposition of anthropogenic active nitrogen significantly influences marine primary productivity and contributes to eutrophication. The form of nitrogen deposition has been evolving annually, alongside changes in human activities. A disparity arises between observation results and simulation conclusions due to the limited field observation and research in the ocean. To address this gap, our study undertook three field cruises in the South China Sea in 2021, the largest marginal sea of China. The objective was to investigate the latest atmospheric particulate inorganic nitrogen deposition pattern and changes in nitrogen sources, employing nitrogen-stable isotopes of nitrate (δ15N-NO3-) and ammonia (δ15N-NH4+) linked to a mixing model. The findings reveal that the N-NH4+ deposition generally surpasses N-NO3- deposition, attributed to a decline in the level of NOx emission from coal combustion and an upswing in the level of NHx emission from agricultural sources. The disparity in deposition between N-NH4+ and N-NO3- intensifies from the coast to the offshore, establishing N-NH4+ as the primary contributor to oceanic nitrogen deposition, particularly in ocean background regions. Fertilizer (33 ± 21%) and livestock (20 ± 6%) emerge as the primary sources of N-NH4+. While coal combustion continues to be a significant contributor to marine atmospheric N-NO3-, its proportion has diminished to 22 (Northern Coast)-35% (background area) due to effective NOx emission controls by the countries surrounding the South China Sea, especially the Chinese Government. As coal combustion's contribution dwindles, the significance of vessel and marine biogenic emissions grows. The daytime higher atmospheric N-NO3- concentration and lower δ15N-NO3- compared with nighttime further underscore the substantial role of marine biogenic emissions.

A comprehensive observation of organic and inorganic nitrogen in gases, particulate matter and precipitation in the northern suburb of Nanjing, East China, with an emphasis on size-resolved particulate nitrogen

China is considered one of the nitrogen deposition hotspots in the world. Measurements to date have focused mainly on inorganic nitrogen, organic nitrogen and the size distribution of particulate nitrogen has yet to be studied. Lack of dry deposition observations imposes a reliance on models, resulting in a much larger uncertainty relative to wet deposition which is routinely measured. In this study, dry and wet organic and inorganic nitrogen deposition were comprehensively measured in Nanjing in 2019, and nitrogen species in size-segregated particles were investigated. Total annual nitrogen deposition flux was up to 29.96 kg N ha−1 yr−1 in Nanjing, with wet deposition contributed slightly more (51.90%) to total deposition than dry deposition (48.10%). Observations revealed that reduced nitrogen (NHx = NH3 + NH4+) contributed 65.49% of the total inorganic nitrogen deposition budget, which implied that the control of reduced N in urban environments is needed to improve local air quality. Dry deposition of ammonia played an especially key role in dry deposition, contributing from 18.17% to 55.91% in different seasons. The deposition flux of water-soluble organic nitrogen (WSON) dominated by wet deposition, with the wet WSON about 4.6 times higher than that of dry deposition and contributed 41.18% of the wet nitrogen deposition. The wet N deposition in summer was significantly higher than in the other three seasons. The seasonal patterns in dry deposition were driven mostly by seasonal concentration patterns. The highest deposition of ammonia occurred in summer, while the deposition of nitrogen dioxide, particulate ammonium and nitrate peaked in winter. The maximum dry and wet WSON deposition appeared in autumn. Higher NH4+ concentration almost distributed in each size bin and centered in ranges of 0.18–1.0 μm, while higher NO3− concentration scattered in particle size ranges of 1.8–5.6 μm and 0.18–1.0 μm.

