Aridity threshold for alpine soil nitrogen isotope signature and ecosystem nitrogen cycling.

Determination of tipping points in nitrogen (N) isotope (δ15N) natural abundance, especially soil δ15N, with increasing aridity, is critical for estimating N-cycling dynamics and N limitation in terrestrial ecosystems. However, whether there are linear or nonlinear responses of soil δ15N to increases in aridity and if these responses correspond well with soil N cycling remains largely unknown. In this study, we investigated soil δ15N and soil N-cycling characteristics in both topsoil and subsoil layers along a drought gradient across a 3000-km transect of drylands on the Qinghai-Tibetan Plateau. We found that the effect of increasing aridity on soil δ15N values shifted from negative to positive with thresholds at aridity index (AI) = 0.27 and 0.29 for the topsoil and subsoil, respectively, although soil N pools and N transformation rates linearly decreased with increasing aridity in both soil layers. Furthermore, we identified markedly different correlations between soil δ15N and soil N-cycling traits above and below the AI thresholds (0.27 and 0.29 for topsoil and subsoil, respectively). Specifically, in wetter regions, soil δ15N positively correlated with most soil N-cycling traits, suggesting that high soil δ15N may result from the "openness" of soil N cycling. Conversely, in drier regions, soil δ15N showed insignificant relationships with soil N-cycling traits and correlated well with factors, such as soil-available phosphorus and foliage δ15N, demonstrating that pathways other than typical soil N cycling may dominate soil δ15N under drier conditions. Overall, these results highlight that different ecosystem N-cycling processes may drive soil δ15N along the aridity gradient, broadening our understanding of N cycling as indicated by soil δ15N under changing drought regimes. The aridity threshold of soil δ15N should be considered in terrestrial N-cycling models when incorporating 15N isotope signals to predict N cycling and availability under climatic dryness.

Dryness limits vegetation pace to cope with temperature change in warm regions.

Climate change leads to increasing temperature and more extreme hot and drought events. Ecosystem capability to cope with climate warming depends on vegetation's adjusting pace with temperature change. How environmental stresses impair such a vegetation pace has not been carefully investigated. Here we show that dryness substantially dampens vegetation pace in warm regions to adjust the optimal temperature of gross primary production (GPP) ( T opt GPP ) in response to change in temperature over space and time. T opt GPP spatially converges to an increase of 1.01°C (95% CI: 0.97, 1.05) per 1°C increase in the yearly maximum temperature (Tmax ) across humid or cold sites worldwide (37o S-79o N) but only 0.59°C (95% CI: 0.46, 0.74) per 1°C increase in Tmax across dry and warm sites. T opt GPP temporally changes by 0.81°C (95% CI: 0.75, 0.87) per 1°C interannual variation in Tmax at humid or cold sites and 0.42°C (95% CI: 0.17, 0.66) at dry and warm sites. Regardless of the water limitation, the maximum GPP (GPPmax ) similarly increases by 0.23 g C m-2 day-1 per 1°C increase in T opt GPP in either humid or dry areas. Our results indicate that the future climate warming likely stimulates vegetation productivity more substantially in humid than water-limited regions.

Dryness weakens the positive effects of plant and fungal ß diversities on above- and belowground biomass.

Plant and microbial diversity are key to determine ecosystem functioning. Despite the well-known role of local-scale α diversity in affecting vegetation biomass, the effects of community heterogeneity (ß diversity) of plants and soil microbes on above- and belowground biomass (AGB and BGB) across contrasting environments still remain unclear. Here, we conducted a dryness-gradient transect survey over 3000 km across grasslands on the Tibetan Plateau. We found that plant ß diversity was more dominant than α diversity in maintaining higher levels of AGB, while soil fungal ß diversity was the key driver in enhancing BGB. However, these positive effects of plant and microbial ß diversity on AGB and BGB were strongly weakened by increasing climatic dryness, mainly because higher soil available phosphorus caused by increasing dryness reduced both plant and soil fungal ß diversities. Overall, these new findings highlight the critical role of above- and belowground ß diversity in sustaining grassland biomass, raising our awareness to the ecological risks of large-scale biotic homogenization under future climate change.

