Mowing exacerbates the loss of ecosystem stability under nitrogen enrichment in a temperate grassland

Labile Carbon Input Mitigates the Negative Legacy Effects of Nitrogen Addition on Arbuscular Mycorrhizal Symbiosis in a Temperate Grassland

Nitrogen (N) deposition and carbon (C) addition significantly influence the dynamics of plant-microbe interactions, particularly altering the symbiotic relationship between plants and arbuscular mycorrhizal fungi (AMF). However, the effects and underlying mechanisms of labile C input on the relationship between AMF and various plant species in a nitrogen-enriched environment remain a knowledge gap. A seven-year field experiment was conducted to examine how six levels of N and three levels of labile C addition impact AMF colonization in four key plant species: Leymus chinensis (Trin. ex Bunge) Tzvelev, Stipa baicalensis Roshev., Thermopsis lanceolata R. Br. and Potentilla bifurca Linn. Our results showed that N and C additions exert significantly different effects on the relationship between AMF and various plant species. Labile C addition mitigated historical N negative effects, particularly for S. baicalensis, enhancing AMF infection and promoting nutrient exchange under high-N and low-C conditions. The relationship between AMF and both L. chinensis and T. lanceolata changed to weak mutualism under low-N and high-C conditions, with significant decreases in vesicular and arbuscular abundance. Plant root stoichiometry plays a critical role in modulating AMF symbiosis, particularly under high-N and -C conditions, as reflected in the increased AMF infection observed in T. lanceolata and P. bifurca. Our findings emphasize the species-specific and nutrient-dependent AMF symbiosis, revealing that targeted C input can mitigate the legacy effects of N enrichment. Effective nutrient management is of crucial importance for ecological restoration efforts in temperate grasslands affected by long-term N enrichment.

Moderate grazing reduces while mowing increases greenhouse gas emissions from a steppe grassland: Key modulating function played by plant standing biomass

Grassland represents one of the most expansive terrestrial ecosystems, exerting a profound influence on atmospheric greenhouse gas (GHG) levels within the broader context of global change. Both climate and land use changes play important roles in modulating grassland GHG emissions by directly or indirectly altering soil physical and chemical properties, especially soil temperature and inorganic nitrogen content. The optimal grassland management practices need to simultaneously meet the requirements of reducing GHG emissions, maintaining biological biodiversity, and ensuring productivity. However, the information on the management effects on GHG emissions from natural grasslands is still insufficient. Here we conducted a six-year grazing and mowing experiment in a semi-arid steppe grassland in central Inner Mongolia, and employed the static chamber method to investigate the effects of three major management measures, fencing, grazing and mowing, on ecosystem respiration (CO2 emission), methane uptake (CH4), and nitrous oxide emission (N2O) patterns in the experimental grassland. The results demonstrated that: (i) moderate grazing reduced plant aboveground standing biomass and CO2 emissions, but promoted belowground nutrient cycling and CH4 uptake; (ii) mowing enhanced plant biomass production, increased soil carbon and nitrogen content, and also increased CO2 emission; (iii) reducing grazing frequency reduced plant biomass loss and N2O emissions. We conclude that grazing at a moderate intensity and frequency is the best for mitigating GHG emissions while maintaining grassland production, and that mowing enhancement of plant production and GHG emissions should be considered in optimizing grassland management.

The spatiotemporal stability of plant diversity is disconnected from biomass stability in response to human activities in a South American temperate grassland

Human activities alter biomass, nutrient availability, and species dominance in grasslands, impacting their richness, composition, and biomass production. Stability (invariability in time or space) can inform the predictability of plant communities in response to human activities. However, this measure has been simplistically analyzed for temporal (interannual) changes in live biomass, disregarding their spatial stability and the temporal stability of other plant community attributes. Moreover, the simultaneous analysis of temporal and spatial stabilities of plant communities has been scarcely assessed. Here, we test how biomass removal and nutrient addition simultaneously modify the temporal and spatial stabilities of plant richness (α diversity), composition dissimilarity (β diversity), aboveground live biomass, and the role of plant species dominance in the stability responses. We conducted a factorial experiment of biomass removal (grazing, mowing, or intact -no removal-) and nutrient addition (unfertilized or fertilized with nitrogen, phosphorus, and potassium) in a temperate grassland of Argentina, South America. We replicated the experiment in 6 blocks over 10 years to estimate the temporal and spatial stabilities of the plant community. The spatiotemporal stability of plant richness and composition dissimilarity decreased in the intact grassland, while the temporal stability of live biomass increased, compared to the grazed and mowed grasslands. Nutrient addition reduced the spatiotemporal stability of live biomass and the spatial stability of plant richness. The stabilities of species richness as well as that of composition dissimilarity were negatively associated with plant dominance, while the live biomass stability was not. Our results suggest that simplifying the effect of biomass removal and nutrient addition on grassland stability is not feasible, as plant diversity stability responses are not surrogates for biomass stability. The contrasting spatiotemporal stability responses of plant diversity and biomass represent a step forward in predicting human activities' impact over time and across space in temperate grasslands.