Atmospheric deposition and soil water chemistry in Swedish forests since 1985 – Effects of reduced emissions of sulphur and nitrogen

Trends for the atmospheric deposition of sulphur (S) and inorganic nitrogen (inorg-N) to forests and changes in the forest soil water chemistry in Sweden have been assessed since 1985, with special focus on the last 25 years, based on measurements within the Swedish Throughfall Monitoring Network (SWETHRO). The reductions in the deposition of S and inorg-N in the southern part of Sweden corresponded relatively well with the pollutant emission reductions for S and inorg-N from both EU27 + UK and Sweden during 1996/97–2021/22. For northern Sweden the deposition of S and inorg-N decreased to a lesser extent than both European and Swedish emissions. The bulk deposition of NO3-N has decreased more than the deposition of NH4-N over the last 25-year period, which is consistent with the much larger emission reductions for NOx compared to NH3 from EU27 + UK and Sweden. The S concentrations in the soil water, at 50 cm below soil surface, have decreased during the last 25 years, however somewhat less than the S deposition. At sites with low ANC and pH in the beginning of the period, the increase in ANC was generally greater and the increase in pH was smaller, but at sites with high pH and ANC above zero, the increase in pH was dominant, in line with the nonlinear relationship between pH and ANC in the soil water. The incidence of elevated concentrations of NO3-N in the soil water was highest in southwest Sweden, ranging between 4 and 19 % of all measuring occasions since 1985/86. The reduced deposition of N over the 35-year period was not reflected in the incidence of elevated concentrations of NO3-N in the soil water over time.

Comparison of inorganic nitrogen concentrations in airborne particles at inshore and offshore sites in the Yellow Sea (2017–2019): Long-range transport and potential impact on marine productivity

Atmospheric deposition of nitrogen is one of the most important external nutrient sources. We investigated the concentrations of NO3− and NH4+ in airborne particles at both an offshore and an inshore site in the Yellow Sea. At the offshore site, devoid of local sources and located downwind from the highly developed areas of Korea and China, the concentrations of atmospheric particulate NO3− and NH4+ were ∼88 ± 101 nmol m−3 and ∼102 ± 102 nmol m−3, respectively, likely due to the transboundary long-range transport of pollutants. The inshore site showed a concentration ∼2 times higher than the offshore site. Considering not only dry inorganic nitrogen deposition but also wet and organic material deposition, the total atmospheric nitrogen deposition was estimated to contribute roughly 10 % to the new production in the Yellow Sea.

Proximity to Roads Does Not Modify Inorganic Nitrogen Deposition in a Topographically Complex, High Traffic, Subalpine Forest

Proximity to Roads Does Not Modify Inorganic Nitrogen Deposition in a Topographically Complex, High Traffic, Subalpine Forest

Patterns and drivers of atmospheric inorganic nitrogen deposition in Northeast Asia

Elevated nitrogen (N) deposition due to intensified emissions of NH3 and NOx is a global problem with profound consequences on living organisms and the environment. Although N emission rates are currently considered to be high in East Asia, reports on the current N deposition level and composition are still limited, especially in northeastern China, where official N deposition monitoring sites are unavailable. This limits our understanding of the spatio-temporal N deposition patterns and their influencing factors at regional to continental scales. Here, we used data collected mostly during 2019 at 38 sites, comprising 7 sites in northeastern China and 31 EANET (Acid Deposition Monitoring Network in East Asia) sites in middle and east Russia, Mongolia, central and southern China, South Korea and Japan to explore the spatial-seasonal variations and drivers of ammonium and nitrate deposition across the Northeast Asia. Total bulk inorganic N (TIN) deposition was 3.7–24.5 kg N ha−1 yr−1 and NH4+-N/NO3−-N ratio in the TIN was 0.8–2.8 in northeastern China. The bulk/wet TIN deposition averaged 7.5 kg N ha−1 yr−1 (predominantly in the form of ammonium-N: NH4+-N/NO3−-N = 1.4) over the Northeast Asia region, with the highest rates being observed in northeastern China (11.6), as well as central and southern China (10.7), followed by east Russia, South Korea and Japan (5.6), and the lowest in middle Russia and Mongolia (1.5). This regional bulk/wet TIN deposition level is about twice of the wet TIN deposition level in Europe and the United States. The TIN deposition in summer and spring was 45–467% higher than in autumn and winter. Out of the ten land uses considered, only agricultural and urban land uses significantly positively correlated with NH4+-N and NO3−-N deposition rates across all monitored sites. This study suggests that the ongoing agricultural and urban expansions are likely to enhance N deposition and its associated effects across global ecosystems.