Variations and controlling factors of soil denitrification rate.

The denitrification process profoundly affects soil nitrogen (N) availability and generates its byproduct, nitrous oxide, as a potent greenhouse gas. There are large uncertainties in predicting global denitrification because its controlling factors remain elusive. In this study, we compiled 4301 observations of denitrification rates across a variety of terrestrial ecosystems from 214 papers published in the literature. The averaged denitrification rate was 3516.3 ± 91.1 µg N kg-1  soil day-1 . The highest denitrification rate was 4242.3 ± 152.3 µg N kg-1  soil day-1 under humid subtropical climates, and the lowest was 965.8 ± 150.4 µg N kg-1 under dry climates. The denitrification rate increased with temperature, precipitation, soil carbon and N contents, as well as microbial biomass carbon and N, but decreased with soil clay contents. The variables related to soil N contents (e.g., nitrate, ammonium, and total N) explained the variation of denitrification more than climatic and edaphic variables (e.g., mean annual temperature (MAT), soil moisture, soil pH, and clay content) according to structural equation models. Soil microbial biomass carbon, which was influenced by soil nitrate, ammonium, and total N, also strongly influenced denitrification at a global scale. Collectively, soil N contents, microbial biomass, pH, texture, moisture, and MAT accounted for 60% of the variation in global denitrification rates. The findings suggest that soil N contents and microbial biomass are strong predictors of denitrification at the global scale.

Ecosystem restoration and belowground multifunctionality: A network view.

Ecological restoration is essential to reverse land degradation worldwide. Most studies have assessed the restoration of ecosystem functions individually, as opposed to a holistic view. Here we developed a network-based ecosystem multifunctionality (EMF) framework to identify key functions in evaluating EMF restoration. Through synthesizing 293 restoration studies (2900 observations) following cropland abandonment, we found that individual soil functions played different roles in determining the restoration of belowground EMF. Soil carbon, total nitrogen, and phosphatase were key functions to predict the recovery of belowground EMF. On average, abandoned cropland recovered ~19% of EMF during 18 years. The restoration of EMF became larger with longer recovery time and higher humidity index, but lower with increasing soil depth and initial soil carbon. Overall, this study presents a network-based EMF framework, effectively helping to evaluate the success of ecosystem restoration and identify the key functions.

Fine-root functional trait responses to experimental warming: a global meta-analysis.

Whether and how warming alters functional traits of absorptive plant roots remains to be answered across the globe. Tackling this question is crucial to better understanding terrestrial responses to climate change as fine-root traits drive many ecosystem processes. We carried out a detailed synthesis of fine-root trait responses to experimental warming by performing a meta-analysis of 964 paired observations from 177 publications. Warming increased fine-root biomass, production, respiration and nitrogen concentration as well as decreased root carbon : nitrogen ratio and nonstructural carbohydrates. Warming effects on fine-root biomass decreased with greater warming magnitude, especially in short-term experiments. Furthermore, the positive effect of warming on fine-root biomass was strongest in deeper soil horizons and in colder and drier regions. Total fine-root length, morphology, mortality, life span and turnover were unresponsive to warming. Our results highlight the significant changes in fine-root traits in response to warming as well as the importance of warming magnitude and duration in understanding fine-root responses. These changes have strong implications for global soil carbon stocks in a warmer world associated with increased root-derived carbon inputs into deeper soil horizons and increases in fine-root respiration.

Vital roles of soil microbes in driving terrestrial nitrogen immobilization.

