Nitrogen Deposition Rates Research Articles

Changes in the Phylogenetic Structure of Alpine Grassland Plant Communities on the Qinghai-Tibetan Plateau with Long-Term Nitrogen Deposition.

Nitrogen (N) deposition rates have notably increased around the world, especially in high-altitude regions like the Qinghai-Tibetan Plateau (QTP). We conducted a six-year comprehensive experiment to simulate nitrogen deposition in an alpine grassland area near Qinghai Lake. Four levels of nitrogen depositions, i.e., 0 (CK), 8 kg N ha-1year-1 (N1), 40 kg N ha-1year-1 (N2), and 72 kg N ha-1year-1 (N3), with three replicates for each N treatment, were tested annually in early May and early July, with the meticulous collection of plant and soil samples during the peak growth period from 15 July to 15 August. We used the null model to evaluate the impact of environmental filtration and interspecific competition on the dynamics of the plant community was assessed based on the level of discrete species affinities within the plant community by constructing a phylogenetic tree. The results showed that the environmental filter was the predominant driver for the change of community's genealogical fabric. The N2 and N3 treatments increased the influence of soil factors on the change of plant community structure. Climatic factors played a crucial role on the change of plant community in the CK grassland area, while soil factors were dominant in the N1- and N3-treated grasslands.

Optimal glycine allowance levels in low-protein diets and the dynamic requirement model for broilers

This study aimed to investigate the effects of different glycine levels in low-protein diets on the growth, nitrogen deposition, and expression of intestinal amino acid and glucose transporters in broilers from 29 to 42 days of age, in order to determine the optimal glycine supplementation level. A total of 240 male broilers at 29 days old were randomly assigned to 5 groups: the control group with a crude protein level of 20%, and experimental groups with low-protein diets (LP130) containing 18% crude protein, supplemented with glycine to achieve standardized ileal digestible (SID) glycine + serine to lysine ratios of 134% (LP134), 140% (LP140), and 145% (LP145). The results showed that the LP134 group had similar growth performance and slaughter performance compared to the control group (P > 0.05), whereas other low-protein diet groups had significantly lower growth performance (P < 0.05). Regression analysis determined that the optimal ratio for SID glycine + serine to lysine was 137%. A dynamic model for glycine + serine requirements was established through binary regression analysis: y = 599.051 × BW^0.75 + 8.381 × ADG (R2 = 0.998, P < 0.001). Feeding LP134, LP140, and LP145 diets significantly improved nitrogen deposition rates in broilers (P < 0.05). Low-protein diets significantly upregulated mRNA levels of b0,+AT, EAAT3, and SGLT1 genes in the duodenum (P < 0.05). In conclusion, appropriate glycine supplementation in low-protein diets can enhance growth performance, and nitrogen deposition efficiency, and regulate the expression of intestinal amino acid and glucose transporters. The optimal ratio of SID glycine + serine to lysine in low-protein diets for broilers aged 29 to 42 days is 137%.

Chronic increasing nitrogen and endogenous phosphorus release from sediment threaten to the water quality in a semi-humid region reservoir

The water quality in the drinking water reservoir directly affects people's quality of life and health. When external pollution input is effectively controlled, endogenous release is considered the main cause of water quality deterioration. As the major nitrogen (N) and phosphorus (P) sources in reservoirs, sediment plays a vital role in affecting the water quality. To understand the spatial and temporal variation of N and P in the sediment, this study analyzed the current characteristics and cumulative effects of a semi-humid reservoir, Yuqiao Reservoir, in North China. The N and P concentrations in the reservoir sediment were decreased along the flow direction, while the minimum values were recorded at the central sediment profile. External input and algal deposition were the main factors leading to higher sediment concentrations in the east (Re-E) and west (Re-W) areas of reservoir sediment profiles. According to the long-term datasets, the peaks of both sediment total nitrogen content and deposition rate were observed in the 2010s, which has increased about three times and six times than in the1990s, respectively. Therefore, the increase in phosphorus concentration may be the main reason for eutrophication in water in recent years. The mineralization of organic matter has a significant promoting effect on releasing N and P from sediments, which will intensify eutrophication in water dominated by P and bring huge challenges to water environment management. This study highlights that the current imbalance in N and P inputs into reservoirs and the endogenous P release from sediment will have a significant impact on water quality.

