AbstractThe sensitivity of soil organic carbon (SOC) decomposition in seasonally frozen soils, such as alpine ecosystems, to climate warming is a major uncertainty in global carbon cycling. Here we measure soil CO2 emission during four years (2018–2021) from the whole-soil warming experiment (4 °C for the top 1 m) in an alpine grassland ecosystem. We find that whole-soil warming stimulates total and SOC-derived CO2 efflux by 26% and 37%, respectively, but has a minor effect on root-derived CO2 efflux. Moreover, experimental warming only promotes total soil CO2 efflux by 7-8% on average in the meta-analysis across all grasslands or alpine grasslands globally (none of these experiments were whole-soil warming). We show that whole-soil warming has a much stronger effect on soil carbon emission in the alpine grassland ecosystem than what was reported in previous warming experiments, most of which only heat surface soils.

Global climate warming could affect the methane (CH4 ) and nitrous oxide (N2 O) fluxes between soils and the atmosphere, but how CH4 and N2 O fluxes respond to whole-soil warming is unclear. Here, we for the first time investigated the effects of whole-soil warming on CH4 and N2 O fluxes in an alpine grassland ecosystem on the Tibetan Plateau, and also studied the effects of experimental warming on CH4 and N2 O fluxes across terrestrial ecosystems through a global-scale meta-analysis. The whole-soil warming (0-100 cm, +4°C) significantly elevated soil N2 O emission by 101%, but had a minor effect on soil CH4 uptake. However, the meta-analysis revealed that experimental warming did not significantly alter CH4 and N2 O fluxes, and it may be that most field warming experiments could only heat the surface soils. Moreover, the warming-induced higher plant litter and available N in soils may be the main reason for the higher N2 O emission under whole-soil warming in the alpine grassland. We need to pay more attention to the long-term response of greenhouse gases (including CH4 and N2 O fluxes) from different soil depths to whole-soil warming over year-round, which could help us more accurately assess and predict the ecosystem-climate feedback under realistic warming scenarios in the future.

Globally increasing nitrogen (N) deposition is recognized as an important regulator of soil microbial commu-nities. However, how N enrichment affects soil microbial diversity, richness and community structure remains unclear at the global scale. Here, by focusing on high-throughput amplicon sequencing data from field experi-ments using N fertilizers only, we conducted a meta-analysis of a global dataset assessing the responses of mi-crobial diversity (Shannon index), richness (Chao1, OTU richness) and community structure to N addition. Our results showed that N addition significantly reduced soil bacterial diversity (-2.3%), and such effect was sig-nificant in cropland (rather than grassland and forest) and with urea addition (rather than ammonium nitrate). However, there was no significant effect of N addition on soil fungal diversity and microbial (bacterial or fungal) richness. Moreover, N addition shifted microbial community structure likely due to the microbial adaptation to N-excess, but had no significant effect on microbial beta-diversity. Model-selection analysis further showed that the change in soil pH was the most important factor regulating the responses of soil bacterial diversity and richness to N addition. Overall, our results contribute to an in-depth understanding of the effects of N addition on soil microbial diversity and community structure in terrestrial ecosystems at the global scale.

Plants live in association with a diversity of soil microorganisms, which are extremely important in affecting plant growth and soil biogeochemical cycling. By adopting plant trait‐based approaches, we explored the linkages between rhizosphere soil microbial activity and plant resource acquisition strategy of above‐ and below‐ground across a range of tree species in a subtropical evergreen mixed forest. The microbial activities were represented by diverse extracellular enzymes relevant to carbon, nitrogen and phosphorus cycling and soil organic carbon (SOC) mineralization. At the species level, leaf and root traits were mainly represented by two leading dimensions, that is, the ‘fast‐slow’ economics spectrum on which leaf and root traits were well aligned and the orthogonal collaboration gradient in the root. Both extracellular enzymes and SOC mineralization in the rhizosphere varied greatly across plant species. We found that diverse rhizosphere soil microbial activities positively correlated with the classical ‘fast‐slow’ conservation gradient of plant resource acquisition (especially above‐ground), that is, the rhizosphere soil microbes associated with fast‐growing plant species feature higher metabolism than that of slow‐growing plant species. In comparison, rhizosphere soil microbial activities were independent of the plant collaboration gradient in the root, and it might be an alternative exploitative strategy in foraging soil nutrients for plants. Synthesis . Our study strengthens the multivariate nature of plant resource acquisition in adapting to above‐ and below‐ground stresses. The findings on the linkages between rhizosphere soil microbial activity and plant resource acquisition strategy have the potential to improve our understanding and prediction of plant species turnover impacts on soil biogeochemical cycles.

