Atmospheric nitrogen (N) deposition has been substantially reduced due to declines in the reactive N emission in major regions of the world. Nevertheless, the impact of reduced N deposition on soil microbial communities and the mechanisms by which they are regulated remain largely unknown. Here, we examined the effects of N addition and cessation of N addition on plant and soil microbial communities through a 17-year field experiment in a temperate grassland. We found that extreme N input did not irreversibly disrupt the ecosystem, but ceasing high levels of N addition led to greater resilience in bacterial and fungal communities. Fungi exhibited diminished resilience compared to bacteria due to their heightened reliance on changes in plant communities. Neither bacterial nor fungal diversity fully recovered to their original states. Their sensitivity and resilience were mainly steered by toxic metal ions and soil pH differentially regulating on functional taxa. Specifically, beneficial symbiotic microbes such as N-fixing bacteria and arbuscular mycorrhizal fungi experienced detrimental effects from toxic metal ions and lower pH, hindering their recovery. The bacterial functional groups involved in carbon decomposition, and ericoid mycorrhizal and saprotrophic fungi were positively influenced by soil metals, and demonstrated gradual recovery. These findings could advance our mechanistic understanding of microbial community dynamics under ongoing global changes, thereby informing management strategies to mitigate the adverse effects of N enrichment on soil function.

The phenomenon of nitrogen deposition resulting in species loss in terrestrial ecosystems has been demonstrated in several experiments. Nitrogen (N) and phosphorus (P), as major nutrients required for plant growth, exhibit ecological stoichiometric coupling in many ecosystems. The increased availability of nitrogen can exacerbate the ecological effects of phosphorus. To reveal the ecological effects of phosphorus under nitrogen-limiting and non-limiting conditions, we conducted a controlled N-P interaction experiment over 5 years in the Hulunbuir meadow steppe, where two nitrogen addition levels were implemented: 0 g N·m-2·a-1 (nitrogen-limiting condition) and 10 g N·m-2·a-1 (nitrogen-non-limiting condition), together with six levels of phosphorus addition (0, 2, 4, 6, 8, and 10 g P·m-2·a-1). The results showed that nitrogen addition (under nitrogen-non-limiting conditions) significantly decreased species diversity in the steppe community, which was exacerbated under phosphorus addition. Under nitrogen-limiting conditions, phosphorus addition had no marked impact on species diversity compared to the control; however, there were substantial differences between different levels of phosphorus addition, exhibiting a unimodal change. Under both experimental nitrogen conditions, the addition of 6 g P·m-2·a-1 was the threshold for affecting the community species diversity. Nitrogen addition reduced the relative biomass of legumes, bunch grasses, and forbs, but substantially increased the relative biomass of rhizomatous grasses. In contrast, phosphorus addition only markedly affected the relative biomass of forbs and rhizomatous grasses, with the former showing a unimodal pattern of first increasing and then decreasing with increasing phosphorus addition level, and the latter exhibiting the opposite pattern. The different responses of rhizomatous grasses and other functional groups to nitrogen and phosphorus addition were observed to have a regulatory effect on the changes in grassland community structure. Phosphorus addition may increase the risk of nitrogen deposition-induced species loss. Both nitrogen and phosphorus addition lead to soil acidification and an increase in the dominance of the already-dominant species, and the consequent species loss in the forb functional group represents the main mechanism for the reduction in community species diversity.

Light availability profoundly influences plant communities, especially below dense tree canopies in forests. Canopy disturbances, altering forest floor light conditions, together with other environmental changes such as climate change, nitrogen deposition and legacy effects from previous land-use will simultaneously impact forest understorey communities. Yet, knowledge on the individual effects of these drivers and their potential interactions remains scarce. Here we performed a forest mesocosm experiment to assess the influence of warming, illumination (simulating canopy opening), nitrogen deposition and soil land-use history (comparing ancient and post-agricultural forest soil) on understorey community composition trajectories over a 7-year period. Strikingly, understorey communities primarily evolved in response to the deeply shaded ambient forest conditions, with experimental treatments exerting only secondary influences. The overruling trajectory steered all mesocosms towards slow-colonizing forest specialist communities dominated by spring geophytes with lower nutrient-demand. The illumination treatment and, to a lesser extent, warming and agricultural land-use legacy slowed down this trend by advancing fast-growing resource-acquisitive generalist species. Warm ambient temperatures induced thermophilization of plant communities in all treatments, including control plots, towards higher dominance of warm-adapted species. Nitrogen addition accelerated this thermophilization process and increased the community light-demand signature. Land-use legacy effects were limited in our study. Our findings underscore the essential role of limited light availability in preserving forest specialists in understorey communities and highlight the importance of maintaining a dense canopy cover to attenuate global change impacts. It is crucial to integrate this knowledge in forest management adaptation to global change, particularly in the face of increasing demands for wood and wood products and intensified natural canopy disturbances.