A database of atmospheric inorganic nitrogen deposition fluxes in China from satellite monitoring

Over the past century, atmospheric inorganic nitrogen (IN) deposition to terrestrial ecosystems has significantly increased and caused various environmental issues. China has been one of the hotspot regions for IN deposition, yet limited data exist regarding IN deposition fluxes in China at the regional scale. In this study, based on NO2 and NH3 columns acquired by satellite sensors, coupled with atmospheric chemical transport model (CTM), mixed-effects model and site observations, we constructed regional-scale IN dry and wet deposition models respectively, and finally proposed a spatially explicit database of IN deposition fluxes in China. The database includes the dry, wet and total deposition fluxes in China during 2011–2020, and the data are presented in raster form with a resolution of 0.25° × 0.25°. Overall, the database is of great importance for monitoring and simulating the trends of IN deposition over a long time series in China.

Simulated organic–inorganic nitrogen deposition changes the growth rate, leaf stoichiometry, and phenolic content of Cyclocarya paliurus

Atmospheric nitrogen (N) deposition is composed of both organic nitrogen (ON) and inorganic nitrogen (IN), and these sources of N affect plant growth, the balance of elements, and the accumulation of secondary metabolism substances; however, how these plant traits respond to ON–IN N deposition remains unclear. Here, we explored the effects of N addition with different ON:IN ratios on the growth rate, biomass allocation, leaf N and phosphorus (P) concentration, and leaf phenolic content of Cyclocarya paliurus in a short-term field experiment. Compared with the control, the growth rate (μ) and biomass accumulation of C. paliurus showed a decreasing trend in the 2 years after different N addition treatments. Higher ON:IN ratios not only resulted in higher leaf N content but also significantly increased the leaf P content. In addition, N addition with different ON:IN ratios induced more biomass accumulation allocated to the underground part of C. paliurus, which may be related to the nutrient acquisition strategy. Increased leaf N:P ratios indicated that stoichiometric imbalance occurred and that N and P limitation conditions of C. paliurus changed after different ON–IN N treatments. The growth rate (μ) of C. paliurus showed a trend of first decreasing (14 < leaf N:P ratios < 16) and then increasing (16 < leaf N:P ratios). However, compared with the growth rate, the leaf phenolic content showed an opposite trend to that of leaf stoichiometry, which indicated that leaf stoichiometry significantly affected both the primary growth and secondary metabolism of plants. Our results suggest that simulated N addition with different ON:IN ratios caused stoichiometric imbalance and changed the plant growth rate and accumulation of biomass and secondary metabolism substances. In addition, our study highlights that the type of nutrient limitation and the variation in N deposition components should be considered when predicting the future responses of plant functional traits to N deposition.

Anthropogenic pollutants induce enhancement of aerosol acidity at a mountainous background atmosphere in southern China

Aerosol acidity plays a crucial role in atmospheric physicochemical processes, climate change and human health, particularly in the formation of secondary organic aerosols (SOA). However, understanding the characteristics and driving factors of aerosol acidity in background mountains has been limited. In this study, we conducted intensive field measurements in the Nanling mountains during the dry and wet seasons to analyze aerosol pH characteristics and their driving factors using sensitivity tests. The mean aerosol pH in the background mountains was found to be 2.68 ± 0.55, with values ranging from 0.38 to 4.44, significantly lower than predicted values in northern China. Sensitivity tests revealed that aerosol acidity in the background atmosphere was more responsive to dominant chemical species (T-NH3 (= NH4+ + NH3) and SO42−) rather than relative humidity and temperature. Additionally, we observed that sulfate and ammonium, transported occasionally by dryer northern air masses, had a substantial impact on decreasing aerosol pH at the site. Similar to the southeastern United States, NH4+/NH3 also dominated the total buffer capacity of aerosol acidity in the Nanling mountains. The strong aerosol acidity in this area is expected to have adverse effects on regional air quality and climate by enhancing SOA formation and regulating the dry deposition of inorganic reactive nitrogen.