The structure and function of plant communities in alpine meadow ecosystems are potentially susceptible to climate warming. Here, we utilized a unique field manipulation experiment in an alpine meadow on the Qinghai-Tibetan Plateau and investigated the responses of plant species diversity, composition, biomass, and net primary productivity (NPP) at both community and functional group levels to whole-soil-profile warming (3–4 °C across 0–100 cm) during 2018–2021. Plant species diversity, biomass and NPP (both above- and belowground) at the community level showed remarkable resistance to warming. However, plant community composition gradually shifted over time. Over the whole experimental warming period, aboveground biomass of legumes significantly decreased by 45%. Conversely, warming significantly stimulated aboveground biomass of forbs by 84%, likely because of better growth and competitive advantages from the warming-induced stimulation of soil water and other variables. However, warming showed minor effects on aboveground biomass of grasses and sedges. Overall, we emphasize that experimental warming may significantly affect plant community composition in a short term by triggering adjustments in plant interspecific competition or survival strategies, which may cause potential changes in plant productivity over a more extended period and lead to changes in carbon source-sink dynamics in the alpine meadow ecosystem.

As the largest reservoir in North China, the Miyun Reservoir is the most important surface drinking water source in Beijing. Bacteria are key regulators of reservoir ecosystem structure and function, and exploring the community distribution characteristics of bacteria is important for maintaining water quality safety in reservoirs. The spatiotemporal distribution and environmental factors of bacterial communities in the water and sediment of the Miyun Reservoir were explored using the high-throughput sequencing method. The results showed a higher α-diversity and non-significant seasonal variation of the bacterial community in the sediment, and the abundant sedimental species were affiliated with Proteobacteria. For planktonic bacteria, Actinobacteriota was the dominant phylum, and the seasonal variance was represented by CL500-29_marine_group and hgcI_clade in the wet season and Cyanobium_PCC-6307 in the dry season. Additionally, obvious differences in key species were also found in water and sediment, and more indicator species were obtained in sedimental bacteria. Further, a more complex co-existence network was identified in water compared to that in sediment, indicating the strong ability of planktonic bacteria to resist environmental changes. Environmental factors had a significantly higher effect on the bacterial community of the water column than that of the sediment. Furthermore, SO2-4 and TN were the main factors affecting planktonic bacteria and sedimental bacteria, respectively. These findings revealed the distribution patterns and driving forces of the bacterial community in the Miyun Reservoir, which will provide important guidance for reservoir management and water-quality assurance.

Fungi possess prominent tolerance and detoxification capacities in highly metal(loid)-polluted systems, yet little is known about their responding behaviors under different contamination conditions. Here, we systematically investigated the structure and function profiles of fungal communities in an abandoned reservoir mainly contaminated by multiple metal(loid)s such as Al, Be, Cd, Co, Cr, and Cu. This abandoned reservoir consisted of three distinct zones, i.e., Zone I with the shortest deprecation time and the highest metal(loid) contamination; Zone II with the medium deprecation time and medium metal(loid) contamination; and Zone III with the longest abandonment time and the lowest metal(loid)contamination. The lowest pH and the highest contents of OM, TN, and TP were also observed for the high-contamination Zone I, followed by the moderate-contamination Zone II and the low-contamination Zone III. Fungal biodiversity was found to be robust and dominated by many endurable genera in Zone I, and notable cooperative relationships among fungal species facilitated their viability and prosperity under severe metal(loid) contaminations. Differently, the lowest biodiversity and fragile co-occurrence network were identified in Zone II. As metal(loid) contaminations reduced from Zone I to Zone III, dominant fungal functions gradually changed from undefined saprotroph guild to parasites or pathogens of plant-animal (i.e. animal pathogen, endophyte, and plant pathogen). Moreover, metal(loid)s combined with physicochemical properties jointly mediated the fungal taxonomic and functional responses to different metal(loid) contamination levels. Overall, this study not only broadens the understanding of taxonomic and functional repertoires of fungal communities under different metal(loid) contaminated conditions, but also highlights the crucial contributions of specific fungi to bioremediation and management in varying metal(loid)-polluted environments.