Global warming and increased atmospheric nitrogen (N) deposition can adversely impact Sphagnum moss populations and ecological functions in peatlands. Based on the anticipated increases in temperature and N levels at global scale, we investigated the effects of simultaneous warming and N treatment on growth and ecophysiological activity of Sphagnum papillosum, a predominant moss at mid-latitudes, utilizing a growth chamber experiment. Warming treatments increased the maximum yield of photosystem II (Fv/Fm) of S. papillosum while decreasing the stable carbon isotope ratio. However, warming treatment alone did not cause significant changes in the biomass increase from that of the control. Regarding N treatment, the low N treatment decreased Fv/Fm under the current temperature but did not affect the biomass increase. In contrast to these results, a simultaneous warming and high N treatment significantly enhanced the biomass production compared to that of the control, exhibiting additive effect of warming and high N treatment on Sphagnum biomass production. These responses were attributed to the improved photosynthetic performances by warming and N treatment. The results of this study contribute to the prediction of Sphagnum responses to warming and changes in N deposition.

Long-term atmospheric nitrogen (N) deposition has been known to enhance plant productivity by increasing available N in terrestrial ecosystems. However, the response of carbon process to N deposition in terrestrial ecosystems remains unclear, particularly about different climate regions and biomes. In this study, we synthesized 1281 pairwise observations from 218 published articles on experimental N addition globally, aiming to quantify the responses of the carbon process and its mechanisms to N addition. Our results revealed a significant overall increase in net ecosystem productivity (NEP) by 31.42 % following N addition treatment, owing to increased aboveground net primary productivity (ANPP, 16.46 %), belowground net primary productivity (BNPP, 27.74 %), and reduced soil respiration (Rs, -2.56 %), soil heterotrophic respiration (Rh, -6.24 %). Furthermore, the effects of N addition on NEP varied with biomes and climate regions. The positive effect of N addition on NEP was more pronounced in arid regions (28.67 %) compared to humid regions (21.92 %), primarily driven by a higher increase in vegetation productivity. Additionally, N addition exhibited a higher increase in NEP (33.95 %) in forest compared to grassland (31.80 %), resulting from a more reduction in respiratory processes. However, the positive effects of N addition on NEP diminished with increasing experimental duration. Furthermore, ANPP and BNPP displayed a convex relationship with N addition rates, with the optimum BNPP addition rate exceeding that of ANPP. In contrast, Rs exhibited a concave response to addition rates. These findings suggest that carbon sink in terrestrial ecosystems could be enhanced under future atmospheric N deposition, especially in arid regions and forest ecosystems. Our study provided insight for predicting how N deposition influences terrestrial ecosystem carbon process.

Atmospheric nitrogen (N) deposition in forests can affect soil microbial growth and turnover directly through increasing N availability and indirectly through altering plant-derived carbon (C) availability for microbes. This impacts microbial residues (i.e., amino sugars), a major component of soil organic carbon (SOC). Previous studies in forests have so far focused on the impact of understory N addition on microbes and microbial residues, but the effect of N deposition through plant canopy, the major pathway of N deposition in nature, has not been explicitly explored. In this study, we investigated whether and how the quantities (25 and 50 kg N ha-1 year-1) and modes (canopy and understory) of N addition affect soil microbial residues in a temperate broadleaf forest under 10-year N additions. Our results showed that N addition enhanced the concentrations of soil amino sugars and microbial residual C (MRC) but not their relative contributions to SOC, and this effect on amino sugars and MRC was closely related to the quantities and modes of N addition. In the topsoil, high-N addition significantly increased the concentrations of amino sugars and MRC, regardless of the N addition mode. In the subsoil, only canopy N addition positively affected amino sugars and MRC, implying that the indirect pathway via plants plays a more important role. Neither canopy nor understory N addition significantly affected soil microbial biomass (as represented by phospholipid fatty acids), community composition and activity, suggesting that enhanced microbial residues under N deposition likely stem from increased microbial turnover. These findings indicate that understory N addition may underestimate the impact of N deposition on microbial residues and SOC, highlighting that the processes of canopy N uptake and plant-derived C availability to microbes should be taken into consideration when predicting the impact of N deposition on the C sequestration in temperate forests.