Understory N application overestimates the effect of atmospheric N deposition on soil N2O emissions

Deposition of atmospheric inorganic and organic nitrogen (N) may increase soil nitrous oxide (N2O) emissions. To simulate and quantify such effects in forest ecosystems, understory N application, usually directly to the soil surface, has been used. However, this approach overlooks N interception by forest canopies, resulting in overestimates of actual impacts from N deposition. To test our assumptions, we monitored soil N2O emissions over one year in a Moso bamboo (Phyllostachys edulis) forest using different N deposition simulation approaches (canopy vs. understory application) and forms of N (inorganic vs. organic N). Our results revealed that cumulative N2O emissions from understory N application treatments were 20–50% higher than the corresponding canopy treatments, with greater N2O emissions under inorganic N than organic N application. Our study underscores the importance of considering canopy processes in future studies on N deposition and soil N2O emissions.

An increasing trend of inorganic nitrogen deposition across montane regions of China

China has experienced a dramatic increase in the total inorganic nitrogen (TIN) deposition rate over the past three decades and has become one of the hotspots of N deposition worldwide. However, the status and trends of TIN deposition across Chinese mountain regions, which occupy over 60% of the land area of China, are still unclear. Here, we established a time series dataset of TIN deposition across Chinese mountains by data collection to evaluate the status and tendency of TIN deposition in mountain regions. The average TIN deposition flux in the mountains was approximately 9.62 kg N/ha/yr, with a maximum value of 23.49 kg N/ha/yr found in northern montane regions. There was a significant increasing trend of TIN deposition during the last two decades across China's montane regions (p < 0.05), different from the decreasing temporal trend from cropland/city sites. The temporal pattern of TIN deposition in montane areas was spatial and elevation dependent, showing an obvious increasing trend in both Southwest and Northeast Chinese montane areas and in the montane areas with lower (<1000 m. asl) and middle (1000–3000 m. asl) elevations. Our results highlighted the different temporal trends of TIN deposition in China's mountain regions from cropland/city areas, which should be considered in earth system models and environmental policy-making.

Assessment of Wet Inorganic Nitrogen Deposition in an Oil Palm Plantation-Forest Matrix Environment in Borneo

Nitrogen (N) deposition significantly affects forest dynamics, carbon stocks and biodiversity, and numerous assessments of N fluxes and impacts exist in temperate latitudes. In tropical latitudes, however, there are few such assessments. In this study, we measured the inorganic N concentration (wet deposition) deposited in rainfall and rainfall pH throughout one year at the boundary of a forest reserve in Malaysian Borneo. We considered that the N deposition may be either from forest and agricultural fires or derived from agricultural fertiliser. Therefore, we determined the wind trajectories using the HYSPLIT model provided by NOAA, the location of fires throughout the landscape throughout one year using NASA’s FIRM system, and obtained the land use cover map of Malaysia and Indonesia. We then correlated our monthly cumulative wet N deposition with the cumulative number of fires and the cumulative area of oil palm plantation that wind trajectories arriving at our study site passed over before reaching the rainfall sampling site. At 7.45 kg N ha−1 year−1, our study site had the highest annual wet inorganic N deposition recorded for a Malaysian forest environment. The fire season and the cumulative agricultural area crossed by the winds had no significant effect on N deposition, rainfall N concentration, or rainfall pH. We suggest that future research should use 15N isotopes in rainfall to provide further information on the sources of N deposition in tropical forests such as this.

Nitrogen atmospheric deposition driven by seasonal processes in a Brazilian region with agricultural background.

