Chronic Nitrogen Research Articles

Chronic nitrogen legacy in the aquifers of China

About half of the global drinking water comes from groundwater, yet groundwater quality is threatened by high nitrate concentrations globally. Our understanding of groundwater nitrate concentrations is often limited by inaccessibility of groundwater and scarcity of nitrate data in groundwater. Here we used machine learning and decision tree-heatmap analysis by compiling nitrate concentrations and isotope data from 4047 groundwater sites across China to understand their dynamics and drivers across gradients of geographical, climate, and human factors. Results show that nitrate concentrations vary substantially over depth and are generally lower in deeper groundwater, indicating potentially higher nitrate removal rates according to nitrate isotopic pattern such as denitrification at depth. At similar groundwater aquifer depths, nitrate concentrations are highest in urban regions with high population density. In addition, nitrate concentrations are generally higher in arid northern China than humid southern China. Interestingly, while groundwater nitrate concentrations are lower at deeper depths, slow groundwater flow also indicates prolonged nitrogen legacy. Although there has been an overall decline in groundwater nitrogen pollution in China since 2016, persistent pollution has lingered. Future strategies for groundwater quality protection in China should address the long-term legacy of nitrate in different aquifers and rising nitrogen levels in groundwater.

Deciphering the active bacteria involving glucose-triggered priming effect in soils with gradient N inputs

The soil priming effect, which refers to the alteration of soil organic matter (SOM) decomposition due to labile carbon (C) inputs, is widely acknowledged for its impact on C storage in terrestrial ecosystems. However, the impact of chronic nitrogen (N) fertilizer on soil priming effect, particularly in agroecological systems, remains unclear. Here, we utilized soils subjected to varying levels of N fertilization (0, 140, 280, 470, and 660 kg N ha−1 y−1), which were collected from a long-term experimental site. Enzyme activity related to C, N, and phosphorus (P) acquisition was measured using the fluorometric method. Additionally, DNA-based stable isotope probing with 13C-labeled glucose was conducted to explore the role of active bacterial communities (16S rRNA gene analysis) on the priming effect in soils with different N fertilization histories. Glucose addition enhanced the decomposition of native SOM and induced positive priming effects in all soils, which were amplified by the historical N application level. Activity of C-related enzymes essential for soil C decomposition increased following glucose addition, which was positively correlated with the soil priming effect. Active bacterial taxa, primarily Firmicutes, Actinobacteria, and Proteobacteria, were capable of assimilating exogenous glucose-C or native SOM-C. Notably, bacteria assimilating glucose exhibited higher abundance-weighted average ribosomal RNA gene operon copy number than those assimilating SOM, indicating the role of r-strategists in accelerating SOC turnover and increasing C loss. These findings highlight the role of active microbial community attributes on the soil priming effects. This study provides new insights into the intricate processes of C transformation in soils subjected to long-term N management in agroecosystems.

Long-Term Nitrogen Dioxide Exposure as a Possible 5-Year Mortality Risk Factor in Diabetic Patients Treated Using Off-Pump Surgical Revascularization-A Retrospective Analysis.

Background: There is mounting evidence that diabetic-related cardiac metabolism abnormalities with oxidative stress and inflammatory mechanism activation align with the functional impairments that result in atherosclerotic lesion formation. Among the possible non-traditional coronary lesion risk factors, environmental exposure may be significant, especially in diabetic patients. Methods: A total of 140 diabetic patients (115 (82%) males and 25 (18%) females) with a mean age of 65 (60-71) underwent surgical revascularization due to multivessel coronary disease. The possible all-cause mortality risk factors, including demographical and clinical factors followed by chronic air pollution exposure, were identified. Results: All patients were operated on using the off-pump technique and followed for 5.6 (5-6.1) years. The multivariable model for 5-year mortality prediction presented the nitrogen dioxide chronic exposure (HR: 3.99, 95% CI: 1.16-13.71, p = 0.028) and completeness of revascularization (HR: 0.19, 95% CI: 0.04-0.86, p = 0.031) as significant all-cause mortality risk factors. Conclusions: Ambient air pollutants such as an excessive chronic nitrogen dioxide concentration (>15 µg/m3) may increase 5-year all-cause mortality in diabetic patients following surgical revascularization.

