Levels Of Nitrogen Deposition Research Articles

Drought effects on growth and density of temperate tree regeneration under different levels of nitrogen deposition

Temperate forests in Central Europe suffer from climate change-induced productivity and vitality reductions and increased tree mortality. Most field work assessing climate change effects in forests refers to mature trees and does not cover interaction between climate change and nitrogen deposition. Here we show in a study of 54 forest sites representing different combinations of climatic conditions and atmospheric nitrogen deposition across Germany that nitrogen significantly affects the drought tolerance of tree regeneration in the field. We compared shoot length increment and regeneration density of Central Europe’s naturally most dominant tree species, European beech (Fagus sylvatica) with three species, which are discussed as potentially more drought-tolerant replacement tree species for climate change adaptation of forestry. Growth of beech was reduced with increasing nitrogen deposition identifying high reactive nitrogen loads as an additional threat for this species, but between-site variation of beech growth was not dependent on climatic parameters despite growth reductions after the drought summer 2018 across all study sites. Remarkably, the only species in the between-site comparisons where low growth was attributable to dry climate was Douglas fir (Pseudotsuga menziesii), as shoot length increment was reduced at sites with dry spring climate. Silver fir (Abies alba) showed increasing growth with increasing nitrogen deposition in combination with increasing drought, probably due to reduced ammonium uptake. Shoot length increment in sessile oak (Quercus petraea) was not affected by the between-site variation of either drought or nitrogen. Our results (combined with results of regeneration density) indicate that high atmospheric nitrogen deposition, which is primarily found in regions with intensive livestock farming in Central Europe, modifies the climate change response of temperate tree species. Interaction of drought and nitrogen deposition should thus be addressed in the debate on climate change adaptation of forestry.

Decomposition and Carbon and Nitrogen Releases of Twig and Leaf Litter Were Inhibited by Increased Level of Nitrogen Deposition in a Subtropical Evergreen Broad-Leaved Forest in Southwest China

Decomposition and Carbon and Nitrogen Releases of Twig and Leaf Litter Were Inhibited by Increased Level of Nitrogen Deposition in a Subtropical Evergreen Broad-Leaved Forest in Southwest China

Adaptation Strategies of Seedling Root Response to Nitrogen and Phosphorus Addition.

The escalation of global nitrogen deposition levels has heightened the inhibitory impact of phosphorus limitation on plant growth in subtropical forests. Plant roots area particularly sensitive tissue to nitrogen and phosphorus elements. Changes in the morphological characteristics of plant roots signify alterations in adaptive strategies. However, our understanding of resource-use strategies of roots in this environment remains limited. In this study, we conducted a 10-month experiment at the Castanopsis kawakamii Nature Reserve to evaluate the response of traits of seedling roots (such as specific root length, average diameter, nitrogen content, and phosphorus content) to nitrogen and phosphorus addition. The aim was to reveal the adaptation strategies of roots in different nitrogen and phosphorus addition concentrations. The results showed that: (1) The single phosphorus and nitrogen-phosphorus interaction addition increased the specific root length, surface area, and root phosphorus content. In addition, single nitrogen addition promotes an increase in the average root diameter. (2) Non-nitrogen phosphorus addition and single nitrogen addition tended to adopt a conservative resource-use strategy to maintain growth under low phosphorus conditions. (3) Under the single phosphorus addition and interactive addition of phosphorus and nitrogen, the roots adopted an acquisitive resource-use strategy to obtain more available phosphorus resources. Accordingly, the adaptation strategy of seedling roots can be regulated by adding appropriate concentrations of nitrogen or phosphorus, thereby promoting the natural regeneration of subtropical forests.

