Canopy nitrogen deposition enhances soil ecosystem multifunctionality in a temperate forest.

Nitrogen (N) deposition affects ecosystem functions crucial to human health and well-being. However, the consequences of this scenario for soil ecosystem multifunctionality (SMF) in forests are poorly understood. Here, we conducted a long-term field experiment in a temperate forest in China, where N deposition was simulated by adding N above and under the canopies. We discover that canopy N addition promotes SMF expression, whereas understory N addition suppresses it. SMF was regulated by fungal diversity in canopy N addition treatments, which is largely due to the strong resistance to soil acidification and efficient resource utilization characteristics of fungi. While in understory N addition treatments, SMF is regulated by bacterial diversity, which is mainly because of the strong resilience to disturbances and fast turnover of bacteria. Furthermore, rare microbial taxa may play a more important role in the maintenance of the SMF. This study provides the first evidence that N deposition enhanced SMF in temperate forests and enriches the knowledge on enhanced N deposition affecting forest ecosystems. Given the divergent results from two N addition approaches, an innovative perspective of canopy N addition on soil microbial diversity-multifunctionality relationships is crucial to policy-making for the conservation of soil microbial diversity and sustainable ecosystem management under enhanced N deposition. In future research, the consideration of canopy N processes is essential for more realistic assessments of the effects of atmospheric N deposition in forests.

Short-versus long-term effects of nitrogen addition and warming on soil nitrogen mineralization and leaching in a grass-dominated old field.

Increased atmospheric nitrogen (N) deposition and climate warming are both anticipated to influence the N dynamics of northern temperate ecosystems substantially over the next century. In field experiments with N addition and warming treatments, cumulative treatment effects can be important for explaining variation in treatment effects on N dynamics over time; however, comparisons between data collected in the early vs. later years potentially can be confounded with interactions between treatment effects and inter-annual variation in environmental conditions or other factors. We compared the short-term versus long-term effects of N addition and warming on net N mineralization and N leaching in a grass-dominated old field using in situ soil cores. We added new N addition and warming plots (3 years old) to an existing field experiment (16 years old), which enabled comparison of the treatment effects at both time scales while controlling for potential inter-annual variation in other factors. For net N mineralization, there was a significant interaction between plot age and N addition over the growing season, and for extractable inorganic N there was a significant interaction between plot age and warming over winter. In both cases, the directions of the treatment effects differed among old and new plots. Moreover, the responses in the new plots differed from the responses observed previously when the 16-year-old plots had been new. These results demonstrate how inter-annual variation in responses, independent from cumulative treatment effects, can play an important role in interpreting long-term effects on soil N cycling in global change field experiments.

Nitrogen deposition accelerates soil carbon sequestration in tropical forests.

Terrestrial ecosystem carbon (C) sequestration plays an important role in ameliorating global climate change. While tropical forests exert a disproportionately large influence on global C cycling, there remains an open question on changes in below-ground soil C stocks with global increases in nitrogen (N) deposition, because N supply often does not constrain the growth of tropical forests. We quantified soil C sequestration through more than a decade of continuous N addition experiment in an N-rich primary tropical forest. Results showed that long-term N additions increased soil C stocks by 7 to 21%, mainly arising from decreased C output fluxes and physical protection mechanisms without changes in the chemical composition of organic matter. A meta-analysis further verified that soil C sequestration induced by excess N inputs is a general phenomenon in tropical forests. Notably, soil N sequestration can keep pace with soil C, based on consistent C/N ratios under N additions. These findings provide empirical evidence that below-ground C sequestration can be stimulated in mature tropical forests under excess N deposition, which has important implications for predicting future terrestrial sinks for both elevated anthropogenic CO2 and N deposition. We further developed a conceptual model hypothesis depicting how soil C sequestration happens under chronic N deposition in N-limited and N-rich ecosystems, suggesting a direction to incorporate N deposition and N cycling into terrestrial C cycle models to improve the predictability on C sink strength as enhanced N deposition spreads from temperate into tropical systems.