Sensitivity of root production to long‐term aridity under environmental perturbations in Chihuahuan Desert ecosystems

Abstract Root production influences carbon and nutrient cycles and subsidizes soil biodiversity. However, the long‐term dynamics and drivers of belowground production are poorly understood for most ecosystems. In drylands, fire, eutrophication, and precipitation regimes could affect not only root production but also how roots track interannual variability in climate. We manipulated the intra‐annual precipitation regime, soil nitrogen, and fire in four common Chihuahuan Desert ecosystem types (three grasslands and one shrubland) in New Mexico, USA, where the 100‐year record indicates both long‐term drying and increasing interannual variability in aridity. First, we evaluated how root production tracked aridity over 10–17 years using climate sensitivity functions, which quantify long‐term, nonlinear relationships between biological processes and climate. Next, we determined the degree to which perturbations by fire, nitrogen addition or intra‐annual rainfall altered the sensitivity of root production to both mean and interannual variability in aridity. All ecosystems had nonlinear climate sensitivities that predicted declines in production with increases in the interannual variance of aridity. However, root production was the most sensitive to aridity in Chihuahuan Desert shrubland, with reduced production under drier and more variable aridity. Among the perturbations, only fire altered the sensitivity of root production to aridity. Root production was more than twice as sensitive to declines with aridity following prescribed fire than in unburned conditions. Neither the intra‐annual seasonal rainfall regime nor chronic nitrogen fertilization altered the sensitivity of roots to aridity. Synthesis. Our results yield new insight into how dryland plant roots respond to climate change. Our comparison of dryland ecosystems of the northern Chihuahuan Desert predicted that root production in shrublands would be more sensitive to future climates that are drier and more variable than root production in dry grasslands. Field manipulations revealed that fire could amplify the climate sensitivity of dry grassland root production, but in contrast, the climate sensitivity of root production was largely resistant to changes in the seasonal rainfall regime or increased soil fertilization.

Revisiting the bucket model: Long‐term effects of rainfall variability and nitrogen enrichment on net primary production in a desert grassland

Abstract The predicted intensification of the North American Monsoon is expected to alter growing season rainfall patterns in the southwestern United States. These patterns, which have historically been characterized by frequent small rain events, are anticipated to shift towards a more extreme precipitation regime consisting of fewer, but larger rain events. Furthermore, human activities are contributing to increased atmospheric nitrogen deposition throughout this dryland region. Alterations in rainfall size and frequency, along with changes in nitrogen availability, are likely to have significant consequences for above‐ground net primary production (ANPP) and plant community dynamics in drylands. The conceptual bucket model predicts that a shift towards fewer, but larger rain events could promote greater rates of ANPP in these regions by maintaining soil moisture availability above drought stress thresholds for longer periods during the growing season. However, only a few short‐term studies have tested this hypothesis, and none have explored the interaction between altered rainfall patterns and nitrogen enrichment. To address this knowledge gap, we conducted a 14‐year rainfall addition and nitrogen fertilization experiment in a northern Chihuahuan Desert grassland to explore the long‐term impacts of changes in monsoon rainfall size and frequency, along with chronic nitrogen enrichment, on ANPP (measured as peak biomass) and plant community dynamics. Contrary to bucket model predictions, small frequent rain events promoted comparable rates of ANPP to large infrequent rain events in the absence of nitrogen enrichment. It was only when nitrogen limitation was alleviated that large infrequent rain events resulted in the greatest ANPP. Furthermore, we found that nitrogen enrichment had the greatest impact on plant community composition under the small frequent rainfall regime. Synthesis. Our long‐term field experiment highlights limitations of the bucket model by demonstrating that water and nitrogen availability sequentially limit dryland ecological processes. Specifically, our findings suggest that while water availability is the primary limiting factor for above‐ground net primary production in these ecosystems, nitrogen limitation becomes increasingly important when water is not limiting. Moreover, our findings reveal that small frequent rain events play an important but underappreciated role in driving dryland ecosystem dynamics.

Does chronic nitrogen deposition have effects on grass physiology of natural habitats?