Simulated Nitrogen Deposition Decreases the Ratios of Soil C to P and N to P, Changes Soil Enzyme Activity, and Reduces Soil Microbial Biomass in Paddy Soil in Southern China

There have been few studies on the impact of nitrogen deposition on paddy field ecosystem; therefore, we evaluated the effects of different N deposition levels (0, 40, and 120 kg N·ha−1) with the conventional nitrogen rate (180 kg N·ha−1) on rice field ecosystem through two-season experiments. The results showed that 40 and 120 kg·ha−1 nitrogen deposition had no significant effect on rice yield, although the rice grains per panicle and the 1000-grain weight increased. The 40 and 120 kg·ha−1 nitrogen deposition levels had no significant effect on rice and soil total carbon/total nitrogen (TC/TN) in the two-season experiment; however, 40 and 120 kg·ha−1 nitrogen deposition significantly increased TP content of the rice root and soil in a short time, and continuous 120 kg·ha−1 nitrogen deposition significantly decreased TP content of the rice root and significantly increased TP content of the rice stem. In addition, nitrogen deposition significantly reduced total carbon/total phosphorus (TC/TP) and total nitrogen/total phosphorus (TN/TP) in the soil. The activities of soil acid phosphatase (S-ACP), β-glucosidase (S-β-GC), and N-acetyl-β-D-glucosidase (S-NAG) increased under 40 kg·ha−1 nitrogen deposition, while the activities of S-β-GC and S-NAG decreased under 120 kg·ha−1 nitrogen deposition compared with 40 kg·ha−1. The microbial carbon, microbial nitrogen, microbial phosphorus, and fungal microbial biomass reduced under 40 and 120 kg·ha−1 nitrogen deposition. These findings suggest that, under short-term N deposition, rice and soil can adjust the C, N, P, and even the nutrient balance by themselves; however, continuous nitrogen deposition may have adverse reactions to microorganisms, thereby disrupting this balance and ultimately leading to the deterioration of paddy soil environment and a reduction in rice yield in the long term.

Resource-acquisitive species have greater plasticity in leaf functional traits than resource-conservative species in response to nitrogen addition in subtropical China

The evergreen broad-leaf forest is subtropical zonal vegetation in China, and its species diversity and stability are crucial for maintaining forest ecosystem functions. The region is generally affected by global changes such as high levels of nitrogen deposition. Therefore, it is critical to determine the adaptation strategies of subtropical dominant species under nitrogen addition. Here, we conducted two-year field experiments with nitrogen addition levels as 0 kg N ha−1 yr−1 (CK), 50 kg N ha−1 yr−1 (LN) and 100 kg N ha−1 yr−1 (HN). We investigated the effects of nitrogen addition on leaf functional traits (including nutrition, structural and physiological characteristics) of five dominant species in subtropical evergreen broad-leaf forest. Results suggested that the effect of nitrogen addition on leaf functional traits was species-specific. Contrary to Rhododendron delavayi and Eurya muricata, Quercus glauca, Schima superba and Castanopsis eyrei all responded more to the HN treatment than LN treatment. Compared to other leaf functional traits, leaf anatomical structure traits had the highest average plasticity (0.246), and the relative effect of leaf photosynthetic property was highest (7.785) under N addition. Among the five species, S. superba was highest in terms of the index of plasticity for leaf functional traits under nitrogen addition, followed by Q. glauca, E. muricata, C. eyrei and R. delavayi. The major leaf functional traits representing the economic spectrum of leaves (LES) showed resource acquisitive strategy (high SLA, LNC, LPC, Pn) and conservative strategy (high LTD, LDMC, C/N) clustering on the opposite ends of the PCA axis. The PCA analysis indicated that species with high leaf plasticity adopt resource acquisitive strategy (S. superba and Q. glauca), whereas species with low leaf plasticity adopt resource conservative strategy (E. muricata, C. eyrei and R. delavayi). In aggregate, resource-acquisitive species benefit from nitrogen addition more than resource-conservative species, suggesting that S. superba and Q. glauca will occupy the dominant position in community succession under persistently elevated nitrogen deposition.