The adoption of diverse resource acquisition strategies is critical for plant growth and species coexistence. Root phosphatase is of particular importance in the acquisition of soil phosphorus (P), yet it is often overlooked in studies of root trait syndromes. Here, we evaluated the role of root phosphatase activity (RPA) within the root economics space and the order-based variation of RPA, as well as the correlations between RPA and a suite of leaf traits and soil properties over a range of evergreen tree species in a subtropical forest. Root phosphatase activity exhibited a high degree of inter-specific variation. We found that there were two leading dimensions of the multidimensional root economics space, the root diameter-specific root length axis (collaboration trait gradient) and the root tissue density-root nitrogen concentration axis (classical trait gradient), and RPA aligned with the former. Root phosphatase activity is used as a 'do it yourself' strategy of soil P acquisition, and was found to be inversely correlated with mycorrhizal colonization, which suggests a trade-off in plant P acquisition strategies. Compared with soil and foliar nutrient status, root traits mattered most for the large inter-specific changes in RPA. Furthermore, RPA generally decreased from first- to third-order roots. Taken together, such diverse P-acquisition strategies are conducive to plant coexistence within local forest communities. The use of easily measurable root traits and their tight correlations with RPA could be a feasible and promising approach to estimating species-specific RPA values, which would be helpful for better understanding plant P acquisition and soil P cycling.

The bacteria in the water column and surface sediments are inherently intertwined and inseparable in aquatic ecosystems, yet little is known about the integrated spatiotemporal dynamics and driving mechanisms of both planktonic and sedimentary bacterial communities in reservoirs. By investigating the planktonic and sedimentary bacteria during four seasons from 88 samples of 11 representative sites across the Danjiangkou reservoir, we depicted an integrated biogeographic pattern of bacterial communities in the water source of the world's largest water diversion project. Our study revealed both planktonic (mantel r = 0.502, P = 0.001) and sedimentary (mantel r = 0.131, P = 0.009) bacterial communities were significantly correlated with environmental heterogeneity, but a weak disparity along spatial heterogeneity, and the significant seasonal dynamics of planktonic (mantel r = 0.499, P = 0.001) rather than sedimentary bacteria. Particularly, rare biosphere played a main role in determining the community succession in the reservoir. It not only exhibited a more striking environmental separation than abundant taxa but also was an essential part in mediating spatiotemporal shifts of planktonic bacteria and maintaining the stability of bacterial community. These rare bacteria were respectively mediated by stochastic (62.68%) and selective (79.60%) processes in water and sediments despite abundant taxa being largely determined by stochastic processes (86.88-93.96%). Overall, our study not only fills a gap in understanding the bacterial community dynamics and underlying drivers in source water reservoirs, but also highlights the particular importance of rare bacteria in mediating biogeochemical cycles in world's large reservoir ecosystems.

Archaea are important participants in biogeochemical cycles of metal(loid)-polluted ecosystems, whereas archaeal structure and function in response to metal(loid) contamination remain poorly understood. Here, the effects of multiple metal(loid) pollution on the structure and function of archaeal communities were investigated in three zones within an abandoned sewage reservoir. We found that the high-contamination zone (Zone I) had higher archaeal diversity but a lower habitat niche breadth, relative to the mid-contamination zone (Zone II) and low-contamination zone (Zone III). Particularly, metal-resistant species represented by potential methanogens were markedly enriched in Zone I (cumulative relative abundance: 32.24%) compared to Zone II (1.93%) and Zone III (0.10%), and closer inter-taxon connections and higher network complexity (based on node number, edge number, and degree) were also observed compared to other zones. Meanwhile, the higher abundances of potential metal-resistant and methanogenic functions in Zone I (0.24% and 9.24%, respectively) than in Zone II (0.08% and 7.52%) and Zone III (0.01% and 1.03%) suggested archaeal functional adaptation to complex metal(loid) contamination. More importantly, six bioavailable metal(loid)s (titanium, tin, nickel, chromium, cobalt, and zinc) were the main contributors to archaeal community variations, and metal(loid) pollution reinforced the role of deterministic processes, particularly homogeneous selection, in the archaeal community assembly. Overall, this study provides the first integrated insight into the survival strategies of archaeal communities under multiple metal(loid) contamination, which will be of significant guidance for future bioremediation and environmental governance of metal(loid)-contaminated environments.