Nitrogen Addition Promotes the Accumulation of Soil Particulate Organic Carbon in a Subtropical Forest

Nitrogen (N) deposition rates of terrestrial ecosystems have gradually declined but are still high in some areas. Previous studies have reported that N addition elicits diverse impacts on soil organic carbon (SOC) pools. SOC can be divided into different functional fractions, namely, particulate organic carbon (POC) and mineral-associated organic carbon (MAOC). The responses of these fractions to N addition should be elucidated to better understand the changes in SOC pools. Here, we conducted a N addition experiment (0, 40, and 80 kg N ha−1 yr−1) in a subtropical Castanopsis fabri forest to simulate N deposition. The surface (0−10 cm) SOC fractions, aboveground litter product, fine root (diameter &lt; 2 mm) biomass, soil exchangeable cation content, and soil enzyme activity under different N addition treatments were measured. The results showed the following: (1) N addition showed a positive effect on POC and SOC contents but did not significantly affect MAOC content; (2) POC content was negatively correlated with pH and soil enzyme activity and positively correlated with aboveground litter product, suggesting that POC accumulation was influenced by aboveground litter input and microbial decomposition; (3) a close negative relationship was observed between exchangeable Al3+ and Ca2+ or K+ contents, indicating that there is likely to be a trade-off between the mineral sorption and desorption, thus resulting in an insignificant reaction of MAOC to N addition. Overall, the accumulation of SOC under short-term N addition was found to be primarily driven by POC, and the response of different SOC functional fractions to N addition was inconsistent. By incorporating these nuances into ecosystem models, it is possible to predict SOC dynamics more accurately in response to global change.

Chemical and C and N stable isotope compositions of three species of epiphytic Tillandsia in a Caribbean coastal zone: air pollution sources and biomonitoring implications

ABSTRACT We characterized the elemental and C and N stable isotope compositions of Tillandsia fasciculata Sw., Tillandsia balbisiana Schult. & Schult.f. and Tillandsia recurvata (L.) L. samples collected in Cienfuegos (Cuba). Results showed high enrichment factors for S, Hg, Cd, Pb, P, Zn, Cu, Mo, Sb and Ca in all Tillandsia species, indicating inputs from local anthropogenic activities (road traffic, industries and cement production). Carbon concentrations and δ 13C varied from 38.3–47.7 % and –20.4 to –13.4 ‰ within the three species, respectively. δ 13C showed seasonal dependence with the dry and wet periods and more 13C-depleted values in urban/industrial areas, coherent with the input of anthropogenic emissions. Nitrogen concentrations (0.4–1.3 %) and δ 15N values (–9.9–4.4 ‰) exhibit larger variations and are positively correlated in the three species. The most positive δ 15N in T. recurvata (–0.2–4.4 ‰) are attributed to contributions from industrial activities and road traffic. In fact, both δ 15N and total nitrogen (TN) values increase in sites with higher road traffic and show significant correlations with typical road traffic and industrial tracers. Finally, we calculate an average total nitrogen deposition rate of 4.4 ± 2.3 kg ha–1 a–1 from N content in T. recurvata, similar to the existing values determined in the region by field measurements, but higher than the global terrestrial average.

Combining multiple investigative approaches to unravel functional responses to global change in the understorey of temperate forests