Anthropogenic activities increase nitrogen deposition, which can have major consequences for biodiversity and ecosystem function. We experimentally investigated how a community of grass species would respond to three levels of nitrogen enrichment, considering their biomass and efficiency of photosynthesis. The experiments were conducted in an open savanna area with sparse trees (cerrado sensu stricto). Fifteen plots were divided considering high nitrogen supplementation, low nitrogen supplementation, and control (without nitrogen application) as treatments. The nitrogen dosages applied in each treatment were based on deposition estimates for 2050 in the Cerrado biome. Three native species had their photosynthetic metabolism evaluated 45 days after the last supplementation. Except for Echinolaena inflexa, all the 16 species found in the area expressed C4 metabolism. Echinolaena inflexa and Loudetiopsis chrysothrix showed an increase in leaf nitrogen with supplementation, with differences in chloroplastidic pigment contents. Differently from L. chrysothrix which increased its chlorophyll content, E. inflexa showed a decrease in chlorophyll and carotenoid contents under N supplementation. E. inflexa is a small species and its leaf expansion under higher contents of nitrogen suggests that this species was affected by shading caused by other community species that increased their biomass. Supplemental nitrogen did not affect the efficiency of photosynthesis of these species or severely change the grass community biomass. However, we noted only a tendency for higher biomass and biomass per tiller for Urochloa decumbens, which suggests a concern about exotic species' performance under conditions of greater availability of nitrogen.

Shifts in bacterial traits under chronic nitrogen deposition align with soil processes in arbuscular, but not ectomycorrhizal-associated trees.

Nitrogen (N) deposition increases soil carbon (C) storage by reducing microbial activity. These effects vary in soil beneath trees that associate with arbuscular (AM) and ectomycorrhizal (ECM) fungi. Variation in carbon C and N uptake traits among microbes may explain differences in soil nutrient cycling between mycorrhizal associations in response to high N loads, a mechanism not previously examined due to methodological limitations. Here, we used quantitative Stable Isotope Probing (qSIP) to measure bacterial C and N assimilation rates from an added organic compound, which we conceptualize as functional traits. As such, we applied a trait-based approach to explore whether variation in assimilation rates of bacterial taxa can inform shifts in soil function under chronic N deposition. We show taxon-specific and community-wide declines of bacterial C and N uptake under chronic N deposition in both AM and ECM soils. N deposition-induced reductions in microbial activity were mirrored by declines in soil organic matter mineralization rates in AM but not ECM soils. Our findings suggest C and N uptake traits of bacterial communities can predict C cycling feedbacks to N deposition in AM soils, but additional data, for instance on the traits of fungi, may be needed to connect microbial traits with soil C and N cycling in ECM systems. Our study also highlights the potential of employing qSIP in conjunction with trait-based analytical approaches to inform how ecological processes of microbial communities influence soil functioning.

Microbial controls over soil priming effects under chronic nitrogen and phosphorus additions in subtropical forests.

The soil priming effect (PE), defined as the modification of soil organic matter decomposition by labile carbon (C) inputs, is known to influence C storage in terrestrial ecosystems. However, how chronic nutrient addition, particularly in leguminous and non-leguminous forests, will affect PE through interaction with nutrient (e.g., nitrogenandphosphorus) availability is still unclear. Therefore, we collected soils from leguminous and non-leguminous subtropical plantations across a suite of historical nutrient addition regimes. We added 13C-labeled glucose to investigate how background soil nutrient conditions and microbial communities affect priming and its potential microbial mechanisms. Glucose addition increased soil organic matter decomposition and prompted positive priming in all soils, regardless of dominant overstory tree species or fertilizer treatment. In non-leguminous soil, only combined nitrogen and phosphorus addition led to a higher positive priming than the control. Conversely, soils beneath N-fixing leguminous plants responded positively to P addition alone, as well as to joint NP addition compared to control. Using DNA stable-isotope probing, high-throughput quantitative PCR, enzyme assays and microbial C substrate utilization, we found that positive PE was associated with increased microbial C utilization, accompanied by an increase in microbial community activity, nutrient-related gene abundance, and enzyme activities. Our findings suggest that the balance between soil available N and P effects onthe PE, wasdependent on rhizosphere microbial community composition. Furthermore, these findings highlight the roles of the interaction between plants and their symbiotic microbial communities in affecting soil priming and improve our understanding of the potential microbial pathways underlying soil PEs.

Ammonia oxidation and nitrate reduction marker genes are key indicators of nitrogen losses in temperate forest catchments.