Nitrogen immobilization usually leads to nitrogen retention in soil and, thus, influences soil nitrogen supply for plant growth. Understanding soil nitrogen immobilization is important for predicting soil nitrogen cycling under anthropogenic activities and climate changes. However, the global patterns and drivers of soil nitrogen immobilization remain unclear. We synthesized 1350 observations of gross soil nitrogen immobilization rate (NIR) from 97 articles to identify patterns and drivers of NIR. The global mean NIR was 8.77 ± 1.01 mg N kg-1  soil day-1 . It was 5.55 ± 0.41 mg N kg-1  soil day-1 in croplands, 15.74 ± 3.02 mg N kg-1  soil day-1 in wetlands, and 15.26 ± 2.98 mg N kg-1  soil day-1 in forests. The NIR increased with mean annual temperature, precipitation, soil moisture, soil organic carbon, total nitrogen, dissolved organic nitrogen, ammonium, nitrate, phosphorus, and microbial biomass carbon. But it decreased with soil pH. The results of structural equation models showed that soil microbial biomass carbon was a pivotal driver of NIR, because temperature, total soil nitrogen, and soil pH mostly indirectly influenced NIR via changing soil microbial biomass. Moreover, microbial biomass carbon accounted for most of the variations in NIR among all direct relationships. Furthermore, the efficiency of transforming the immobilized nitrogen to microbial biomass nitrogen was lower in croplands than in natural ecosystems (i.e., forests, grasslands, and wetlands). These findings suggested that soil nitrogen retention may decrease under the land use change from forests or wetlands to croplands, but NIR was expected to increase due to increased microbial biomass under global warming. The identified patterns and drivers of soil nitrogen immobilization in this study are crucial to project the changes in soil nitrogen retention.

Global patterns and controlling factors of soil nitrification rate.

Soil nitrification, an important pathway of nitrogen transformation in ecosystems, produces soil nitrate that influences net primary productivity, while the by-product of nitrification, nitrous oxide, is a significant greenhouse gas. Although there have been many studies addressing the microbiology, physiology, and impacting environment factors of soil nitrification at local scales, there are very few studies on soil nitrification rate over large scales. We conducted a global synthesis on the patterns and controlling factors of soil nitrification rate normalized at 25°C by compiling 3,140 observations from 186 published articles across terrestrial ecosystems. Soil nitrification rate tended to decrease with increasing latitude, especially in the Northern Hemisphere, and varied largely with ecosystem types. The soil nitrification rate significantly increased with mean annual temperature (MAT), soil nitrogen content, microbial biomass carbon and nitrogen, soil ammonium, and soil pH, but decreased with soil carbon:nitrogen and carbon:nitrogen of microbial biomass. The total soil nitrogen content contributed the most to the variations of global soil nitrification rate (total coefficient = 0.29) in structural equation models. The microbial biomass nitrogen (MBN; total coefficient = 0.19) was nearly of equivalent importance relative to MAT (total coefficient = 0.25) and soil pH (total coefficient = 0.24) in determining soil nitrification rate, while soil nitrogen and pH influenced soil nitrification via changing soil MBN. Moreover, the emission of soil nitrous oxide was positively related to soil nitrification rate at a global scale. This synthesis will advance our current understanding on the mechanisms underlying large-scale variations of soil nitrification and benefit the biogeochemical models in simulating global nitrogen cycling.

Global changes alter plant multi-element stoichiometric coupling.

Plant stoichiometric coupling among all elements is fundamental to maintaining growth-related ecosystem functions. However, our understanding of nutrient balance in response to global changes remains greatly limited to plant carbon : nitrogen : phosphorus (C : N : P) coupling. Here we evaluated nine element stoichiometric variations with one meta-analysis of 112 global change experiments conducted across global terrestrial ecosystems and one synthesis over 1900 species observations along natural environment gradients across China. We found that experimentally increased soil N and P respectively enhanced plant N : potassium (K), N : calcium (Ca) and N : magnesium (Mg), and P : K, P : Ca and P : Mg, and natural increases in soil N and P resulted in qualitatively similar responses. The ratios of N and P to base cations decreased both under experimental warming and with naturally increasing temperature. With decreasing precipitation, these ratios increased in experiments but decreased under natural environments. Based on these results, we propose a new stoichiometric framework in which all plant element contents and their coupling are not only affected by soil nutrient availability, but also by plant nutrient demand to maintain diverse functions under climate change. This study offers new insights into understanding plant stoichiometric variations across a full set of mineral elements under global changes.