AbstractPlant communities are being exposed to changing environmental conditions all around the globe, leading to alterations in plant diversity, community composition, and ecosystem functioning. For herbaceous understorey communities in temperate forests, responses to global change are postulated to be complex, due to the presence of a tree layer that modulates understorey responses to external pressures such as climate change and changes in atmospheric nitrogen deposition rates. Multiple investigative approaches have been put forward as tools to detect, quantify and predict understorey responses to these global‐change drivers, including, among others, distributed resurvey studies and manipulative experiments. These investigative approaches are generally designed and reported upon in isolation, while integration across investigative approaches is rarely considered. In this study, we integrate three investigative approaches (two complementary resurvey approaches and one experimental approach) to investigate how climate warming and changes in nitrogen deposition affect the functional composition of the understorey and how functional responses in the understorey are modulated by canopy disturbance, that is, changes in overstorey canopy openness over time. Our resurvey data reveal that most changes in understorey functional characteristics represent responses to changes in canopy openness with shifts in macroclimate temperature and aerial nitrogen deposition playing secondary roles. Contrary to expectations, we found little evidence that these drivers interact. In addition, experimental findings deviated from the observational findings, suggesting that the forces driving understorey change at the regional scale differ from those driving change at the forest floor (i.e., the experimental treatments). Our study demonstrates that different approaches need to be integrated to acquire a full picture of how understorey communities respond to global change.

Impact of patch age and size on forest soil characteristics in European agricultural landscapes

Many landscapes worldwide are characterized by the presence of a mosaic of forest patches with contrasting age and size embedded in a matrix of agricultural land. However, our understanding of the effects of these key forest patch features on the soil nutrient status (in terms of nitrogen, carbon, and phosphorus) and soil pH is still limited due to a lack of large-scale data. To address this research gap, we analyzed 830 soil samples from nearly 200 forest patches varying in age (recent versus ancient forests) and size (small versus larger patches) along a 2500-km latitudinal gradient across Europe. We also considered environmental covariates at multiple scales to increase the generality of our research, including variation in macroclimate, nitrogen deposition rates, forest cover in a buffer zone, basal area and soil type. Multiple linear mixed-effects models were performed to test the combined effects of patch features and environmental covariates on soil nutrients and pH. Recent patches had higher total soil phosphorus concentrations and stocks in the mineral soil layer, along with a lower nitrogen to phosphorus ratio within that layer. Small patches generally had a higher mineral soil pH. Mineral soil nitrogen stocks were lower in forest patches with older age and larger size, as a result of a significant interactive effect. Additionally, environmental covariates had significant effects on soil nutrients, including carbon, nitrogen, phosphorus, and their stoichiometry, depending on the specific covariates. In some cases, the effect of patch age on mineral soil phosphorus stocks was greater than that of environmental covariates. Our findings underpin the important roles of forest patch age and size for the forest soil nutrient status. Long-term studies assessing edge effects and soil development in post-agricultural forests are needed, especially in a context of changing land use and climate.

Lactobacillus rhamnosus GG ATCC53103 and Lactobacillus plantarum JL01 improved nitrogen metabolism in weaned piglets by regulating the intestinal flora structure and portal vein metabolites.

At present, most studies have shown that probiotics have a positive regulatory effect on the nutritional metabolism of the body, but the mechanism is still unclear. Here, 48 piglets were divided into four groups. The control group was not fed probiotics, the Lac group was fed L. Rhamnosus GG ATCC53103, the Rha group was fed L. Plantarum JL01, and the mix group was fed two types of probiotics. Nitrogen metabolism and mRNA levels of mTOR and S6K in skeletal muscle were observed in each group. Then, metagenome and non-targeted metabonomics were used to observe the changes of intestinal microorganisms and plasma metabolites in portal channels after probiotics feeding. Finally, we combined the results of omics analysis to reveal the mechanism of probiotics on nitrogen metabolism in weaned piglets. The results showed that L. Rhmnosus GG ATCC53103 and L. Plantarum JL01 increased nitrogen apparent digestibility, nitrogen deposition rate, and nitrogen utilization rate of weaned piglets (P < 0.05); the relative expression of mTOR and SK6 mRNA in skeletal muscle increased significantly (P < 0.05). When L. rhamnosus GG ATCC53103 and L. plantarum JL01 were combined, we found that Clostridium and Prevotella significantly increased in the jejunum (P < 0.05). The relative abundance of Lactobacillus, Ruminococcus, Streptococcus, and Prevotella in the ileum increased significantly (P < 0.05). Compared with the control group, L-Tryptophan, 3-Phosphonyloxypyruvate, cis-Aconitate, and Carbamoyl phosphate were significantly increased in the mixed group portal vein. The result of the combinatorial analysis showed that the significantly increased microorganisms could encode the enzyme genes for the synthesis of L-Tryptophan, 3-Phosphonooxypyruvate, cis-Aconitate, and Carbamoyl phosphate. In summary, our results demonstrated that L. Rhamnosus GG ATCC53103 and L. Plantarum JL01 could stimulate the expression of skeletal muscle protein synthesis genes of weaned piglets by modulating the structure of the gut microbiota and its metabolites, thereby improving nitrogen metabolism in weaned piglets.