Arbuscular mycorrhizal fungi inoculation exerts weak effects on species- and community-level growth traits for invading or native plants under nitrogen deposition

Nitrogen deposition and biological invasion are two major components of global environmental change. Nitrogen deposition has been considered to enhance the resource availability of recipient habitats, which influences the invasiveness of plant invader and the invasibility of recipient native communities. Nitrogen deposition has been shown to reduce the relative abundances of arbuscular mycorrhizal fungi (AMF) globally. AMF have been found to mutualistically symbiose with approximately 75% of plant species and act as a nutrient supplier. AMF may modify the structure of native plant communities, collaborate with alien plant invaders and thus promote their invasion. The alien woody invader, Rhus typhina L. has been introduced into North China as a horticultural species, invaded the native plant community and outperformed the native competitors in growth and in photosynthetic efficiency. Nevertheless, little is known about if nitrogen deposition and AMF inoculation synergistically alter the invasibility of native plant community. In this study, R. typhina was subjected to the artificial plant community assembled by four co-existing native species – Chenopodium album L., Vitex negundo var. heterophylla (Franch.) Rehd., Rhus chinensis Mill. and Acer truncatum Bunge in a mesocosm experiment. Nitrogen deposition and AMF inoculation were simulated as environmental and biotic filters respectively. Aboveground biomass and biomass proportion, reflecting plant growth and performance, and specific leaf area and chlorophyll concentration correlated with carbon use and photosynthetic capacity of both the alien invader and the native plants were measured and calculated after harvest. We found that AMF inoculation did not alter the trait variation of alien and native species to increasing nitrogen deposition level in general, although AMF inoculation impeded the increase of aboveground biomass for C. album, V. negundo and native community with increasing nitrogen deposition level. In the scenario of nitrogen deposition and AMF inoculation, a stable status of invasion dynamic may be maintained and needs to be checked with integration of traits at extended temporal scale.

Morphological and physiological responses of critically endangered Acer catalpifolium to nitrogen deposition levels

Morphological and physiological responses of critically endangered Acer catalpifolium to nitrogen deposition levels

Effects of nitrogen deposition on soil enzymatic activity and soil microbial community in a Mediterranean holm oak forest

Ongoing increases in global levels of nitrogen (N) deposition are predicted to continue, causing important shifts in the structure and function of ecosystems worldwide. Soil microbes and enzymes are key components of biogeochemical cycles and ecosystem functioning, so understanding their response to N deposition is crucial. We studied the effects of N addition on the activities of five extracellular enzymes and microbial biomass and composition in a mesic Mediterranean holm oak forest characterized by moderate soil N- and P-limitation. Addition of N increased photosynthetic potential of higher plants, activities of P- and C-, but not N- related enzymes, and biomass of total microbes, bacteria and fungi, and decreased ratios of fungi:bacteria. Overall, N deposition increased general biological activity of the mesic Mediterranean forest, enhanced ecosystem investment in P mobilization and affected soil microbial community structure.

Effect of nitrogen deposition on seed germination of ephemeral species in the cold desert

Seed germination is the key initial stage in the plant life cycle and strongly regulated by the external environment. Nitrogen deposition is an important environmental change factor that is increasing and affecting the structure and function of many ecosystems. This research aimed to investigate the effect of nitrogen deposition on the seed germination of nine ephemeral plants, including seed germination percentage, germination index and seed viability under four levels of nitrogen deposition. The germination percentage of the studied species differed significantly in response to nitrogen deposition, but nitrogen deposition has almost no significant effect on germination index. This study can provide information on the germination strategy of ephemeral plants in response to increasing nitrogen deposition in future, and the results can guide ecological conservation in arid areas.