Both climate warming and plateau pika (Ochotona curzoniae) exert considerable impacts on carbon cycling in alpine ecosystems on the Tibetan Plateau. However, the interaction of warming and pika disturbance on soil organic carbon (SOC) dynamics in alpine ecosystems remains largely unknown. Here, we measured plant, soil and microbial properties after four-year warming in a swamp meadow (with or without pika disturbance) on the Tibetan Plateau. Our results showed that in non-pika plots, warming not only increased plant belowground biomass (48%), but also enhanced microbial biomass, hydrolytic enzyme activities, and bacterial functional genes (41-46%). More plant inputs likely outweighed faster microbial decomposition, leading to the accumulation of fast-cycling particulate organic carbon (POC, 49%) and bulk SOC (22%), but not slow-cycling mineral-associated organic carbon (MAOC). In pika-disturbed plots, however, warming did not significantly change plant belowground biomass (which dominates total plant biomass), microbial biomass and hydrolytic enzyme activities. Although bacterial functional genes were suppressed (-40%) and plant aboveground biomass was increased (118%), oxidative enzymes were significantly stimulated (17%), which likely counteracted the higher aboveground plant inputs and led to minor changes in soil carbon pools (SOC, POC and MAOC) with warming. Moreover, bacterial and fungal community structure were significantly altered by pika disturbance, but not warming. Overall, these findings demonstrated that pika disturbance could offset the short-term positive effect of warming on soil organic carbon in the alpine swamp meadow ecosystem.

Wastewater treatment plant (WWTP) effluents carrying plenty of nutrients and micropollutants pose serious threats to receiving rivers, however, the response of microbial community structure and function to WWTP effluents discharge is still poorly understood. To address this knowledge gap, paired water and sediment samples from 17 sites of the Huangshui River, and effluents from 6 WWTPs were collected to investigate the effect of WWTP discharge on riverine microbial communities. Our results revealed that WWTP effluents exerted significant effects on planktonic rather than sedimentary microorganisms in the receiving river. Notably, lower diversity and richness of planktonic communities were observed in the effluent-influenced section (WRW) than other river sections (RW) along the urban river. Meanwhile, network analysis potentially revealed lower stability of co-occurrence patterns of microbial communities in WRW. The remarkably higher antibiotics, nitrate-nitrogen, and water temperature in WRW samples caused by WWTPs played essential roles in shaping the structure and function of planktonic microbial communities. This study suggested the enrichment of multiple-drug resistance genes and destruction of energy metabolisms were caused by sewage effluents, and highlighted the importance of effective management strategies for protecting the ecological health of the receiving river.

Microbial biomass and extracellular enzyme activities control the rate of soil organic carbon decomposition, thereby affecting soil carbon pool. However, seasonal dynamics of soil microbial properties at different depths of the soil profile remain unclear. In this study, we sampled soils in the early, middle and late growing season at different soil depths (0–100 cm) in two alpine ecosystems (meadow and shrubland) on the Tibetan Plateau. We measured plant belowground biomass, soil properties, microbial biomass and extracellular enzyme activities. We found that soil properties changed significantly with sampling time and soil depth. Specifically, most of soil properties consistently decreased with increasing soil depth, but inconsistently varied with sampling time. Moreover, root biomass and microbial biomass decreased with increasing soil depth and increased with sampling time during the growing season. However, microbial extracellular enzyme activities and their vector properties all changed with depth, but did not vary significantly with time. Taken together, these results show that soil properties, microbial biomass and extracellular enzyme activities mostly decline with increasing depth of the soil profile, and soil properties and microbial biomass are generally more variable during the growing season than extracellular enzyme activities across the soil profile in these alpine ecosystems. Further studies are needed to investigate the changes in soil microbial community composition and function at different soil depths over the growing season, which can enhance our mechanistic understanding of whole-profile soil carbon dynamics of alpine ecosystems under climate change.