Understanding the seasonal patterns and influencing factors of nitrogen atmospheric deposition is essential to evaluate human impacts on the air quality and nitrogen biogeochemical cycle. However, evaluation of the nitrogen deposition flux, especially in South America agricultural regions, has not been fully investigated. In this paper, we quantified the atmospheric wet deposition fluxes of total dissolved nitrogen (TDN), dissolved organic nitrogen (DON), and dissolved inorganic nitrogen (DIN), in a region with agricultural and livestock predominance in the Southern Minas Gerais region, Brazil, from May 2018 to April 2019. Deposition fluxes of nitrogen species in the wet season (October-March) were on average 4.8-fold higher than those in the dry season, which revealed significant seasonal variations driven largely by the seasonality of rainfall and agricultural operations. We also found high NO3-/NH4+ ratios (average = 8.25), with higher values in dry season (NO3-/NH4+ = 12.8) in comparison with wet season (NO3-/NH4+ = 4.48), which revealed a higher relative contribution of NOx emissions from traffic sources in dry season. We also estimated the influence of atmospheric deposition of inorganic nitrogen (N-DIN) on environmental ecosystems, being 2.01 kgNha-1year-1 with potential risk of acidification and eutrophication of 30%. Therefore, attention should be paid to the role of wet atmospheric deposition of nitrogen as a source of nitrogen environmental pollution in agricultural regions.

Chemical Composition and Deposition Characteristics of Precipitation into a Typical Temperate Forest in Northeastern China

The chemical compositions and deposition characteristics of atmospheric precipitation affect the structure and function of forest ecosystems and reflect regional air quality. Although northeastern China constitutes a vital forested area, few relevant studies reveal the chemical composition and the nitrogen (N) and sulphur (S) deposition characteristics within precipitation. In this study, we monitor precipitation chemistry during 2018–2020 at a rural forested site in northeastern China (the Qingyuan site) and compare it with those from background sites (Mondy in Russia and Ochiishi in Japan) and highly anthropogenically influenced areas (Beijing). The precipitation pH range was 4.7–8.0 (volume-weighted average 6.2). The average concentration of total ions in precipitation was 459 μmol L−1, representing a moderate pollution level. Nitrate (NO3−, 73 μmol L−1) and ammonium (NH4+, 133 μmol L−1) were the major anions and cations in the precipitation. Total inorganic nitrogen (TIN) deposition was 12.3–15.9 kg N ha−1 year−1 (NH4+-N deposition accounted for 54–67%), lower than the average level in China (19.4 kg N ha−1 year−1). Annual precipitation sulphate (SO42−) deposition was 4.9–6.7 kg S ha−1 year−1. Seventy-two percent of the precipitation ions at our site originated from human activities. This work has revealed that N and S deposition is an important ion deposition component in atmospheric precipitation in the study of temperate forests in northeastern China. Nitrogen deposition, as a source of vital nutrients in the forest ecosystem, may promote forest growth and, thereby, forest carbon sequestration.

Isotopic components and source analysis of inorganic nitrogen in coastal aerosols of the Yellow Sea

The coastal atmospheric environment is one of the most complex environments on earth. It is shaped by terrestrial, marine, and atmospheric processes and acts as an external nutrient source for coastal waters. At present, there are few observations of inorganic nitrogen isotopes of China coastal aerosols, let alone the Yellow Sea. In this study, a weekly collection of total suspended particulate aerosols was conducted on the Qianliyan Island in 2018 for the measurements of inorganic nitrogen species (NO3− and NH4+) and their isotopic ratios (δ15N-NO3−, δ18O-NO3−, and δ15N-NH4+). At the Qianliyan Island, the average NO3− and NH4+ concentrations were 2.49 ± 2.12 and 3.33 ± 2.68 μg·m−3, respectively; the average δ15N-NO3−, δ18O-NO3−, and δ15N-NH4+ were 2.4‰ ± 5.7‰, 78.7‰ ± 8.0‰, and −2.6‰ ± 6.3‰, respectively. The major nitrate formation pathways were •OH oxidation and N2O5 hydrolysis paths, and the dominant sources of inorganic nitrogen aerosols were coal combustion (29% ± 7%), marine (19% ± 15%), and fertilizer (16% ± 13%). Aerosol δ15N-NO3− and δ18O-NO3− were obviously higher in winter and lower in summer; conversely, aerosol δ15N-NH4+ was slightly higher in summer and slightly lower in winter. The difference in nitrogen sources was considered to be the best explanation for the aerosol δ15N-NO3− and δ15N-NH4+ differences between summer and winter, of which coal combustion contributed the most. The seasonal difference in nitrate formation paths was considered to be the best explanation for the difference of Qianliyan aerosol nitrate δ18O-NO3− between summer and winter. Aerosol inorganic nitrogen deposition flux was estimated to be 3.4 nmol N·m−2·s−1, which induced less than 1% to marine primary production, and aerosol inorganic nitrogen deposition, compared with N2 fixation, contributed some 80% of δ15N-NO3− depression of the summer Yellow Sea thermocline.