Microbes drive global soil nitrogen mineralization and availability.

Soil net nitrogen mineralization rate (Nmin ), which is critical for soil nitrogen availability and plant growth, is thought to be primarily controlled by climate and soil physical and/or chemical properties. However, the role of microbes on regulating soil Nmin has not been evaluated on the global scale. By compiling 1565 observational data points of potential net Nmin from 198 published studies across terrestrial ecosystems, we found that Nmin significantly increased with soil microbial biomass, total nitrogen, and mean annual precipitation, but decreased with soil pH. The variation of Nmin was ascribed predominantly to soil microbial biomass on global and biome scales. Mean annual precipitation, soil pH, and total soil nitrogen significantly influenced Nmin through soil microbes. The structural equation models (SEM) showed that soil substrates were the main factors controlling Nmin when microbial biomass was excluded. Microbe became the primary driver when it was included in SEM analysis. SEM with soil microbial biomass improved the Nmin prediction by 19% in comparison with that devoid of soil microbial biomass. The changes in Nmin contributed the most to global soil NH4+ -N variations in contrast to climate and soil properties. This study reveals the complex interactions of climate, soil properties, and microbes on Nmin and highlights the importance of soil microbial biomass in determining Nmin and nitrogen availability across the globe. The findings necessitate accurate representation of microbes in Earth system models to better predict nitrogen cycle under global change.

Global patterns and substrate-based mechanisms of the terrestrial nitrogen cycle.

Nitrogen (N) deposition is impacting the services that ecosystems provide to humanity. However, the mechanisms determining impacts on the N cycle are not fully understood. To explore the mechanistic underpinnings of N impacts on N cycle processes, we reviewed and synthesised recent progress in ecosystem N research through empirical studies, conceptual analysis and model simulations. Experimental and observational studies have revealed that the stimulation of plant N uptake and soil retention generally diminishes as N loading increases, while dissolved and gaseous losses of N occur at low N availability but increase exponentially and become the dominant fate of N at high loading rates. The original N saturation hypothesis emphasises sequential N saturation from plant uptake to soil retention before N losses occur. However, biogeochemical models that simulate simultaneous competition for soil N substrates by multiple processes match the observed patterns of N losses better than models based on sequential competition. To enable better prediction of terrestrial N cycle responses to N loading, we recommend that future research identifies the response functions of different N processes to substrate availability using manipulative experiments, and incorporates the measured N saturation response functions into conceptual, theoretical and quantitative analyses.

Effects of functional diversity loss on ecosystem functions are influenced by compensation.

Understanding the impacts of biodiversity loss on ecosystem functioning and services has been a central issue in ecology. Experiments in synthetic communities suggest that biodiversity loss may erode a set of ecosystem functions, but studies in natural communities indicate that the effects of biodiversity loss are usually weak and that multiple functions can be sustained by relatively few species. Yet, the mechanisms by which natural ecosystems are able to maintain multiple functions in the face of diversity loss remain poorly understood. With a long-term and large-scale removal experiment in the Inner Mongolian grassland, here we showed that losses of plant functional groups (PFGs) can reduce multiple ecosystem functions, including biomass production, soil NO3 -N use, net ecosystem carbon exchange, gross ecosystem productivity, and ecosystem respiration, but the magnitudes of these effects depended largely on which PFGs were removed. Removing the two dominant PFGs (perennial rhizomatous grasses and perennial bunchgrasses) simultaneously resulted in dramatic declines in all examined functions, but such declines were circumvented when either dominant PFG was present. We identify the major mechanism for this as a compensation effect by which each dominant PFG can mitigate the losses of others. This study provides evidence that compensation ensuing from PFG losses can mitigate their negative consequence, and thus natural communities may be more resilient to biodiversity loss than currently thought if the remaining PFGs have strong compensation capabilities. On the other hand, ecosystems without well-developed compensatory functional diversity may be much more vulnerable to biodiversity loss.

Extreme precipitation causes divergent responses of root respiration to nitrogen enrichment in an alpine meadow.