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

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

Nitrate and Ammonium Nitrogen Addition Did Not Alter Nutrient Resorption of Dominant Plant in an Alpine Grassland

Worldwide increases in nitrogen deposition rates are influenced by human activities. Although the total amount of N deposition tends to be stable in our country, atmospheric N deposition of the reactive N forms (NO4+-N, NH4+; NO3−-N, NO3−) is remarkably different, and the ratios of NH4+ to NO3− change continuously. Nutrient resorption is a crucial driver of plant nutrient conservation strategies and litter quality. Therefore, the plant nutrient resorption pattern has remarkable ecological significance for nutrient cycling and the community structure of the ecosystem. However, previous studies have found that plants have different preferences in NH4+ and NO3−, and the response of nutrient resorption to different N forms remains unclear. In 2017 and 2018, we conducted field experiments simulating five NO3− and NH4+ addition ratios to examine the responses of the nutrient resorption of the dominant species, Stipa capillata, in the alpine grassland of northwest China. Results showed that N addition treatments did not affect the nutrient resorption efficiencies. The results showed that N application had little effect on soil inorganic N composition, so there was no significant change in the nutrient resorption efficiencies. In addition, the nutrient resorption efficiencies were unaffected across the five different ratios of nitrate and NH4+ addition, suggesting that nutrient resorption was not only controlled by the nutrient supply and nutrient form. Our results have important implications in understanding the significance of the frequency of N addition due to N loss through leaching and denitrification. The results also highlight that low intensity and high frequency N addition are required to investigate the response of plant nutrient resorption to the N supply in the future study.

Patterns and controlling factors of soil carbon sequestration in nitrogen-limited and -rich forests in China-a meta-analysis.

Soil organic carbon (SOC) management has the potential to contribute to climate change mitigation by reducing atmospheric carbon dioxide (CO2). Understanding the changes in forest nitrogen (N) deposition rates has important implications for C sequestration. We explored the effects of N enrichment on soil carbon sequestration in nitrogen-limited and nitrogen-rich Chinese forests and their controlling factors. Our findings reveal that N inputs enhanced net soil C sequestration by 5.52-18.46 kg C kg-1 N, with greater impacts in temperate forests (8.37-13.68 kg C kg-1 N), the use of NH4NO3 fertilizer (7.78 kg Ckg-1 N) at low N levels (<30 kg Ckg-1 N; 9.14 kg Ckg-1 N), and in a short period (<3 years; 12.95 kg C kg-1 N). The nitrogen use efficiency (NUE) varied between 0.24 and 13.3 (kg C kg-1 N) depending on the forest type and was significantly controlled by rainfall, fertilizer, and carbon-nitrogen ratio rates. Besides, N enrichment increased SOC concentration by an average of 7% and 2% for tropical and subtropical forests, respectively. Although soil carbon sequestration was higher in the topsoil compared to the subsoil, the relative influence indicated that nitrogen availability strongly impacts the SOC, followed by dissolved organic carbon concentration and mean annual precipitation. This study highlights the critical role of soil NUE processes in promoting soil C accumulation in a forest ecosystem.