Chronic nitrogen inputs can alleviate N limitation and potentially impose N losses in forests, indicated by soil enrichment in 15 N over 14 N. However, the complexity of the nitrogen cycle hinders accurate quantification of N fluxes. Simultaneously, soil ecologists are striving to find meaningful indicators to characterise the "openness" of the nitrogen cycle. We integrate soil δ15 N with constrained ecosystem N losses and the functional gene potential of the soil microbiome in 14 temperate forest catchments. We show that N losses are associated with soil δ15 N and that δ15 N scales with the abundance of soil bacteria. The abundance of the archaeal amoA gene, representing the first step in nitrification (ammonia oxidation to nitrite), followed by the abundance of narG and napA genes, associated with the first step in denitrification (nitrate reduction to nitrite), explains most of the variability in soil δ15 N. These genes are more informative than the denitrification genes nirS and nirK, which are directly linked to N2 O production. Nitrite formation thus appears to be the critical step associated with N losses. Furthermore, we show that the genetic potential for ammonia oxidation and nitrate reduction is representative of forest soil 15 N enrichment and thus indicative of ecosystem N losses.

As above, not so below: Long-term dynamics of net primary production across a dryland transition zone.

Drylands are key contributors to interannual variation in the terrestrial carbon sink, which has been attributed primarily to broad-scale climatic anomalies that disproportionately affect net primary production (NPP) in these ecosystems. Current knowledge around the patterns and controls of NPP is based largely on measurements of aboveground NPP (ANPP), particularly in the context of altered precipitation regimes. Limited evidence suggests belowground NPP (BNPP), a major input to the terrestrial carbon pool, may respond differently than ANPP to precipitation, as well as other drivers of environmental change, such as nitrogen deposition and fire. Yet long-term measurements of BNPP are rare, contributing to uncertainty in carbon cycle assessments. Here, we used sixteen years of annual NPP measurements to investigate responses of ANPP and BNPP to several environmental change drivers across a grassland-shrubland transition zone in the northern Chihuahuan Desert. ANPP was positively correlated with annual precipitation across this landscape; however, this relationship was weaker within sites. BNPP, on the other hand, was weakly correlated with precipitation only in Chihuahuan Desert shrubland. Although NPP generally exhibited similar trends among sites, temporal correlations between ANPP and BNPP within sites were weak. We found chronic nitrogen enrichment stimulated ANPP, whereas a one-time prescribed burn reduced ANPP for nearly a decade. Surprisingly, BNPP was largely unaffected by these factors. Together, our results suggest that BNPP is driven by a different set of controls than ANPP. Furthermore, our findings imply belowground production cannot be inferred from aboveground measurements in dryland ecosystems. Improving understanding around the patterns and controls of dryland NPP at interannual to decadal scales is fundamentally important because of their measurable impact on the global carbon cycle. This study underscores the need for more long-term measurements of BNPP to improve assessments of the terrestrial carbon sink, particularly in the context of ongoing environmental change.

CO2 Uptake Offsets Other Greenhouse Gas Emissions from Salt Marshes with Chronic Nitrogen Loading

CO2 Uptake Offsets Other Greenhouse Gas Emissions from Salt Marshes with Chronic Nitrogen Loading

Chronic nitrogen enrichment decreases soil gross nitrogen mineralization by acidification in topsoil but by carbon limitation in subsoil

Continuously increasing nitrogen (N) deposition alters soil N mineralization, which is of vital importance in regulating soil N availability and satisfying plant N demands. Neither the response pattern of soil N mineralization to the chronic N enrichment nor the related mechanism has been well studied, particularly in deep soils. In this study, we measured the gross N mineralization rate (GNMR) as well as the corresponding soil abiotic and biotic properties along a chronic N addition gradient (7 years) at different soil depths in an alpine meadow. The results showed that GNMR was negatively correlated with the N addition rate and soil available NO3– content at each soil depth (P < 0.05). GNMR was positively correlated with soil pH, which was significantly decreased by N addition in the topsoil (0–10 cm, P < 0.05), but GNMR was not related to pH in the subsoil (10–20 or 20–40 cm). Instead, it was positively correlated with the soil carbon/nitrogen ratio (C/N), which was significantly decreased by N addition in subsoil (P < 0.05). Soil pH and C/N mainly explained the decreased GNMR with N enrichment in the topsoil and subsoil, respectively, and the contribution of the soil C/N to the decreased GNMR at the 20–40-cm soil depth was larger than that at the 10–20-cm soil depth. The results demonstrated that acidification, increased N availability, and C limitation all decreased soil N mineralization under chronic N deposition, but acidification dominated the decrease in the topsoil, while C limitation dominated the decrease in the subsoil. These findings extend our understanding of the response pattern and mechanism of soil N mineralization with chronic N enrichment at different soil depths. The diverse mechanisms underlying soil N mineralization in response to chronically increasing N deposition at different soil depths should be considered when predicting soil N mineralization in the context of global change.