Grassland roots are fundamental to obtain the most limiting soil water and nitrogen (N) resources. However, this natural pattern could be significantly changed by recent co-occurrence of N deposition and extreme precipitations, likely with complex interactions on grassland root production and respiration. Despite this nonlinearity, we still know little about how extreme precipitation change nonlinearly regulates the responses of root respiration to N enrichment. Here, we conducted a 6-year experiment of N addition in an alpine meadow, coincidently experiencing extreme precipitations among experimental years. Our results demonstrated that root respiration showed divergent responses to N addition along with extreme precipitation changes among years. Under normal rainfall year, root respiration was significantly stimulated by N addition, whereas it was depressed under high or low water. Moreover, we revealed that both root biomass and traits (i.e. specific root length) were critical mechanisms in affecting root respiration response, but their relative importance changed with water condition. For example, specific root length and specific root respiration were more dominant than root biomass in determining root respiration response under low water, or vice versa. Overall, this study comprehensively reveals the nonlinearity of root respiration responses to the interactions of N enrichment and extreme water change. These new findings help to reconcile previously conflicting results that obtain in a specific episode of water gradient, with important implications for understanding grassland belowground carbon dynamics in facing combined N deposition and extreme precipitation events.

Nitrogen enrichment delays the drought threshold responses of leaf photosynthesis in alpine grassland plants.

Extreme drought is found to cause a threshold response in photosynthesis in ecosystem level. However, the mechanisms behind this phenomenon are not well understood, highlighting the importance of revealing the drought thresholds for multiple leaf-level photosynthetic processes. Thus, we conducted a long-term experiment involving precipitation reduction and nitrogen (N) addition. Moreover, an extreme drought event occurred within the experimental period. We found the presence of drought thresholds for multiple leaf-level photosynthetic processes, with the leaf light-saturated carbon assimilation rate (Asat) displaying the highest threshold (10.76 v/v%) and the maximum rate of carboxylation by Rubisco (Vcmax) showing the lowest threshold (5.38 v/v%). Beyond the drought thresholds, the sensitivities of leaf-level photosynthetic processes to soil water content could be greater. Moreover, N addition lowered the drought thresholds of Asat and stomatal conductance (gs), but had no effect on that of Vcmax. Among species, plants with higher leaf K concentration traits had a lower drought threshold of Asat. Overall, this study highlights that leaf photosynthesis may be suppressed abruptly as soil water content surpasses the drought threshold. However, N enrichment helps to improve the resistance via delaying drought threshold response. These new findings have important implications for understanding the nonlinearity of ecosystem productivity response and early warning management in the scenario of combined extreme drought events and continuous N deposition.

Opposing effects of warming on the stability of above- and belowground productivity in facing an extreme drought event.

Climate warming, often accompanied by extreme drought events, could have profound effects on both plant community structure and ecosystem functioning. However, how warming interacts with extreme drought to affect community- and ecosystem-level stability remains a largely open question. Using data from a manipulative experiment with three warming treatments in an alpine meadow that experienced one extreme drought event, we investigated how warming modulates resistance and recovery of community structural and ecosystem functional stability in facing with extreme drought. We found warming decreased resistance and recovery of aboveground net primary productivity (ANPP) and structural resistance but increased resistance and recovery of belowground net primary productivity (BNPP), overall net primary productivity (NPP), and structural recovery. The findings highlight the importance of jointly considering above- and belowground processes when evaluating ecosystem stability under global warming and extreme climate events. The stability of dominant species, rather than species richness and species asynchrony, was identified as a key predictor of ecosystem functional resistance and recovery, except for BNPP recovery. In addition, structural resistance of common species contributed strongly to the resistance changes in BNPP and NPP. Importantly, community structural resistance and recovery dominated the resistance and recovery of BNPP and NPP, but not for ANPP, suggesting the different mechanisms underlie the maintenance of stability of above- versus belowground productivity. This study is among the first to explain that warming modulates ecosystem stability in the face of extreme drought and lay stress on the need to investigate ecological stability at the community level for a more mechanistic understanding of ecosystem stability in response to climate extremes.