Atmospheric reactive nitrogen (Nr) deposition has been modified significantly by human activities such as agriculture and fossil fuel combustion. Understanding the changes in Nr deposition is essential for maintaining the functionality and sustainability of ecosystems. Taking Beijing as a case study, we report long-term measurements of wet Nr deposition from 1999 to 2022 and dry Nr deposition from 2010 to 2022 and their relationship with China\'s air pollution control. Total Nr deposition to Beijing decreased by 34 % during 2010-2022, mainly caused by a decrease in dry N deposition by 54.27 %, from 47.86 kg N ha-1 yr-1 in 2010-2014 to 21.89 kg N ha-1 yr-1 in 2018-2022; reduced and oxidized N in dry deposition decreased by 29.93 % and 72.05 %, respectively. This was a result of the \"Action Plan for Prevention and Control of Air Pollution (APCP)\" and the implementation of the \"Zero Growth in Fertilizer Use by 2020\" in 2015. Our ground-based measurements provide evidence to support recent achievements in air pollution control and a reference and guidance for other regions of China and other countries for abating Nr pollution.

The primary mechanisms contributing to nitrogen (N) addition induced grassland biodiversity loss, namely light competition and soil cation toxicity, are often examined separately in various studies. However, their relative significance in governing biodiversity loss along N addition gradient remains unclear. We conducted a 4-yr field experiment with five N addition rates (0, 2, 10, 20, and 50 g N m-2 yr-1) and performed a meta-analysis using global data from 239 observations in N-fertilized grassland ecosystems. Results from our field experiment and meta-analysis indicate that both light competition and soil cation (e.g. Mn2+ and Al3+) toxicity contribute to plant diversity loss under N enrichment. The relative importance of these mechanisms varied with N enrichment intensity. Light competition played a more significant role in influencing species richness under low N addition (≤ 10 g m-2 yr-1), while cation toxicity became increasingly dominant in reducing biodiversity under high N addition (>10 g m-2 yr-1). Therefore, a transition from light competition to cation toxicity occurs with increasing N availability. These findings imply that the biodiversity loss along the N gradient is regulated by distinct mechanisms, necessitating the adoption of differential management strategies to mitigate diversity loss under varying intensities of N enrichment.

Exploring the influences of nitrogen deposition on soil carbon (C) flux is necessary for predicting C cycling processes; however, few studies have investigated the effects of nitrogen deposition on soil respiration (Rs), autotrophic respiration (Ra) and heterotrophic respiration (Rh) across urban-rural forests. In this study, a 4-year simulated nitrogen deposition experiment was conducted by treating the experimental plots with 0, 50, or 100 kg·ha-1·year-1 of nitrogen to check out the mechanisms of nitrogen deposition on Rs, Ra, and Rh in urban-rural forests. Our finding indicated a positive association between soil temperature and Rs. Soil temperature sensitivity was significantly suppressed in the experimental plots treated with 100 kg·ha-1·year-1 of nitrogen only in terms of the urban forest Rs and Ra and the rural forest Ra. Nitrogen treatment did not significantly increase Rs and had different influencing mechanisms. In urban forests, nitrogen addition contributed to Rh by increasing soil microbial biomass nitrogen and inhibited Ra by increasing soil ammonium‑nitrogen concentration. In suburban forests, the lack of response of Rh under nitrogen addition was due to the combined effects of soil ammonium‑nitrogen and microbial biomass nitrogen; the indirect effects from nitrate‑nitrogen also contributed to a divergent effect on Ra. In rural forests, the soil pH, dissolved organic C, fine root biomass, and microbial biomass C concentration were the main factors mediating Rs and its components. In summary, the current rate of nitrogen deposition is unlikely to result in significant increases in soil C release in urban-rural forests, high nitrogen deposition is beneficial for reducing the temperature sensitivity of Rs in urban forests. The findings grant a groundwork for predicting responses of forest soil C cycling to global change in the context of urban expansion.

Increased nitrogen deposition is a key feature of global climate change, however, its effects on the structure and assembling mechanisms of the nitrogen-fixing bacteria present at the root surface remain to be elucidated. In this pursuit, we used NH4NO3 to simulate nitrogen deposition in a 10-year-old Camellia oleifera plantation, and set up four deposition treatments, including control N0 (0 kg N hm-2 a-1), low nitrogen N20 (20 kg N hm-2 a-1), medium nitrogen N40 (40 kg N hm-2 a-1) and high nitrogen N160 (160 kg N hm-2 a-1). The results showed that nitrogen deposition affected the soil nitrogen content and the structure of the nitrogen-fixing bacterial community. Low nitrogen deposition was conducive for nitrogen fixation in mature C. oleifera plantation. With increasing nitrogen deposition, the dominant soil nitrogen-fixing bacterial community shifted from Desulfobulbaceae to Bradyrhizobium. When nitrogen deposition was below 160 kg N hm-2 a-1, the soil organic matter content, total nitrogen content, nitrate nitrogen content, ammonium nitrogen content, urease activity, soil pH and nitrate reductase activity influenced the composition of the nitrogen-fixing bacterial community, but the stochastic process remained the dominant factor. The results indicate that the strains of Bradyrhizobium japonicum and Bradyrhizobium sp. ORS 285 can be used as indicator species for excessive nitrogen deposition.