Airway inflammation in adolescents and elderly women: Chronic air pollution exposure and polygenic susceptibility

Background and aimThe fractional exhaled nitric oxide (FeNO) concentration in the exhaled breath is a biomarker for eosinophilic airway inflammation. We explored the interplay between chronic air pollution exposure and polygenic susceptibility to airway inflammation at different critical age stages. MethodsAdolescents (15 yr) enrolled in the GINIplus/LISA birth cohorts (n = 2434) and 220 elderly women (75 yr on average) enrolled in the SALIA cohort with FeNO measurements available were investigated. Environmental main effects of the mean of ESCAPE land-use regression air pollutant concentrations within a time window of 15 years and main effects of the polygenic risk scores (PRS) using internal weights from elastic net regression of genome-wide derived single nucleotide polymorphisms were investigated. Furthermore, we examined gene-environment interaction (GxE) effects on natural log-transformed FeNO levels by adjusted linear regression models. ResultsWhile we observed no significant environmental and polygenic main effects on airway inflammation in either age group, we found robust harmful effects of chronic nitrogen dioxide (NO2) exposure in the GxE models for elderly women (16.2 % increase in FeNO, p-value = 0.027). Stratified analyses found GxE effects between the PRS and chronic NO2 exposure in never-smoker elderly women and in adolescents without any inflammatory respiratory conditions. ConclusionsFeNO measurement is a useful biomarker to detect higher risk of NO2-induced eosinophilic airway inflammation in the elderly. There was limited evidence for GxE effects on airway inflammation in adolescents or the elderly. Further GxE studies in subpopulations should be conducted to investigate the assumption that susceptibility to airway inflammation differs between age stages.

Chronic nitrogen deposition drives microbial community change and disrupts bacterial-fungal interactions along a subtropical urbanization gradient

Microbial responses to nitrogen (N) enrichment under chronic ambient N deposition conditions are understudied, especially in subtropical forests which are often not limited by N. We investigated variation in subtropical forest soil microbial biomass and composition, bacterial-fungal interactions (BFI), and their linkages to N cycling along a gradient of high-rate N deposition in Shanghai, China (estimated N deposition >40–100 kg N ha−1 yr−1). In contrast to global and temperate findings, arbuscular mycorrhizal (AM) fungal biomass and the ratio of AM to saprotrophic fungi increased with ammonium, indicating possibly increased P limitation following N enrichment in this subtropical ecosystem. Positive BFI (positive relationships between a particular fungal OTU and bacterial OTU) mostly decreased (by >85%) with increasing N availability, as did negative BFI (by > 85%), suggesting that bacterial-fungal cooperation and competition both tended to weaken under nutrient-rich conditions. Ammonium and nitrate were significantly related to overall microbial community composition and microorganisms at different taxonomic levels. Increasing ammonium seemed to favor taxa from bacterial phyla Acidobacteria, Actinobacteria, and Nitrospirae. Most pathogenic fungal OTUs increased with nitrate. Other microbial groups and taxa involved in litter decomposition and N cycling also had significant relationships with ammonium and/or nitrate. We found that N availability may drive significant microbial community change even when present in excess. Overall, we observed strong microbial linkages to N availability in subtropical forests under chronic high-rate N deposition, and specific relationships were often contrary to previous observations from temperate N addition experiments.

Integration of Dual Stress Transcriptomes and Major QTLs from a Pair of Genotypes Contrasting for Drought and Chronic Nitrogen Starvation Identifies Key Stress Responsive Genes in Rice