Contextualized response of carbon-use efficiency to warming at the plant and ecosystem levels.

Carbon-use efficiency (CUE) has been widely used as a constant value in many earth system models to simulate how assimilated C is partitioned in ecosystems, to estimate ecosystem C budgets, and investigate C feedbacks to climate warming. Although correlative relationships from previous studies indicated that CUE could vary with temperature, and relying on a fixed CUE value could cause large uncertainty in model projections, however, due to the lack of manipulative experiment, it remains unclear how CUE at the plant (CUEp) and ecosystem (CUEe) levels respond to warming. Based on a 7-year manipulative warming experiment in an alpine meadow ecosystem on the Qinghai-Tibet Plateau, we quantitatively distinguished various C flux components of CUE, including gross ecosystem productivity, net primary productivity, net ecosystem productivity, ecosystem respiration, plant autotrophic respiration, and microbial heterotrophic respiration and explored how CUE at different levels responded to climate warming. We found large variations in both CUEp (0.60 to 0.77) and CUEe (from 0.38 to 0.59). The warming effect on CUEp was positively correlated with ambient soil water content (SWC) and the warming effect on CUEe was negatively correlated with ambient soil temperature (ST), but was positively correlated with warming-induced changes in ST. We also found that the direction and magnitude of the warming effects on different CUE components scaled differently with changes in the background environment, which explained the variation in CUE's warming response under environmental changes. Our new insights have important implications for reducing modelling uncertainty of ecosystem C budgets and improving our ability to predict ecosystem C-climate feedbacks under climate warming.

Nitrogen enrichment differentially regulates the response of ecosystem stability to extreme dry versus wet events.

Extreme climate events, such as severe droughts and heavy rainfall, have profound impacts on the sustainable provision of ecosystem functions and services. However, how N enrichment interacts with discrete extreme climate events to affect ecosystem functions is largely unknown. Here, we investigated the responses of the temporal stability (i.e., resistance, recovery, and resilience) of aboveground net primary productivity (ANPP) in an alpine meadow to extreme dry and wet events under six N addition treatments (0, 2, 4, 8, 16, 32 g N m-2 year-1). We found that N addition had contrasting effects on the responses of ANPP to the extreme dry versus wet events, which resulted in no overall significant effects on ANPP stability across 2015-2019. Specifically, high N addition rates reduced the stability, resistance, and resilience of ANPP in response to extreme drought, whereas medium N addition rates increased ANPP stability and recovery in response to the extreme wet event. The main mechanisms underlying the response of ANPP to extreme drought and wet events were discrepant. Species richness, asynchrony, and dominant species resistance contributed most to the reduction of ANPP resistance to extreme drought, while species asynchrony and dominant and common species resilience contributed most to the decrease of ANPP resilience from extreme drought with N enrichment. The ANPP recovery from the extreme wet event was mainly explained by dominant and common species recovery. Our results provide strong evidence that N deposition mediates ecosystem stability in response to extreme dry and wet events in different ways and modulates the provisioning of grassland ecosystem functions under increasing extreme climate events.

Water causes divergent responses of specific carbon sink to long-term grazing in a desert grassland.

Heavy grazing generally reduces grassland biomass, further decreasing its carbon sink. Grassland carbon sink is determined by both plant biomass and carbon sink per unit biomass (specific carbon sink). This specific carbon sink could reflect grassland adaptative response, because plants generally tend to adaptively enhance the functioning of their remaining biomass after grazing (i.e. higher leaf nitrogen content). Though we know well about the regulation of grassland biomass on carbon sink, little attention is paid to the role of specific carbon sink. Thus, we conducted a 14-year grazing experiment in a desert grassland. Ecosystem carbon fluxes, including net ecosystem CO2 exchange (NEE), gross ecosystem productivity (GEP) and ecosystem respiration (ER), were measured frequently during five consecutive growing seasons with contrasting precipitation events. We found that heavy grazing reduced NEE more in drier (-94.0 %) than wetter (-33.9 %) years. However, grazing did not reduce community biomass much more in drier (-70.4 %) than wetter years (-66.0 %). These meant a positive response of specific NEE (NEE per unit biomass) to grazing in wetter years. This positive response of specific NEE was mainly caused by a higher biomass ratio of other species versus perennial grasses with greater leaf nitrogen content and specific leaf area in wetter years. In addition, we also detected a shift of grazing effects on specific NEE from positive in wetter years to negative in drier years. Overall, this study is among the first to reveal the adaptive response of grassland specific carbon sink to experimental grazing in plant trait view. The stimulation response of specific carbon sink can partially compensate for the loss of grassland carbon storage under grazing. These new findings highlight the role of grassland adaptive response in decelerating climate warming.