Nitrogen deposition enhances the deterministic process of the prokaryotic community and increases the complexity of the microbial co-network in coastal wetlands

Global nitrogen deposition has increased significantly in recent years. At present, research on the effects of different amounts and types of nitrogen deposition on soil microorganisms in coastal wetlands is scarce. In this study, based on 7 years of simulated nitrogen deposition at multiple levels (low, medium, high) and of multiple types (NH4NO3, NH4Cl, KNO3), the effects of different nitrogen deposition conditions on the diversity, community assembly processes, co-networks, and community function of soil prokaryotes in coastal wetlands were examined. The results showed that, compared with that in control, the microbial α diversity increased significantly under nitrogen deposition (P < 0.05). However, it decreased significantly in the high-NH4NO3 and high-NH4Cl treatments (P < 0.05). The deterministic process of community assembly was strengthened under the different types of nitrogen deposition. Compared with that under NH4+-N deposition, the microbial co-network under NO3−-N deposition was more complex. Network stability significantly decreased under different NH4+-N deposition levels. In addition, the results of FAPROTAX functional prediction showed that microbial community functional groups associated with carbon and nitrogen cycling changed significantly (P < 0.05). In conclusion, our results emphasize that nitrogen deposition environments cause changes in soil microbial community structure and interactions, and may also affect soil carbon and nitrogen cycling, but the effects of different forms and levels of nitrogen deposition are not consistent. This study provides new insights for evaluating the changes in soil microbial communities in coastal wetlands caused by different types of long-term nitrogen deposition, and has scientific significance for assessing the ecological effects of long-term nitrogen deposition.

The beneficial effect of Epichloë endophytes on the growth of host grasses was affected by arbuscular mycorrhizal fungi, pathogenic fungi and nitrogen addition

Native grasslands are often limited by nitrogen, so even a low level of nitrogen deposition may significantly impact the growth of plants as well as plant-associated microbes. Epichloë endophytes inhabit the aboveground tissues of healthy plants, and their beneficial effects on the growth of host grasses have been widely reported. In this study, the abundance of the endophyte and its effect on the host Achnatherum sibiricum when cooccurring with arbuscular mycorrhizal fungi (AMF) or pathogens was investigated with increasing nitrogen supply. The results demonstrated that endophyte abundance increased significantly with increasing leaf nitrogen concentration and plant biomass under low nitrogen conditions, but there was no such correlation for AMF or pathogen abundance. The individual effect of endophytes on plant performance was positive under both low ( P = 0.002) and high ( P = 0.013) nitrogen conditions; only the promotion degree was even greater under low nitrogen conditions ( P = 0.004). When endophytes and AMF were both present, the combined effects of the two symbionts on plant growth were antagonistic under low nitrogen levels ( P = 0.048), while the combined effects of the two symbionts on plant growth were independent under high nitrogen levels ( P = 0.109). When endophytes and pathogens were both present, the net effect on plants was greater than the sum of the separate effects of the two microbes and was not affected by environmental nitrogen. Endophytes could inhibit pathogen abundance by increasing total phenolic content of the host A. sibiricum . Overall, the impact of endophytes on host plants was affected not only by abiotic environmental nitrogen but also by biotic factors AMF and pathogens. Therefore, clarifying the impact factors of endophytes on host growth is important for predicting the population dynamics and community diversity of host plants. • Host plant biomass increased with increasing abundance of endophytes only under low nitrogen treatment. • When present alone, endophytes promoted host growth under both low and high nitrogen conditions. • When present with AMF, the net effect on plants was antagonistic under low nitrogen while independent under high nitrogen. • When present with pathogens, the net effect on plants was greater than the sum of the separate effects of the two microbes.

Effects of Grazing Sheep and Mowing on Grassland Vegetation Community and Soil Microbial Activity under Different Levels of Nitrogen Deposition

Increasing nitrogen deposition plays a critical role in the material circulation of grassland. Mowing and grazing sheep are important means of utilizing grassland. This study investigated the effects of nitrogen deposition, sheep grazing and mowing on the soil, vegetation and soil microorganisms of grassland. N deposition increased soil inorganic nitrogen, SOM and microbial activity, and decreased soil pH, while grazing sheep and mowing had opposing effects. Compared with mowing, grazing sheep decreased the range of grass groups in the community. N deposition increased the proportion of Leymus chinensis in the community and decreased community diversity. N deposition enhanced the contribution rate of soil to the vegetation community, and reduced the effect of microorganisms on the vegetation community. In addition, N deposition significantly interacted with mowing and grazing sheep in terms of effects on soil inorganic nitrogen, soil organic matter (SOM), microbial respiration (Q), microbial mass carbon (MBC), and vegetation diversity. Therefore, appropriate N deposition in sheep grazing and mown grasslands could enhance inorganic N and organic matter, increase microbial activity, offset the adverse effects of grazing sheep and mowing, and contribute to maintaining community diversity and grassland productivity.