Moderate Nitrogen Deposition Alleviates Drought Stress of Bretschneidera sinensis

Droughts are becoming more frequent and intense, and the nitrogen deposition rate is increasing worldwide due to human activities. Young seedlings of Bretschneidera sinensis Hemsl. are susceptible to mortality under drought conditions because their root tips have few root hairs. We studied the effect of nitrogen deposition on the physiological characteristics of two-year-old B. sinensis seedlings under drought stress. Seedlings were grown under no nitrogen deposition (control; N0), low nitrogen deposition (N30, 30 kg·hm−2 year−1), medium nitrogen deposition (N60, 60 kg·hm−2 year−1), and high nitrogen deposition (N90, 90 kg·hm−2 year−1), and were subjected to either the normal watering regime (NW) or drought stress (DW). Under DW, the relative conductivity (RC) of seedlings receiving N60 was not significantly different from that of N0 seedlings, and the RC of seedlings receiving N90 was significantly higher than that of N0 seedlings. Under 10 d DW, N60 treatment increased antioxidant enzymes such as superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) activities and content of soluble protein, chlorophyll a and a + b, with POD activity and soluble protein significantly increasing by 18.89% and 34.66%, respectively. Under DW, the proline (PRO) content of seedlings treated with N90 increased. Our data suggested that moderate nitrogen deposition could alleviate drought stress by decreasing cell membrane permeability, reducing cell membrane peroxidation, increasing the content of osmoregulatory substances, and reducing the tendency for chlorophyll to decline, whereas high nitrogen deposition increased the sensitivity of B. sinensis seedlings to drought stress and aggravated the degree of stress, thereby affecting growth.

Mycorrhizal fungi improve growth of Juniperus communis but only at sufficiently high soil element concentrations

High nitrogen (N) deposition rates can cause soil eutrophication and acidification causing harmful effects to many plant species. One species that seems to be affected is Juniperus communis, populations of which are declining in Western Europe, due to a lack of regeneration. This is primarily caused by limited seedling recruitment and low viable seed production; factors that have both been attributed to increased levels of N deposition. J. communis sapling performance could be dependant on mycorrhizal fungi (MF) in their roots, but how – and under what conditions – MF contribute to the performance of J. communis has not been studied before. Here we experimentally tested the effect of MF on J. communis cuttings along an acidity gradient in situ. To manipulate the interaction with MF, we used ingrowth cores, effectively allowing for root-MF interactions or strongly reducing these. After six months we measured growth and needle nutrient concentrations of cuttings with a normal or reduced mycelial network. MF had a positive effect on absolute growth of our cuttings. However, this effect was weakened as soil Mg2+, Ca2+ and Na+ concentrations declined. Soil pH itself was not directly associated with the effect of MF on J. communis. MF also decreased needle phosphorous concentrations, most likely because the increased plant growth by MF diluted its concentration in the plants. We conclude that MF stimulate the performance of J. communis, but that leaching of cations, accelerated by N deposition, negatively affects their mutualism and thereby the performance of J. communis saplings. This highlights the potential positive role of MF in effective conservation and restoration efforts of J. communis habitats in Western Europe and the importance of prioritising the reduction of ongoing eutrophication and acidification.

Soil clay is a key factor affecting soil phosphorus availability in the distribution area of Masson pine plantations across subtropical China

Evaluating the environmental effects on soil phosphorus (P) availability is the key to understanding P cycling and regulating forest productivity. However, what and how environmental factors affect soil P availability in regional plantation ecosystems remains unclear. The study evaluated the effects of climatic factors, nitrogen deposition rate, stand attributes, litter characteristics, and soil-associated properties on soil P availability in Masson pine plantations across subtropical China using stepwise regression and structural equation modelling (SEM) analyses. At the 0–20 cm and 20–40 cm soil depths, soil total P ranged from 0.24 to 0.43 g kg−1 and from 0.24 to 0.48 g kg−1, respectively. Similarly, soil available P varied from 0.35 to 7.55 mg kg−1 and from 0.14 to 4.34 mg kg−1, respectively. Soil total and available P had high values in the eastern and northern distribution areas of subtropical China. Soil total P linearly decreased with an increase in bulk density, clay and pH but linearly increased with an increase in DBH. These four factors had a similar contribution (15.5–35.1 %) to soil total P. Soil available P linearly decreased with an increase in bulk density and clay. The relative contribution of clay (48.1–75.7 %) to soil available P was higher than that of bulk density (16.2–33.5 %). Soil total P decreased during the initial 30 years of plantations and then increased. But other variables (e.g., climate, nitrogen deposition, and litter) had no significant impacts on soil P. Overall, the SEM explained 35.4 % and 28.1 % of soil available P variations at the 0–20 cm and 20–40 cm soil depths, respectively, and revealed that clay had strong total effects (β = -0.526 and −0.396) on the soil available P. These results reveal the dominant roles of soil texture (especially clay) in affecting soil P availability in Masson pine plantations and suggest that extending the rotation period benefits soil P recovery in pine plantation ecosystems of subtropical China.