Increasing temperature plays important roles in affecting plant and soil microbial communities as well as ecological processes and functions in terrestrial ecosystems. However, mechanisms of warming influencing soil carbon dynamics associated with plant-microbe interactions remain unclear. In this study, open-top chambers (OTCs) experiments were carried out to detect the responses of plants, soil microbes, and SOC contents, physical fractions (by particle-size fractionation) and chemical composition (by solid-state 13C NMR spectroscopy) to warming in two alpine swamp meadows (Kobresia humilis vs K. tibetica) on the Tibetan Plateau. Our results showed that four years of warming had significant influences on plant belowground biomass, microbial community and SOC contents in the K. humilis swamp meadow, but had much weaker or minor effects in the K. tibetica swamp meadow with water-logged status and lower level of warming. In the K. humilis swamp meadow, warming increased microbial biomass, C-hydrolysis gene abundance and N-acetylglucosaminidase enzyme activity. These positive effects of warming on microbial biomass and functions further increased soil dissolved inorganic nitrogen and alleviated the nitrogen limitation for plant growth, potentially leading to higher plant biomass. Therefore, increases in SOC and particulate organic carbon (POC) under warming were likely attributed to the higher C input with promoted plant biomass overweighting the simultaneous higher C degradation and release in the K. humilis swamp meadow. Conversely, warming marginally reduced soil alkyl C, which was likely associated with enhanced decomposition by fungi and gram-positive bacteria. Overall, the increases in unprotected POC and decreases in recalcitrant alkyl C demonstrate the sensitivity of SOC physical fractions as well as chemical composition to climate warming in the K. humilis alpine swamp meadow, and suggest that the overall stability of SOC might be lower despite the gain in the content of SOC after climate warming in this alpine swamp meadow.

Mycorrhizal fungi constitute a considerable sink for plant photo-assimilates, yet poor knowledge about mycorrhizal mycelial respiration (RMyc) hinders the quantification of its role in belowground carbon cycling. We synthesized available field data of RMyc, and found that RMyc contributed 15% of soil respiration and 38% of autotrophic respiration (including RMyc and root respiration, RRoot). We observed no significant differences of RMyc between ectomycorrhiza and arbuscular mycorrhiza dominated ecosystems. Both RMyc/RRoot and RMyc/(RRoot + RMyc) significantly decreased with increasing RRoot, and correlated with absolute latitude, mean annual temperature and precipitation. Collectively, RMyc is a substantial component of soil respiration and exhibits divergent geographical pattern with that of RRoot, and should be considered separately in terrestrial carbon cycling models.

From 2000 to 2010 China experienced rapid economic development and urbanization. Many cities in economically developed areas have developed from a single-center status to polycentricity. In this study, we used exploratory spatial data analysis (ESDA) to identify the population centers, which identified 232 cities in China as having urban centers. COMP was used to represent urban agglomeration, and POLYD (representing how far is the city's sub-centers to the main center), POLYC (representing the number of a city's centers), and POLYP (representing the population distributed between the main center and the sub-centers) were used to indicate urban polycentricity. Night light data were used to determine the CO2 emissions from various cities in China. A mixed model was used to study the impact of urban aggregation and polycentric data on the CO2 emission efficiency in 2000 and 2010. The study found that cities with higher compactness were distributed in coastal areas, and the cities with higher multicentricity were distributed in the Yangtze River Delta and Shandong Province. The more compact the city was, the less conducive it was to improving CO2 emission efficiency. Polycentric development of the city was conducive to improving the CO2 emission efficiency, but the number of urban centers had no significant relationship with the CO2 emission efficiency. Our research showed that the compactness and multicentricity of the city had an impact on the CO2 emission efficiency and provided some planning suggestions for the low carbon development of the city.

Due to the sharp increase in carbon emissions from human activities, global surface air temperature has increased significantly by approximately 1 °C since the Industrial Revolution, and it will continue to increase by up to 4 °C by the end of 21st century.This unprecedented climate change will not only affect the adaptation strategies of terrestrial vegetation, but also profoundly affect the structure and function of terrestrial ecosystems.The feedbacks of terrestrial ecosystem carbon cycling to warming is the key factor controlling the speed of future climate change.Therefore, a large number of ecosystem-scale field warming manipulation experiments have been conducted globally to study the carbon budget of terrestrial ecosystems and to improve the prediction accuracy of earth system models.However, due to differences in techniques and methods of these field warming experiments, results among different studies are difficult to compare and synthesize.This paper reviews the common techniques and methods of field warming manipulation experiments, including active warming and passive warming.It also summarizes advantages and disadvantages, applicable objects and related publications for these techniques and methods.Moreover, it briefly introduces future directions of field warming manipulation experiments-the next-generation field warming techniques, namely whole-soil-profile warming and whole-ecosystem warming, and calls for establishing a coordinated distributed network of field warming manipulation experiments using these techniques.