Effects of experimental nitrogen deposition on soil organic carbon storage in Southern California drylands.

Atmospheric nitrogen (N) deposition is enriching soils with N across biomes. Soil N enrichment can increase plant productivity and affect microbial activity, thereby increasing soil organic carbon (SOC), but such responses vary across biomes. Drylands cover ~45% of Earth's land area and store ~33% of global SOC contained in the top 1 m of soil. Nitrogen fertilization could, therefore, disproportionately impact carbon (C) cycling, yet whether dryland SOC storage increases with N remains unclear. To understand how N enrichment may change SOC storage, we separated SOC into plant-derived, particulate organic C (POC), and largely microbially derived, mineral-associated organic C (MAOC) at four N deposition experimental sites in Southern California. Theory suggests that N enrichment increases the efficiency by which microbes build MAOC (C stabilization efficiency) if soil pH stays constant. But if soils acidify, a common response to N enrichment, then microbial biomass and enzymatic organic matter decay may decrease, increasing POC but not MAOC. We found that N enrichment had no effect on C fractions except for a decrease in MAOC at one site. Specifically, despite reported increases in plant biomass in three sites and decreases in microbial biomass and extracellular enzyme activities in two sites that acidified, POC did not increase. Furthermore, microbial C use and stabilization efficiency increased in a non-acidified site, but without increasing MAOC. Instead, MAOC decreased by 16% at one of the sites that acidified, likely because it lost 47% of the exchangeable calcium (Ca) relative to controls. Indeed, MAOC was strongly and positively affected by Ca, which directly and, through its positive effect on microbial biomass, explained 58% of variation in MAOC. Long-term effects of N fertilization on dryland SOC storage appear abiotic in nature, such that drylands where Ca-stabilization of SOC is prevalent and soils acidify, are most at risk for significant C loss.

Multi-decadal environmental change in the Barents Sea recorded by seal teeth.

Multiple environmental forcings, such as warming and changes in ocean circulation and nutrient supply, are affecting the base of Arctic marine ecosystems, with cascading effects on the entire food web through bottom-up control. Stable nitrogen isotopes (δ15 N) can be used to detect and unravel the impact of these forcings on this unique ecosystem, if the many processes that affect the δ15 N values are constrained. Combining unique 60-year records from compound specific δ15 N biomarkers on harp seal teeth alongside state-of-the-art ocean modelling, we observed a significant decline in the δ15 N values at the base of the Barents Sea food web from 1951 to 2012. This strong and persistent decadal trend emerges due to the combination of anthropogenic atmospheric nitrogen deposition in the Atlantic, increased northward transport of Atlantic water through Arctic gateways and local feedbacks from increasing Arctic primary production. Our results suggest that the Arctic ecosystem has been responding to anthropogenically induced local and remote drivers, linked to changing ocean biology, chemistry and physics, for at least 60 years. Accounting for these trends in δ15 N values at the base of the food web is essential to accurately detect ecosystem restructuring in this rapidly changing environment.

Fungal community structure and function shifts with atmospheric nitrogen deposition.

Fungal decomposition of soil organic matter depends on soil nitrogen (N) availability. This ecosystem process is being jeopardized by changes in N inputs that have resulted from a tripling of atmospheric N deposition in the last century. Soil fungi are impacted by atmospheric N deposition due to higher N availability, as soils are acidified, or as micronutrients become increasingly limiting. Fungal communities that persist with chronic N deposition may be enriched with traits that enable them to tolerate environmental stress, which may trade-off with traits enabling organic matter decomposition. We hypothesized that fungal communities would respond to N deposition by shifting community composition and functional gene abundances toward those that tolerate stress but are weak decomposers. We sampled soils at seven eastern US hardwood forests where ambient N deposition varied from 3.2 to 12.6 kg N ha-1  year-1 , five of which also have experimental plots where atmospheric N deposition was simulated through fertilizer application treatments (25-50 kg N ha-1  year-1 ). Fungal community and functional responses to fertilizer varied across the ambient N deposition gradient. Fungal biomass and richness increased with simulated N deposition at sites with low ambient deposition and decreased at sites with high ambient deposition. Fungal functional genes involved in hydrolysis of organic matter increased with ambient N deposition while genes involved in oxidation of organic matter decreased. One of four genes involved in generalized abiotic stress tolerance increased with ambient N deposition. In summary, we found that the divergent response to simulated N deposition depended on ambient N deposition levels. Fungal biomass, richness, and oxidative enzyme potential were reduced by N deposition where ambient N deposition was high suggesting fungal communities were pushed beyond an environmental stress threshold. Fungal community structure and function responses to N enrichment depended on ambient N deposition at a regional scale.