Nitrogen deposition causes eutrophication in bryophyte communities in central and northern European forests

Key messageOur results indicate that nitrogen deposition is likely to adversely affect forest bryophyte communities, having negative impacts in terms of increased dominance of nitrophilic species at the expense of N-sensitive species and a decrease in evenness.ContextElevated atmospheric deposition of nitrogen (N) has long been recognised as a threat to biodiversity and, despite declines in European emission levels, will remain a threat in the future.AimsIt has proven difficult to show clear large-scale impacts of N deposition on vascular forest understorey species, and few studies have looked at impacts on forest bryophytes. Here, we assess the impact of nitrogen deposition on forest bryophyte communities.MethodsWe used data from 187 plots included in European monitoring schemes to analyse the relationship between levels of throughfall nitrogen deposition and bryophyte taxonomic and functional diversity and community nitrogen preference.ResultsWe found that nitrogen deposition is significantly associated with increased bryophyte community nitrogen preference and decreases in species evenness.ConclusionOur results indicate that nitrogen deposition is likely to adversely affect forest bryophyte communities, having negative impacts in terms of increased dominance of nitrophilic species at the expense of N-sensitive species and a decrease in species evenness.

Biogeochemical and ecosystem properties in three adjacent semi‐arid grasslands are resistant to nitrogen deposition but sensitive to edaphic variability

AbstractDrylands have low nitrogen stocks and are predicted to be sensitive to modest increases in reactive nitrogen availability, but direct evidence that atmospheric nitrogen deposition will have sustained effects on dryland ecosystems is sparse and conflicting.We used three long‐running in situ nitrogen deposition simulation experiments and a complementary laboratory incubation experiment to address fundamental questions about how nitrogen inputs affect drylands: (1) What are the long‐ and short‐term consequences of nitrogen inputs for biogeochemical and ecosystem properties?; (2) Do these consequences depend on soil moisture availability?; and (3) Does soil texture modify the effects of nitrogen inputs and/or soil moisture availability?In 2011, we established three study sites along a soil texture gradient in Arches National Park with plots receiving 0, 2, 5 or 8 kg N ha−1annually (n = 5 per treatment per site). We assessed a suite of biogeochemical metrics over the long term and short term. To assess longer term effects, we sampled annually (2013–2019), just prior to spring nitrogen fertilization. To assess short‐term effects, we sampled immediately before and after spring nitrogen fertilization in 2013. Additionally, we compared foliar chemistry, soil extracellular enzyme activities, heterotrophic respiration rates, and nitrogen trace gas fluxes at select intervals during the study period (2011–2019). Finally, we conducted a laboratory incubation to measure the individual and interacting effects of soil moisture and nitrogen additions on soil microbial activity.We identified some short‐term effects in situ, but no lasting consequences of added nitrogen for any of the metrics measured. In the incubation, soil moisture treatments independently increased heterotrophic respiration rates but did not modify the effects of added nitrogen. In contrast to nitrogen treatments, soil texture was associated with large differences in biogeochemical cycling.Our results oppose the common prediction that coupled dryland biogeochemical cycles are sensitive to nitrogen inputs and suggest that fine‐scale edaphic heterogeneity is a key driver of dryland ecosystem properties.Synthesis. This study synthesizes a rare, long‐term dataset, which provides robust evidence that carbon and nutrient cycling and availability are resistant to realistic levels of atmospheric nitrogen deposition in remote dryland ecosystems.