Quantifying the wet deposition of reactive nitrogen over China: Synthesis of observations and models

Accurate estimation on reaction nitrogen (Nr) deposition is highly demanded for assessing the impacts on the environment and human beings. This study investigated the wet deposition of inorganic nitrogen (IN) in mainland China by measurements from over 500 sites from five observational networks/databases and ensemble results of eleven chemical transport models (CTMs). Each data source has its focus and limitations and together formed a comprehensive view over China. But the inconsistency among different sources may hinder the appropriate usage of data. Model evaluation results demonstrated the models' deficiency in simulating the wet NO3− deposition over Southeast China (40 % underestimation) and showed an overall underestimation of wet NH4+ deposition over the hotspot regions (5–60 % underestimation). A synthesis of this study and twelve reference studies was conducted to quantify the national amount of wet IN deposition. The estimations by CTMs ranged 2.4–3.9 Tg(N) yr−1 for wet NOy deposition and 4–6.7 Tg(N) yr−1 for wet NHx deposition, after adjusting the results with 10–19 % underestimations in wet NOy deposition and 1–40 % underestimations in wet NHx deposition. The estimations by ground observations ranged 7.1–9 Tg(N) yr−1 for wet NOy deposition and 8–13.1 Tg(N) yr−1 for wet NHx deposition, which were 20–275 % higher than the estimation by CTMs, but the results were strongly influenced by the abundances and representative of measurements. Studies using statistical techniques to interpolate site observations predicted 3–5.5 Tg(N) yr−1 for wet NOy deposition and 3.9–7.2 Tg(N) yr−1 for wet NHx deposition. This approach benefited from high accuracy and good robustness of the statistical models, but the uncertainty in the interpolation methods could be a potential drawback.

Deposition Characteristics of Water-soluble Inorganic Nitrogen and Organic Nitrogen in Atmospheric Precipitation in the Northern Suburbs of Nanjing

Based on atmospheric precipitation collected in the northern suburbs of Nanjing from 2019 to 2020, the pH, conductivity, and chemical components of precipitation were analyzed. The seasonal variation in pH and conductivity of atmospheric precipitation in the northern suburbs of Nanjing were studied. The pollution levels and deposition characteristics of water-soluble inorganic nitrogen (WSIN) and organic nitrogen (WSON) in precipitation were also analyzed. The frequency of acid rain (pH<5.6) in atmospheric precipitation in the northern suburbs of Nanjing reached 37.18% during the observation period. Precipitation acidification was more serious in autumn and winter, and the pH value showed a variation trend of spring>summer>autumn>winter. The average conductivity of precipitation was 29.49 μS·cm-1; high pH and conductivity in spring were related to the high dust content in the atmosphere. The seasonal difference between WSIN and WSON in precipitation was significant. The highest and lowest concentrations of NO3--N and NH4+-N appeared in spring and summer, respectively. The concentration of WSON was the highest in autumn (2.63 mg·L-1). The average concentration ratio of WSON to water-soluble total nitrogen (WSTN) in precipitation was approximately 0.47, indicating that WSON played an important role in the study of total nitrogen. The average wet deposition fluxes of WSIN and WSON were 12.10 kg·(hm2·a)-1 and 11.13 kg·(hm2·a)-1, respectively, in which the inorganic nitrogen deposition was mainly NH4+-N.