Significant contributions of combustion-related NH3 and non-fossil fuel NOx to elevation of nitrogen deposition in southwestern China over past five decades.

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 ( NH 4 + ) and nitrate ( NO 3 - ) 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 NH 4 + 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 NO 3 - deposition (by 6.0 and 2.9 kg-N/ha/year, respectively) in the later 30 years. This caused a shifted dominance from NO 3 - to NH 4 + 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 NH 4 + 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 NO 3 - 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.

Nitrogen application and osmotic stress antagonistically affect wheat seed germination and seedling growth.

Atmospheric nitrogen (N) deposition (AtNiDe) and drought stress (DS) have strongly affected plant growth. However, previous research has primarily focused on the effects of AtNiDe with various levels and DS on plant growth (especially seed germination and seedling growth). This study aimed to evaluate the single and combined effects of AtNiDe with four types (compounds: NH4-N, NO3-N, CO(NH2)2-N, and a mixture of the three types of N) and DS (three levels: control, low, and high) on wheat seed germination and seedling growth. The AtNiDe treatment increased wheat seed germination and seedling growth. Mixed N exerted a greater positive effect on wheat seed germination and seedling growth than single N forms. Organic N also had a greater positive effect on wheat seed germination and seedling growth than reduced inorganic N. The DS treatment decreased wheat seed germination and seedling growth. The AtNiDe treatment alleviated the adverse effects of DS on wheat seed germination and seedling growth. Mixed N had the greatest effect on alleviating the adverse effects of DS on wheat seed germination and seedling growth. Thus, AtNiDe and DS antagonistically affected wheat seed germination and seedling growth. NOVELTY STATEMENT This study assessed the single and combined effects of atmospheric nitrogen deposition with four types and drought stress at three levels on wheat seed germination and seedling growth. Generally, nitrogen and drought antagonistically affected wheat seed germination and seedling growth.

Sierra Nevada mountain lake microbial communities are structured by temperature, resources and geographic location.

Warming, eutrophication (nutrient fertilization) and brownification (increased loading of allochthonous organic matter) are three global trends impacting lake ecosystems. However, the independent and synergistic effects of resource addition and warming on autotrophic and heterotrophic microorganisms are largely unknown. In this study, we investigate the independent and interactive effects of temperature, dissolved organic carbon (DOC, both allochthonous and autochthonous) and nitrogen (N) supply, in addition to the effect of spatial variables, on the composition, richness, and evenness of prokaryotic and eukaryotic microbial communities in lakes across elevation and N deposition gradients in the Sierra Nevada mountains of California, USA. We found that both prokaryotic and eukaryotic communities are structured by temperature, terrestrial (allochthonous) DOC and latitude. Prokaryotic communities are also influenced by total and aquatic (autochthonous) DOC, while eukaryotic communities are also structured by nitrate. Additionally, increasing N availability was associated with reduced richness of prokaryotic communities, and both lower richness and evenness of eukaryotes. We did not detect any synergistic or antagonistic effects as there were no interactions among temperature and resource variables. Together, our results suggest that (a) organic and inorganic resources, temperature, and geographic location (based on latitude and longitude) independently influence lake microbial communities; and (b) increasing N supply due to atmospheric N deposition may reduce richness of both prokaryotic and eukaryotic microbes, probably by reducing niche dimensionality. Our study provides insight into abiotic processes structuring microbial communities across environmental gradients and their potential roles in material and energy fluxes within and between ecosystems.