Changes in the root microbiome of four plant species with different mycorrhizal types across a nitrogen deposition gradient in ombrotrophic bogs

Anthropogenic activities have severely altered biogeochemical cycles with far-reaching consequences for biodiversity and ecosystem functioning. The use of artificial fertilizers, increased legume cultivation and fossil fuel combustion has resulted in a twofold increase of inorganic nitrogen input in natural ecosystems worldwide, often with considerable negative effects on plant and microbial communities. However, not all ecosystems are as sensitive to increased nitrogen deposition and effects may vary among ecological and taxonomic groups. Here, we studied how increasing nitrogen deposition affected soil and root-associated microbial communities of plants growing in ombrotrophic bogs. We specifically tested the hypothesis that microbiomes of plants with different mycorrhizal types respond differently to increased nitrogen deposition. We sampled soil and the roots of three plant species of different mycorrhizal types – arbuscular mycorrhizal (Molinia caerulea), ectomycorrhizal (Betula pubescens), ericoid mycorrhizal (Vaccinium oxycoccos) – and a non-mycorrhizal plant species (Eriophorum vaginatum) along a nitrogen deposition gradient in Europe (5–30 kg N ha−1 year−1). For each sample, the fungal and bacterial biomass and community composition were assessed and related to current levels of nitrogen deposition. In general, we found that fungi were more strongly affected by increased nitrogen deposition than bacteria. Fungal biomass, richness and diversity significantly decreased with increasing nitrogen deposition while bacterial biomass, richness and diversity was indifferent. OTU richness, diversity or community composition of ericoid mycorrhizal fungi did not change with increasing nitrogen deposition, while ectomycorrhizal fungal OTU richness and diversity significantly declined and community composition changed. We did not find an increase in arbuscular mycorrhizal fungi biomass along this gradient, despite the strong increase in abundance of the arbuscular mycorrhizal plant M. caerulea with increasing nitrogen deposition. We conclude that atmospheric nitrogen deposition has stronger effects on fungal than on bacterial communities in ombrotrophic bogs and that fungal guilds differ in their response.

Improvement of stomatal resistance and photosynthesis mechanism of Noah-MP-WDDM (v1.42) in simulation of NO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; dry deposition velocity in forests

Abstract. Rapid urbanisation and economic development in China have led to a dramatic increase in nitrogen oxide (NO2) emissions, causing serious atmospheric nitrogen pollution and relatively high levels of nitrogen deposition. However, despite the importance of nitrogen deposition, dry deposition processes in forested areas are still insufficiently represented in current global and regional atmospheric chemistry models, which constrains our understanding and prediction of spatial and temporal patterns of nitrogen transport in forest ecosystems in southern China. The offline 1-D community Noah land surface model with multi-parameterisation options (Noah-MP) is coupled with the WRF-Chem dry deposition module (WDDM) and is applied to further understand and identify the key processes that affect forest canopy dry deposition. The canopy stomatal resistance mechanism and the nitrogen-limiting scheme for photosynthesis in Noah-MP-WDDM are modified to improve the simulation of reactive nitrogen oxide dry deposition velocity. This study finds that the combined improved stomatal resistance mechanism and nitrogen-limiting scheme for photosynthesis (BN-23) agree better with the observed NO2 dry deposition velocity, with the mean bias being reduced by 50.1 %. At the same time, by comparing the different mechanisms of the two processes of canopy stomatal resistance and leaf nitrogen-limiting factors, this study also finds that the diurnal changes in dry deposition velocity simulated by each regional model present four sets of distributions. This is mainly due to the different ways that each integrated mechanism handles the opening and closing of stomata at noon and the way the nitrogen-limiting factor acts.