We report here the genome-wide changes resulting from low N (N-W+), low water (N+W-)) and dual stresses (N-W-) in root and shoot tissues of two rice genotypes, namely, IR 64 (IR64) and Nagina 22 (N22), and their association with the QTLs for nitrogen use efficiency. For all the root parameters, except for root length under N-W+, N22 performed better than IR64. Chlorophyll a, b and carotenoid content were higher in IR64 under N+W+ treatment and N-W+ and N+W- stresses; however, under dual stress, N22 had higher chlorophyll b content. While nitrite reductase, glutamate synthase (GS) and citrate synthase assays showed better specific activity in IR64, glutamate dehydrogenase showed better specific activity in N22 under dual stress (N-W-); the other N and C assimilating enzymes showed similar but low specific activities in both the genotypes. A total of 8926 differentially expressed genes (DEGs) were identified compared to optimal (N+W+) condition from across all treatments. While 1174, 698 and 903 DEGs in IR64 roots and 1197, 187 and 781 in N22 roots were identified, nearly double the number of DEGs were found in the shoot tissues; 3357, 1006 and 4005 in IR64 and 4004, 990 and 2143 in N22, under N-W+, N+W- and N-W- treatments, respectively. IR64 and N22 showed differential expression in 15 and 11 N-transporter genes respectively, under one or more stress treatments, out of which four showed differential expression also in N+W- condition. The negative regulators of N- stress, e.g., NIGT1, OsACTPK1 and OsBT were downregulated in IR64 while in N22, OsBT was not downregulated. Overall, N22 performed better under dual stress conditions owing to its better root architecture, chlorophyll and porphyrin synthesis and oxidative stress management. We identified 12 QTLs for seed and straw N content using 253 recombinant inbred lines derived from IR64 and N22 and a 5K SNP array. The QTL hotspot region on chromosome 6 comprised of 61 genes, of which, five were DEGs encoding for UDP-glucuronosyltransferase, serine threonine kinase, anthocyanidin 3-O-glucosyltransferase, and nitrate induced proteins. The DEGs, QTLs and candidate genes reported in this study can serve as a major resource for both rice improvement and functional biology.

Dissolved and gaseous nitrogen losses in forests controlled by soil nutrient stoichiometry

Global chronic nitrogen (N) deposition to forests can alleviate ecosystem N limitation, with potentially wide ranging consequences for biodiversity, carbon sequestration, soil and surface water quality, and greenhouse gas emissions. However, the ability to predict these consequences requires improved quantification of hard-to-measure N fluxes, particularly N gas loss and soil N retention. Here we combine a unique set of long-term catchment N budgets in the central Europe with ecosystem 15N data to reveal fundamental controls over dissolved and gaseous N fluxes in temperate forests. Stream leaching losses of dissolved N corresponded with nutrient stoichiometry of the forest floor, with stream N losses increasing as ecosystems progress towards phosphorus limitation, while soil N storage increased with oxalate extractable iron and aluminium content. Our estimates of soil gaseous losses based on 15N stocks averaged 2.5 ± 2.2 kg N ha−1 yr−1 and comprised 20% ± 14% of total N deposition. Gaseous N losses increased with forest floor N:P ratio and with dissolved N losses. Our relationship between gaseous and dissolved N losses was also able to explain previous 15N-based N loss rates measured in tropical and subtropical catchments, suggesting a generalisable response driven by nitrate (NO3 −) abundance and in which the relative importance of dissolved N over gaseous N losses tended to increase with increasing NO3 − export. Applying this relationship globally, we extrapolated current gaseous N loss flux from forests to be 8.9 Tg N yr−1, which represent 39% of current N deposition to forests worldwide.

Community change can buffer chronic nitrogen impacts, but multiple nutrients tip the scale.

Anthropogenic nitrogen (N) inputs are causing large changes in ecosystems worldwide. Many previous studies have examined the impact of N on terrestrial ecosystems; however, most have added N at rates that are much higher than predicted future deposition rates. Here, we present the results from a gradient of experimental N addition (0-10g·N·m-2 ) in a temperate grassland. After a decade of N addition, we found that all levels of N addition changed plant functional group composition, likely indicating altered function for plant communities exposed to 10 yr of N inputs. However, N addition only had weak impacts on species composition and this functional group shift was not driven by any particular species, suggesting high levels of functional redundancy among grasslands species. Adding other nutrients (P, K, and micronutrients) in combination with N caused substantially greater changes in the relative abundance of species and functional groups. Together, these results suggest that compositional change within functional groups may buffer grasslands from impacts of N deposition, but concurrent eutrophication with other elements will likely lead to substantial changes in plant composition and biomass.