Drivers of temporal changes in temperate forest plant diversity vary across spatial scales.

Global biodiversity is affected by numerous environmental drivers. Yet, the extent to which global environmental changes contribute to changes in local diversity is poorly understood. We investigated biodiversity changes in a meta-analysis of 39 resurvey studies in European temperate forests (3988 vegetation records in total, 17-75 years between the two surveys) by assessing the importance of (i) coarse-resolution (i.e., among sites) vs. fine-resolution (i.e., within sites) environmental differences and (ii) changing environmental conditions between surveys. Our results clarify the mechanisms underlying the direction and magnitude of local-scale biodiversity changes. While not detecting any net local diversity loss, we observed considerable among-site variation, partly explained by temporal changes in light availability (a local driver) and density of large herbivores (a regional driver). Furthermore, strong evidence was found that presurvey levels of nitrogen deposition determined subsequent diversity changes. We conclude that models forecasting future biodiversity changes should consider coarse-resolution environmental changes, account for differences in baseline environmental conditions and for local changes in fine-resolution environmental conditions.

Soil phosphorus constrains biodiversity across European grasslands.

Nutrient pollution presents a serious threat to biodiversity conservation. In terrestrial ecosystems, the deleterious effects of nitrogen pollution are increasingly understood and several mitigating environmental policies have been developed. Compared to nitrogen, the effects of increased phosphorus have received far less attention, although some studies have indicated that phosphorus pollution may be detrimental for biodiversity as well. On the basis of a dataset covering 501 grassland plots throughout Europe, we demonstrate that, independent of the level of atmospheric nitrogen deposition and soil acidity, plant species richness was consistently negatively related to soil phosphorus. We also identified thresholds in soil phosphorus above which biodiversity appears to remain at a constant low level. Our results indicate that nutrient management policies biased toward reducing nitrogen pollution will fail to preserve biodiversity. As soil phosphorus is known to be extremely persistent and we found no evidence for a critical threshold below which no environmental harm is expected, we suggest that agro-environmental schemes should include grasslands that are permanently free from phosphorus fertilization.

Characteristics of sub-micron aerosols in the Yellow Sea and its environmental implications.

The Yellow Sea, characterized by an influx of both natural marine and anthropogenic pollutants, coupled with favorable photochemical conditions, serve as key sites for potential interactions between atmospheric gases and aerosols. A recent air monitoring campaign in the Yellow Sea revealed aerosol contributions from four sources, with the highest mass concentrations and dominance of NO3- (38.1 ± 0.37 %) during winds from China. Indications of potential secondary aerosol formation were observed through the presence of hydrolysis and oxidation products of nitrate and volatile organic compounds. Correlations between time series distributions of biomass burning organic aerosols and particle number counts (Dp 100-500 nm, R2 = 0.94) further suggest potential size growth through adsorption and scavenging processes. The results from this study provide observational evidence of a shift in atmospheric compositions from sulfate to nitrate, leading to an increased atmospheric nitrogen deposition in the Yellow Sea.

Quantitative assessment of benefits and losses from the influence of atmospheric nitrogen deposition on cropland ecosystems in China.

China is facing severe atmospheric nitrogen (N) deposition. Assessing the economic benefits and damage costs induced by N deposition can help to develop effective mitigation strategies for N emissions. A net economic benefit method was used to assess the economic impact of N deposition in cropland ecosystems in China in 2020. The results showed that atmospheric N deposition gained an economic benefit of $4896.0 million through increased yields of major grain crops and a climate benefit of $1259.3 million through cooling effects. On the other hand, N deposition induced economic losses of $6257.1 and $1063.4 million, respectively to human health and ecosystem health; excessive N deposition induced damages of $137.8 million due to reduced crop yields and $168.4 million due to the increased greenhouse gas emissions. In general, the net economic benefit was -$1471.5 million (-$5324.7 âˆ¼ $921.4 million), indicating that China is suffering economic losses due to N deposition in cropland ecosystems. These results would provide scientific data for the government to enact efficient measures to reduce N pollution.