Plant community re-organization and increased productivity due to multi-year nutrient enrichment of a coastal grassland.

Nutrient enrichment alters plant community structure and function at a global scale. Coastal plant systems are expected to experience increased rates of nitrogen and phosphorus deposition by 2100, caused mostly by anthropogenic activity. Despite high density of studies investigating connections between plant community structure and ecosystem function in response to nutrient addition, inconsistencies in system response based on the ecosystem in question calls for more detailed analyses of nutrient impacts on community organization and resulting productivity response. Here, we focus on nutrient addition impacts on community structure and organization as well as productivity of different lifeforms in a coastal grassland. We established long-term nutrient enrichment plots in 2015 consisting of control (C), nitrogen (N), phosphorus (P), and nitrogen + phosphorus (NP) treatments. In 2017 we collected graminoid and forb productivity, root productivity, and community composition for each plot. We found no N x P interaction, but N enrichment was a significant main effect on productivity, highlighting N limitation in coastal systems. Importantly, nutrient enrichment treatments did not alter root productivity. However, all treatments caused significant differences in community composition. Using rank abundance curves, we determined that community composition differences were driven by increased dominance of nitrophilous graminoids, re-organization of subordinate species, and species absences in N and NP plots. Results of this study highlight how coastal grassland communities are impacted by nutrient enrichment. We show that community re-organization, increased dominance, and absence of critical species are all important mechanisms that reflect community-level impacts of nutrient enrichment in our coastal grassland site.

Trait value and phenotypic integration contribute to the response of exotic Rhus typhina to heterogeneous nitrogen deposition: A comparison with native Rhus chinensis

The temporal heterogeneity of nitrogen availability in soils is increasing due to agricultural deposition. We here compared the effects of gradually increasing nitrogen deposition rate and its increasing temporal heterogeneity patterns on the functional traits of seedlings of exotic species Rhus typhina and the native species Rhus chinensis. Nitrogen deposition rates of 0, 8, 20 g N m−2 year−1 and constant, single-peak, and double-peak nitrogen were added to simulate deposition rate and temporal heterogeneity. After 60 days of treatment, R. typhina seedlings had several advantageous growth trait values, such as higher total biomass production, but lower phenotypic plasticity than R. chinensis seedlings. R. typhina seedlings also had higher phenotypic integration, measured as the correlation among functional traits. The increased nitrogen deposition rate affected several traits of the two species differently. Thus, while R. chinensis seedlings allocated more biomass to leaves and less to roots with increasing N deposition, R. typhina seedlings had stable biomass allocation among all N treatments. Chlorophyll content, leaf phosphorus concentration, and water use efficiency increased, but the maximum net photosynthetic rate decreased, with N availability in R. chinensis, but not in R. typhina. Temporal heterogeneity had no significant effect on the total biomass of R. typhina and R. chinensis seedlings. Overall, the performance of R. typhina is better than that of R. chinensis seedlings under different nitrogen deposition treatments, which is due to the significantly advantageous trait values and greater phenotypic integration of R. typhina seedlings, whereas R. chinensis seedlings have higher phenotypic plasticity.

Decreasing nitrogen deposition rates: Good news for oligotrophic grassland species?