Beneficial effects of nitrogen deposition on carbon and nitrogen accumulation in grasses over other species in Inner Mongolian grasslands

Globally, chronic nitrogen (N) deposition into terrestrial ecosystems has resulted in changes in community composition, depending on the responses of co-existing species to increasing soil N availability. The levels of plant non-structural carbohydrates (NSCs) produced by photosynthesis are associated with leaf N content, and together they represent the capital of a plant for its life including competitive ability. However, the specific ways that concentrations of plant NSCs, and also N, influence the growth of plants, and thus their productivity, are still abscue. Here, we explored the effect of variations in plant leaf NSCs (starch and soluble sugars) and N concentrations on the growth of plants with different soil N uptake capacities, i.e. two grasses (Leymus chinensis and Stipa baicalensis), two forbs (Potentilla tanacetifolia and Galium verum), and one legume (Thermopsis lanceolate) in Inner Mongolian grasslands subjected to three N addition rates (i.e., 0, 10, or 20 g N m−2 yr−1) and three N forms [i.e., NH4NO3, (NH4)2SO4, or CO(NH2)2] for four years. Irrespective of N addition rate and N form, N addition significantly increased aboveground biomass of the grasses, but not that of the forbs or the legume. The grasses had higher increase in N concentrations than the forbs and the legume. N addition increased the starch concentrations of grasses, but decreased that of forbs and the legume. Changes in the aboveground biomass of all tested plant species were significantly positively correlated with changes in concentrations of both NSCs and N. Our results indicate that, irrespective of the N addition rates and N forms, greater increase in NSCs and N concentrations contributed to higher aboveground productivity in grasses, but not in forbs and legumes. These results imply that future nitrogen deposition will benefit grasses over other plant forms, and thus the former may become more dominant in grasslands in northern China.

Chronic nitrogen addition differentially affects gross nitrogen transformations in alpine and temperate grassland soils

Nitrogen (N) deposition can profoundly alter soil N cycling of grassland ecosystems. Substrates and soil acidification are expected to modify soil N transformations in response to elevated N deposition. Here, we carried out 15N tracing studies to test the effects of N addition rates (low: 30 kg N ha−1 and high: 90/120 kg N ha−1) and soil acidification on gross N transformation rates using two typical Chinese grassland soils, an alpine calcareous soil and a temperate neutral soil. We found that N addition significantly increased the ratio of gross nitrification rate to gross ammonia immobilization rate (N/I) in both soils, but gross N transformation rates changed differently as a function of N addition rates and soil types. In the calcareous soil, N addition increased soil gross N transformations, largely due to mineral N substrates, SOC, TN and fungal dominance. In contrast, low N addition did not affect gross N transformation rates in the neutral soil, but high N addition significantly decreased gross N transformation rates. Although both SOC and TN were increased with N addition in the neutral soil, N-induced soil pH decline decreased gross N transformation rates. Our results indicate that the effects of N addition on grassland soil gross N transformations are highly dependent on mineral N substrates, SOC and TN. Soil acidification played a more important role than SOC and TN in gross N transformation rate changes in response to elevated N deposition. These findings suggest that the different changes of gross N transformation rates in response to N deposition and soil properties (e.g. SOC, TN and soil pH) should be integrated into biogeochemical models to better predict grassland ecosystem N cycling in the future scenarios of N deposition.

Nitrous oxide emissions in proportion to nitrification in moist temperate forests

Chronic elevated nitrogen deposition has increased nitrogen availability in many forest ecosystems globally, and this phenomenon has been suggested to increase soil nitrification. Although it is believed that increased nitrogen availability would also increase nitrous oxide (N2O) emissions from forest ecosystems, its impact on N2O flux is poorly known. In this study, 3-years monitoring of N2O emissions was performed in a forested watershed receiving elevated nitrogen deposition and located in the suburbs of Tokyo, Japan. In addition, a comparative field survey was carried out in nine temperate forest sites with varying nitrogen availabilities. In the intensively studied forest site showing typical nitrogen saturation, the average annual N2O emissions from the whole watershed were estimated to be 0.88 kg N ha−1 year−1, comparable to the highest observed levels for temperate forests except for some very high emission sites in Europe. Although no correlation was found for humid spots with WFPS > 60%, a clear positive correlation was noted between N2O flux and net nitrification rate in situ for plots with water-filled pore space (WFPS) < 60%. The N2O flux varied across nine forest sites almost in proportional to the stream water NO3− concentration in the watershed that ranged from 0.14 to 1.64 mg N/L. We conclude that N2O emissions are related to nitrification in moist temperate forest, which may be associated with the magnitude of nitrogen saturation.