Moss differentiating the fluxes and sources of nitrogen deposition between 1984 and 2021 in a mountain area of Northern China.

Anthropogenic reactive nitrogen (N) deposition has increased significantly since the industrial revolution. Northern China has become a global hotspot for N deposition. However, few studies have been conducted to quantify the historical changes of atmospheric N deposition fluxes and source contributions in Northern China. By investigating N contents and δ15N values of mosses at Mount Tai (Northern China) in 1984 and 2021, we reconstructed fluxes and source contributions of wet inorganic N deposition and evaluated their historical changes. Compared with 1984, moss N contents (from 1.7 ± 0.3% to 2.1 ± 0.4%) showed a significant increase in 2021, which was mainly attributed to a significant increase in nitrate N deposition fluxes at Mount Tai. Moss δ15N values (from -5.9 ± 0.9‰ to -5.2 ± 2.4‰) showed a slight increase from 1984 to 2021 at Mount Tai. The importance of combustion-related NH3 (including vehicle exhaust, coal combustion, and biomass burning) in 2021 (51.2%) were higher than those in 1984 (43.9%), while the importance of volatilization NH3 sources (including waste and fertilizers) in 2021 (48.8%) were lower than those in 1984 (56.1%). It was fossil-fuel NOx (from vehicle exhaust and coal combustion) (54.1%) rather than non-fossil fuel NOx (from biomass burning and microbial N cycles) (45.9%) dominated NOx emissions in both 1984 and 2021. Our results revealed significant contributions of combustion-related NH3 and fossil-fuel NOx sources emissions to the elevation of N deposition at Mount Tai in Northern China, which are beneficial for mitigating N emissions and conducting ecological benefit assessments in Northern China.

Climate warming impacts chewing Spodoptera litura negatively but sucking Corythucha marmorata positively on native Solidago canadensis.

Insect-plant interactions are among importantly ecological processes, and rapid environmental changes such as temperature and resource fluctuations can disrupt long-standing insect-plant interactions. While individual impacts of climate warming, atmospheric nitrogen (N) deposition, and plant provenance on insect-plant interactions are well studied, their joint effects on insect-plant interactions are less explored in ecologically realistic settings. To this end, we performed five experiments with native and invasive Solidago canadensis populations from home and introduced ranges and two insect herbivores (leaf-chewing Spodoptera litura and sap-sucking Corythucha marmorata) in the context of climate warming and N deposition. We determined leaf defensive traits, feeding preference, and insect growth and development, and quantified the possible associations among climate change, host-plant traits, and insect performance with structural equation modeling. First, native S. canadensis populations experienced higher damage by S. litura but lower damage by C. marmorata than invasive S. canadensis populations in the ambient environment. Second, warming decreased the leaf consumption, growth, and survival of S. litura on native S. canadensis populations, but did not affect these traits on invasive S. canadensis populations; warming increased the number of C. marmorata on native S. canadensis populations via direct facilitation, but decreased that on invasive S. canadensis populations via indirect suppression. Third, N addition enhanced the survival of S. litura on native S. canadensis populations, and its feeding preference and leaf consumption on invasive S. canadensis populations. Finally, warming plus N addition exhibited non-additive effects on insect-plant interactions. Based on these results, we tentatively conclude that climate warming could have contrasting effects on insect-plant interactions depending on host-plant provenance and that the effects of atmospheric N deposition on insects might be relatively weak compared to climate warming. Future studies should focus on the molecular mechanisms underlying these different patterns.

An in situ 15N labeling experiment unveils distinct responses to N application approaches in a mountain beech forest.