Fluxes, characteristics and influence on the aquatic environment of inorganic nitrogen deposition in the Danjiangkou reservoir

Atmospheric reactive nitrogen (Nr) deposition has caused serious damage to the terrestrial and freshwater ecosystems and also affected human health. Measuring temporal and spatial characteristics of Nr deposition is critical for proposing control strategy to decrease negative effects. We investigated the fluxes of ammonia nitrogen (NH4-N) and nitrate nitrogen (NO3-N) in both dry and wet deposition from October 2017 to September 2020 at six sites around the Danjiangkou reservoir. The results showed that the fluxes of dissolved inorganic nitrogen (DIN) decreased from 24.39 kg ha−1 yr−1 (2017–2018) to 16.11 kg ha−1 yr−1 (2019–2020) for dry deposition, and from 19.71 kg ha−1 yr−1 (2017–2018) to 12.29 kg ha−1 yr−1 (2019–2020) for wet deposition. Both NH4-N and NO3-N in wet deposition exhibited significant (P < 0.01) differences among four seasons, and were markedly influenced by the precipitation. The fluxes of NO3-N deposition showed significant (P < 0.05) difference among six samples. Dry component contributed more to total DIN deposition, and NH4-N was the dominant species in DIN deposition. The ratios of NH4-N to NO3-N in four seasons were higher than 2. A positive matrix factorization (PMF) model estimated that the factors of agriculture and fossil fuel combustion accounted for 77.1 % and 17.0 %, respectively, to the dry NH4-N deposition; and that the factors of agricultural source and biomass burning accounted for 56.2 % and 21.1 %, respectively, to the wet NH4-N deposition. The DIN deposition contributed to 7.7 % of the total Nr input into the reservoir, and the contribution of DIN deposition to the increase in the nitrogen concentration (ΔN) of the Danjiangkou reservoir was 0.13 mg L−1 yr−1. The dry DIN deposition was significantly correlated with the concentration of nitrogen in Danjiangkou reservoir (P < 0.01). This study suggested that the control measures of agricultural activity were essential to reduce Nr deposition, and to decrease the potential risks of water pollution in the reservoir. Furthermore, more long-term study is necessary to understand the relation between control measures, Nr deposition and water quality.

Impact of tree species diversity on throughfall deposition in a young temperate forest plantation

Throughfall deposition is an important pathway via which particles, aerosols and gases can move from the atmosphere to the forest floor, which can greatly impact forest biodiversity and functioning. Although throughfall deposition biochemistry has been well studied in forest ecosystems, less is known about how throughfall deposition is modified by tree species diversity. To disentangle the effects of tree species identity and diversity on throughfall deposition, we installed rain gauges in a 10-year-old tree diversity experiment. With these rain gauges, we monitored throughfall biweekly, and performed chemical analyses monthly for all the major ions (Na+, Cl−, SO42−-S, K+, Ca2+, Mg2+, PO43−-P, NO3−-N and NH4+-N), for a period of one year. Based on these data, we analysed species identity (i.e. whether throughfall deposition depended on the tree species present in the overstorey), and diversity effects (i.e. whether throughfall deposition depended on tree species richness). We confirmed species identity effects on throughfall deposition. Presence of pine led to higher throughfall deposition of inorganic nitrogen, and lower amounts of phosphorus, calcium and magnesium reaching the forest floor. Additionally, species characteristics and stand structural characteristics, i.e. basal area and canopy cover, emerged as key factors driving throughfall deposition of sodium, chloride, nitrate, sulphate and potassium. Tree species diversity effects on throughfall deposition were also present, mostly via increased basal area and canopy cover, leading to increased throughfall deposition of inorganic nitrogen, sulphate, chloride, sodium and potassium. Our findings thus suggest that tree species diversity enhances throughfall deposition of most ions that we analysed, potentially increasing nutrient availability for below-canopy plant communities. Given that inorganic nitrogen and sulphur are major atmospheric pollutants, increased deposition in mixed plots might affect ecosystem functioning. Further research will be needed to determine the extent of this impact.