Anthropogenic activities have caused rapid warming globally, particularly in the alpine regions. The Tibetan Plateau is sensitive to global change and plays an important role in global carbon cycle due to its large area and high carbon density. Although a large number of field manipulative warming experiments have been conducted on the Tibetan Plateau, the general patterns of how warming affects ecosystem carbon (C) and nitrogen (N) cycles remain unclear. Here we conducted a meta-analysis from 65 publications to synthesize the responses of C and N cycles to warming in the alpine grassland ecosystems across the Tibetan Plateau. The results showed that warming significantly increased aboveground (mean 11.0%, 95% CI 1.6-21.4%) and belowground plant biomass (24.3%, 13.7-36.0%), soil dissolved organic C (13.6%, 5.4-22.5%), soil respiration (14.3%, 2.9-27.1%), ecosystem respiration (15.3%, 4.7-27.1%), and soil ammonium N (29.0%, 4.6-59.2%), but did not change soil total organic C (2.4%, -4.4 to 9.7%), soil total N (1.9%, -2.7 to 6.7%), microbial biomass C (5.5%, -4.1 to 16.2%) and microbial biomass N (4.7%, - 7.4 to 18.4%). There were negative relationships between the response ratio of aboveground (P = 0.08, R-2 = 0.05) and particularly below-ground (P < 0.01, R-2 = 0.23) biomass and the duration of warming. Moreover, the effect of warming on ecosystem respiration was positively correlated with the change in soil water content after warming (P = 0.03, R-2 = 0.12). Overall, this study showed that ecosystem C and N cycles were generally stimulated by warming, but soil C and N stocks showed minimal responses to warming in the alpine grassland ecosystems on the Tibetan Plateau. This finding suggests that the faster ecosystem C and N cycles under climate warming may not necessarily lead to the loss of soil C and N stocks in the alpine grassland on the Tibetan Plateau. Future studies should conduct long-term coordinated distributed experiments using consistent methods and pay more attention to subsurface soils and the less-studied central and western regions of the Tibetan Plateau.

Changes of soil organic matter (SOM) stability in alpine ecosystems can alter carbon release from the soil and consequently feed back to climate change. We sought to explore the altitudinal patterns of SOM stability and their relationships with climatic, vegetational and edaphic factors. We combined a number of biological, thermal, chemical, molecular and isotopic indices to characterize SOM stability in two soil layers (0–10 and 10–30 cm) of alpine grassland along two altitudinal gradients of 3200–4200 m (Haibei) and 4400–5200 m (Damxung) on the Tibetan Plateau, respectively. There were significant altitudinal variations in SOM stability between the two transects. Greater biological, chemical and isotopic indices of SOM stability were observed in the middle altitude in Haibei, while opposite results were observed in Damxung. Except for a decreasing trend of SOM stability along altitudinal gradient in subsurface soils (10–30 cm) in Damxung, there was no significant altitudinal trend of thermal and molecular indices of SOM stability. The multiple indices of SOM stability were mutually associated and all showed greater SOM stability in subsurface soils than in surface soils across regions and altitudes. Along the altitudinal gradient, SOM stability showed opposite unimodal patterns in Haibei and Damxung, and had higher values in subsurface soils than in surface soils of the alpine grassland. Climatic, vegetational and edaphic factors collectively control SOM stability rather than just temperature or precipitation.

Background and aims Long-term nitrogen (N) addition can affect soil organic carbon (SOC) pool within different soil fractions with different turnover rates. However, the mechanisms of these effects, particularly in alpine grassland ecosystems, are not clear. Methods We studied the responses of SOC content in different soil fractions to N addition based on a six-year N addition field experiment in an alpine meadow ecosystem on the Tibetan Plateau. We measured soil chemical and microbial properties, and SOC content in bulk soil, particular organic matter (POM) and mineral-associated organic matter (MAOM) fractions in response to N addition. Results N addition increased soil N availability, decreased soil pH and microbial biomass, but had minimal effect on plant biomass, soil enzyme activity, and SOC content in bulk soil. With increasing levels of N addition, SOC in the POM fraction (POC) showed a significant negative trend, while SOC in the MAOM fraction (MAOC) did not change significantly. Conclusions As plant biomass input and soil enzyme activity were not significantly altered with N addition, the decline in POC was likely caused by changes in microbial physiology (carbon use efficiency), while the insignificant change in MAOC may be determined by the balance between input (from microbial necromass) and output (from microbial decomposition). Taken together, our study showed that the less-protected POC fraction is more vulnerable to N addition than the more-protected MAOC fraction in the alpine grassland. This finding may improve the prediction of soil C dynamics in response to N deposition in alpine grassland ecosystems on the Tibetan Plateau.