Atmospheric nitrogen (N) deposition has notably increased since the industrial revolution, doubling N inputs to terrestrial ecosystems. This could mitigate N limitations in forests, potentially enhancing productivity and carbon sequestration. However, excessive N can lead to forest N saturation, causing issues like soil acidification, nutrient imbalances, biodiversity loss, increased tree mortality and a potential net greenhouse gas emission. Traditional experiments often overlook the canopy's role in N fate, focusing instead on direct N addition to the forest floor. In our study, we applied 20 kg N ha y-1 of labeled 15NH415NO3 solution (δ15N = 30‰) both above and below the canopy, maintaining also control plots. We assessed ecosystem components before and after treatment, calculated N stocks, and used mass balance for fertilizer recovery analysis. Findings revealed that the above-canopy N addition intercepted up to 31 ± 4% of added N in foliage, a significant contrast to the negligible recovery in leaves with below-canopy treatment. Overall plant recovery was higher in the above-canopy treatment (43 ± 11%) compared with below (9 ± 24%). Post-vegetative season, about 15 ± 1% of above-canopy added N was transferred to soil via litterfall, indicating substantial N reabsorption or loss through volatilization, stemflow or throughfall. In contrast, the below-canopy approach resulted in just 4.0 ± 0.6% recovery via litterfall. These results highlight a significant difference in N fate based on the application method. Nitrogen applied to the canopy showed distinct recovery in transient compartments like foliage. However, over a few months, there was no noticeable change in N recovery in long-lived tissues across treatments. This implies that N application strategy does not significantly alter the distribution of simulated wet N deposition in high Carbon/N tissues, underscoring the complex dynamics of forest N cycling.

The impact of simulated chronic nitrogen deposition on the biomass and N2-fixation activity of two boreal feather moss-cyanobacteria associations.

Bryophytes achieve substantial biomass and play several key functional roles in boreal forests that can influence how carbon (C) and nitrogen (N) cycling respond to atmospheric deposition of reactive nitrogen (Nr). They associate with cyanobacteria that fix atmospheric N2, and downregulation of this process may offset anthropogenic Nr inputs to boreal systems. Bryophytes also promote soil C accumulation by thermally insulating soils, and changes in their biomass influence soil C dynamics. Using a unique large-scale (0.1 ha forested plots), long-term experiment (16 years) in northern Sweden where we simulated anthropogenic Nr deposition, we measured the biomass and N2-fixation response of two bryophyte species, the feather mosses Hylocomium splendens and Pleurozium schreberi. Our data show that the biomass declined for both species; however, N2-fixation rates per unit mass and per unit area declined only for H. splendens. The low and high treatments resulted in a 29% and 54% reduction in total feather moss biomass, and a 58% and 97% reduction in total N2-fixation rate per unit area, respectively. These results help to quantify the sensitivity of feather moss biomass and N2 fixation to chronic Nr deposition, which is relevant for modelling ecosystem C and N balances in boreal ecosystems.

Environmental significance of atmospheric nitrogen deposition in the transition zone between the Tibetan Plateau and arid region.

The ecological effect of atmospheric N deposition has become a hot research point along with intensive human activities and global climatic change. As the transition zone between the Tibetan Plateau and the arid region, the Qilian Mountains are important ecological barriers and source regions of inland rivers in northwest China. However, the quantification of N deposition in the transition zone between the Tibetan Plateau (TP) and the arid region remains unclear, primarily due to the lack of in situ measurements. Hence, an observational study was conducted on the Qilian Mountains, and precipitation data were collected. Approximately 1382 samples were collected and analyzed regarding their characteristics and environmental effects of the atmospheric N wet deposition. The annual wet deposition of atmospheric N was 10.05 kg/hm2, and showed a decreasing trend from the south to the north of the Qilian Mountains. NO3--N deposition was the main form of wet deposition of atmospheric N on the Qilian Mountains, accounting for 73.83% of the DIN deposition. Additionally, altitude, meteorological factors, and ecosystem types were found to influence the wet deposition of atmospheric N. The contribution of NO3--N to the wet deposition of atmospheric N in meadows, forests, grasslands, farmlands, and towns was 48.38%, 71.55%, 77.54%, 69.61%, and 82.84%, respectively. This study provides a scientific basis for the effective management and sustainable development of environmental protection in the transition zone between the TP and the arid region. The results showed that relevant policies, as well as scientific and governmental measures, could contribute to reducing N deposition effectively. However, the further mitigation measures should be proposed and strictly enforced.