Shifting biomass allocation and light limitation co-regulate the temporal stability of an alpine meadow under eutrophication.

Eutrophication generally promotes but destabilizes grassland productivity. Under eutrophication, plants tend to decrease biomass allocation to roots but increase aboveground allocation and light limitation, likely affecting community stability. However, it remains unclear to understand how shifting plant biomass allocation and light limitation regulate grassland stability in response to eutrophication. Here, using a 5-yr multiple nutrient addition experiment in an alpine meadow, we explored the role of changes in plant biomass allocation and light limitation on its community stability under eutrophication as well as traditionally established mechanisms (i.e., plant Shannon diversity, species asynchrony and grass subcommunity stability). Our results showed that nitrogen (N) addition, rather than phosphorus (P) or potassium (K) addition, significantly reduced the temporal stability of the alpine meadow. In accordance with previous studies, we found that N addition decreased plant Shannon diversity, species asynchrony and grass subcommunity stability, further destabilizing meadow community productivity. In addition, we also found the decrease in biomass allocation to belowground by N addition, further weakening its community stability. Moreover, this shifts in plant biomass allocation from below- to aboveground, intensifying plant light limitation. Further, the light limitation reduced plant species asynchrony, which finally weakened its community stability. Overall, in addition to traditionally established mechanisms, this study highlights the role of plant biomass allocation shifting from belowground to aboveground in determining grassland community stability. These "unseen" mechanisms might improve our understanding of grassland stability in the context of ongoing eutrophication.

Depth-dependent drivers of soil aggregate carbon across Tibetan alpine grasslands.

Elucidating the effects underlying soil organic carbon (SOC) variation is imperative for ascertaining the potential drivers of mitigating climate change. However, the drivers of variations in various SOC fractions (e.g., macroaggregate C, microaggregate C, and silt and clay C) at different soil depths remain poorly understood. Here, we investigated the effects and relative contributions of climatic, plant, edaphic, and microbial factors on soil aggregate C between the topsoil (0-10 cm) and subsoil (20-30 cm) across alpine grasslands on the Tibetan Plateau. Results showed that the C content of macroaggregates, microaggregates, and silt and clay fractions in the topsoil was 128.6 %, 49.6 %, and 242.4 % higher than that in the subsoil, respectively. Overall, plant properties were the most determinants controlling soil macroaggregate, microaggregate, and silt + clay associated C for both two soil depths, accounting for 32.2 %, 37.4 %, and 38.8 % of the variation, respectively, followed by edaphic, microbial, and climatic factors. The aggregate C of both soil depths was significantly related with the climatic, plant, edaphic, and microbial factors, but the relative importance of these determinants was soil-depth dependent. Specifically, the effects of plant root biomass and microbial (e.g., microbial biomass carbon and fungal diversity index) factors on each aggregate C weakened with soil depth, but the importance of edaphic factors (e.g., clay content, pH, and bulk density) strengthened with soil depth, except for the weakened effect of bulk density on the microaggregate C. And the effects of climatic factor (e.g., mean annual precipitation) on macroaggregate and microaggregate C increased with soil depth. Our results highlight differential drivers and their impacts on soil aggregate C between the topsoil and subsoil, which benefits biogeochemical models for more accurately forecasting soil C dynamics and its feedbacks to environmental changes.