Several studies have found that increased nitrogen (N) deposition leads to a decline in species richness in semi-natural grasslands, mainly due to the loss of species typical of nutrient-poor soils. However, after reaching a peak around 1990, N deposition has decreased in Europe over the last 30 years. In this study, we investigated the changes in species number and composition of semi-natural grasslands during this period of declining N deposition. To this end, we compared the data from the first survey (2001-2005) of 147 grassland sites in Switzerland with those from the third survey (2011-2015). We further analysed the vegetation development of a specific hay meadow from 1992 to 2013. In this grassland, total vegetation cover and the cover of graminoid species decreased, while the cover of oligotrophic species increased. At the 147 grassland sites, total species number decreased at sites with still high levels of N deposition and it tended to increase at sites with low N deposition, i. e. below the critical load for N deposition. The number of oligotrophic grassland species increased at sites with a large decrease in N deposition and strong inclination. Thus, the results of this study indicate that the reduction of N emissions had a measurable positive effect on species diversity in these semi-natural grasslands. Most of the grasslands surveyed appear to be quite resilient against N deposition, i. e. they do not shift to an alternative low diversity state dominated by a few competitive species, and recovery of the species composition as a result of the decrease in N deposition seems possible, especially on steep slopes. Furthermore, the study underlines the importance of regular management of semi-natural, unfertilised, low-productivity grassland to maintain the diversity of oligotrophic grassland species.

Low-level nitrogen and short-term addition increase soil carbon sequestration in Chinese forest ecosystems

Soil carbon (C) sequestration plays a vital role in mitigating global climate change. Human activities have vastly increased nitrogen (N) deposition rate in China, which in turn influences belowground C cycle processes. We performed a meta-analysis based on 61 published studies on N addition experiments, including 4072 observations across China, to quantify the responses of belowground soil C dynamics and sequestration to N enrichment in Chinese forest ecosystems. The results showed that, on average, N enrichment significantly enhanced C dynamics in boreal and temperate forests by an average of 24% and 10% while it reduced in subtropical and tropical forests by 11% and 19%. The response of C pools and C input to N addition were more pronounced in boreal forests with +17% and +10% while it was lower in subtropical forests with −0.4% and −19% respectively. The N enrichment enhanced soil C output by in boreal (6%) and temperate (7%) forests and a negative effect in subtropical (−30%) and tropical forests (−10%). This can be explained by the higher reduction levels in C respiration, despite the slightly lower litter and root-derived C inputs. Notably, N addition specifically increased belowground C sequestration, at low N addition rates (<30 kg N ha−1 yr−1) and during a short-term period (<1 year). These C sequestration effects were reversed at higher N deposition levels and /or during a longer time period. This implies that soil C sequestration is currently most likely not enhanced in (large) parts of China, as forests are characterized by long-term elevated N deposition levels.

Two Dominant Herbaceous Species Have Different Plastic Responses to N Addition in a Desert Steppe.

Nitrogen (N) deposition rates are increasing in the temperate steppe due to human activities. Understanding the plastic responses of plant dominant species to increased N deposition through the lens of multiple traits is crucial for species selection in the process of vegetation restoration. Here, we measured leaf morphological, physiological, and anatomical traits of two dominant species (Stipa glareosa and Peganum harmala) after 3-year N addition (0, 1, 3, and 6 g N m−2 year−1, designated N0, N1, N3, and N6, respectively) in desert steppe of Inner Mongolia. We separately calculated the phenotypic plasticity index (PI) of each trait under different N treatments and the mean phenotypic plasticity index (MPI) of per species. The results showed that N addition increased the leaf N content (LNC) in both species. N6 increased the contents of soluble protein and proline, and decreased the superoxide dismutase (SOD) and the peroxidase (POD) activities of S. glareosa, while increased POD and catalase (CAT) activities of P. harmala. N6 increased the palisade tissue thickness (PT), leaf thickness (LT), and palisade-spongy tissue ratio (PT/ST) and decreased the spongy tissue–leaf thickness ratio (ST/LT) of S. glareosa. Furthermore, we found higher physiological plasticity but lower morphological and anatomical plasticity in both species, with greater anatomical plasticity and MPI in S. glareosa than P. harmala. Overall, multi-traits comparison reveals that two dominant desert-steppe species differ in their plastic responses to N addition. The higher plasticity of S. glareosa provides some insight into why S. glareosa has a broad distribution in a desert steppe.

Exotic Rhus Typhina is Superior to Native Rhus Chinensis Due to Trait Values and Phenotypic Integration, But Not Heterogeneous Nitrogen Deposition Rates

Exotic Rhus Typhina is Superior to Native Rhus Chinensis Due to Trait Values and Phenotypic Integration, But Not Heterogeneous Nitrogen Deposition Rates