Types and Distribution of Organic Amines in Organic Nitrogen Deposition in Strategic Water Sources.

Organic nitrogen (ON) is an important part of atmospheric nitrogen deposition, but the content and distribution of components other than urea and amino acids are the blind area of current research. The deposition of organic amines (OA) in strategic water sources poses a great public health risk to unspecified populations. In order to further reveal the composition of about 50% soluble organic nitrogen, besides urea and amino acids, five functional sampling points (such as industrial area, agricultural area, urban area, tourism area and forest area) were set in the reservoir area to detect dissolved total nitrogen (DTN), dissolved organic nitrogen (DON) and OA components. The results show that the total nitrogen concentration was 6.42-10.82 mg/m3 and the DON concentration was 2.77-4.99 mg/m3. Ten kinds of OA were detected: dimethylamine (DMA), diethylamine (DEA), propylamine (PA), butylamine (BA), pyrrolidine (PYR), dibutylamine (DBA), N-methylaniline (NMA), 2-ethylaniline (2-ELA), benzylamine (BMA), and 4-ethylaniline (4-ELA). The average concentrations were 7.64, 26.35, 14.51, 14.10, 18.55, 7.92, 10.56, 12.84, 13.46 and 21.00 ng/m3, respectively. The total concentration of ten OA accounted for 2.28-9.81% of DON in the current month, of which the content of DEA was the highest, reaching 0.71%, the content of 4-ELA, PYR, PA and BA was 0.4-0.56%, and the content of DMA, DBA and NMA was 0.2-0.36%. The sources of OA in the reservoir area have significant seasonal differences. The content is the highest in spring, followed by autumn, and lower in summer and winter. The rainfall in spring and autumn is small, the source of road dust is relatively high, and the rainfall in summer is large. After the particles in the air are washed by rain, the concentration of OA in the sample is the lowest. On account of spring and autumn being the time of frequent agricultural activities, the concentration of OA is significantly higher than that in winter and summer.

Leaf Photosynthetic and Functional Traits of Grassland Dominant Species in Response to Nutrient Addition on the Chinese Loess Plateau.

Leaf photosynthetic and functional traits of dominant species are important for understanding grassland community dynamics under imbalanced nitrogen (N) and phosphorus (P) inputs. Here, the effects of N (N0, N50, and N100, corresponding to 0, 50, and 100 kg ha-1 yr-1, respectively) or/and P additions (P0, P40, and P80, corresponding to 0, 40, and 80 kg ha-1 yr-1) on photosynthetic characteristics and leaf economic traits of three dominant species (two grasses: Bothriochloa ischaemum and Stipa bungeana; a leguminous subshrub: Lespedeza davurica) were investigated in a semiarid grassland community on the Loess Plateau of China. Results showed that, after a three-year N addition, all three species had higher specific leaf area (SLA), leaf chlorophyll content (SPAD value), maximum net photosynthetic rate (PNmax), and leaf instantaneous water use efficiency (WUE), while also having a lower leaf dry matter content (LDMC). The two grasses, B. ischaemum and S. bungeana, showed greater increases in PNmax and SLA than the subshrub L. davurica. P addition alone had no noticeable effect on the PNmax of the two grasses while it significantly increased the PNmax of L. davurica. There was an evident synergetic effect of the addition of N and P combined on photosynthetic traits and most leaf economic traits in the three species. All species had relatively high PNmax and SLA under the addition of N50 combined with P40. Overall, this study suggests that N and P addition shifted leaf economic traits towards a greater light harvesting ability and, thus, elevated photosynthesis in the three dominant species of a semiarid grassland community, and this was achieved by species-specific responses in leaf functional traits. These results may provide insights into grassland restoration and the assessment of community development in the context of atmospheric N deposition and intensive agricultural fertilization.