Impact of shipping emissions on air pollution and pollutant deposition over the Barents Sea

Arctic warming leading to reduced summertime sea-ice is likely to lead to increased local shipping especially along the Northeast Passage near the northern coasts of Norway and Russia, which are shorter than the traditional southerly routes. Here, the regional chemistry-transport model WRF-Chem is used to examine the effects of shipping emissions on levels of air pollutants and deposition fluxes over the Barents Sea both for present-day and future conditions, based on a high growth scenario. Present-day shipping emissions are found to have already substantial effects on ozone concentrations, but limited effects on sulphate and nitrate aerosols. Predicted future changes in ozone are also important, particularly in regions with low nitrogen oxide concentrations, and results are sensitive to the way in which diversion shipping is distributed due to non-linear effects on photochemical ozone production. Whilst modest future increases in sulphate and nitrate aerosols are predicted, large enhancements in dry deposition of sulphur dioxide and wet deposition of nitrogen compounds to the Barents Sea are predicted. Such levels of future nitrogen deposition would represent a significant atmospheric source of oceanic nitrogen affecting sensitive marine ecosystems.

Effect of nitrogen deposition on Soil CO2 emission during freezing-thawing incubation period

Due to the traditional analysis method on the influence of soil nitrogen deposition on soil emissions during the freezing and thawing period, the initial impact of nitrogen deposition on soil emissions during the freezing and thawing incubation period was not analyzed, resulting in insufficient accuracy of the later analysis results. A new method was proposed to analyze the effect of nitrogen deposition on Soil emission during freeze-thaw cultivation.On this basis, the contents of soil temperature, moisture, inorganic nitrogen and soluble carbon were determined. Three freeze-thaw models of nitrogen deposition levels were established. The influence of nitrogen deposition on Soil emission and the effect of nitrogen deposition on emission in alpine wetland were studied by multivariate variance analysis. The effect of nitrogen deposition on emission of alpine wetland was studied. The results showed that different soil temperature and moisture content had great influence on the seasonal variation of soil flux, which was generally consistent with the single peak of soil temperature, but highly consistent with the variation of soil moisture content in different growth periods. Nitrogen treatment changed the DOC content of soil organic matter. DOC content in mineral layer and organic layer increased significantly in low and medium nitrogen treatments. The emission of soil in freezing period is lower than that in normal temperature period, and that in multiple freezing period is less than that in one freezing period. The emission rate of soil under freeze-thaw condition is the smallest, and the emission rate of soil after thaw is the largest. Appropriate nitrogen deposition can promote soil emission, while high nitrogen deposition can inhibit emission.

Stabilization of soil organic carbon in the alpine meadow is dependent on the nitrogen deposition level on the Qinghai-Tibetan Plateau

The Qinghai-Tibet Plateau (QTP) is experiencing increasing nitrogen (N) deposition, which may have great impacts on the physical, chemical, and biological factors that determine the stabilization of soil organic carbon (SOC). However, few studies have been conducted to quantify the relative contributions of these factors to SOC stabilization at various N addition levels in alpine habitats. We established a N manipulation experiment from 2015 to 2016 in an alpine meadow on the QTP to examine the impacts of N addition gradients on SOC stabilization under different scenarios of N deposition including 8, 24, 40, 56, and 72 kg N ha−1 yr−1. Using solid-state 13C NMR spectroscopy, we assessed the changes in the chemical composition of SOC. We measured soil aggregate size fractions, iron ions, soil microbes, and soil chemicals to investigate their contributions to SOC stabilization. We found that there were response thresholds of SOC to N addition in this alpine meadow on the QTP. The critical level of N addition for the change of SOC was close to 16 kg N ha−1 yr−1 plus the natural atmospheric N deposition (approximately 8 kg N ha−1 yr−1). N addition affected SOC stabilization in this alpine meadow mainly by affecting soil aggregates and soil microbes. These results of this study suggest that an exogenous N input lower than the critical load is beneficial for C sequestration in this alpine meadow on the QTP. This is potentially important for understanding the influences of atmospheric N deposition on soil properties, with subsequent effects on SOC stabilization on the alpine meadow of QTP and similar fragile ecosystems worldwide.

Lichen nitrogen concentrations and isotopes for indicating nitrogen deposition levels and source changes

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