Phase-dependent grassland temporal stability is mediated by species and functional group asynchrony: A long-term mowing experiment

The temporal stability of grasslands plays a key role in the stable provisioning of multiple ecosystem goods and services for humankind. Despite recent progress, our knowledge on how long-term mowing influences ecosystem stability remains unclear. Using a dataset from an 18-year-long mowing experiment with different treatment intensities (no-mowing, mowing once per year, and mowing twice per year) in grasslands of Inner Mongolia, China, we aimed to determine whether and how long-term mowing influenced grassland temporal stability in a temperate steppe. We found mowing decreased ecosystem stability in the early and intermediate periods (1-12 years of treatment), but increased stability in the later period (13-18 years of treatment), indicating responses of ecosystem stability to long-term mowing were phase dependent. Bivariate correlation and structural equation modeling analyses revealed that the degree of asynchrony both at the species and functional group levels, as well as dominant species stability, played key roles in stabilizing the whole community. In addition, portfolio effects rather than diversity made significant contributions to ecosystem stability. Our results suggest the phase-dependent temporal stability of grassland under long-term mowing is mainly mediated by species and functional group asynchrony. This finding provides a new insight for understanding how dryland grassland responds to long-term anthropogenic perturbations.

Tradeoff between productivity and stability across above- and below-ground communities

An 11-year nitrogen addition experiment reveals that for both plants and soil microorganisms, the ruderal strategists had higher productivity but lower stability, while the tolerant strategists had higher stability and lower productivity, leading to the tradeoff between productivity and stability within and across above- and below-ground communities.

Mowing increased community stability in semiarid grasslands more than either fencing or grazing

A substantial body of empirical evidence suggests that anthropogenic disturbance can affect the structure and function of grassland ecosystems. Despite this, few studies have elucidated the mechanisms through which grazing and mowing, the two most widespread land management practices, affect the stability of natural grassland communities. In this study, we draw upon 9 years of field data from natural grasslands in northern China to investigate the effects of gazing and mowing on community stability, specifically focusing on community aboveground net primary productivity (ANPP) and dominance, which are two major biodiversity mechanisms known to characterize community fluctuations. We found that both grazing and mowing reduced ANPP in comparison to areas enclosed by fencing. Grazing reduced community stability by increasing the likelihood of single-species dominance and decreasing the relative proportion of nondominant species. In contrast, mowing reduced the productivity of the dominant species but increased the productivity of nondominant species. As a consequence, mowing improved the overall community stability by increasing the stability of nondominant species. Our study provides novel insight into understanding of the relationship between community species fluctuation-stability, with implications for ecological research and ecosystem management in natural grasslands.

Nitrogen supply neutralizes the nanoplastic-plant interaction in a coastal wetland

The presence of nanoplastics posed a potential threat to coastal saline-alkaline wetlands where nitrogen (N) fertilizer is being implemented as an important ecological restoration measure. Notwithstanding, the effects of N inputs on plant community in polypropylene-nanoplastics (PP-NPs) coexistence environments are largely unknown. To address this, we investigated the effects of PP-NPs addition alone or combined N supply on community aboveground biomass, morphological traits, diversity, composition, niche differentiation, interspecific interactions, and assembly. Our results showed that the PP-NPs addition alone reduced community aboveground biomass and morphological traits. However, the addition of high concentration (0.5%) PP-NPs alone favored community α-diversity and reduced community stability, which could be weakened through combined N supply. Overall, the effect of PP-NPs addition alone on plant community composition was greater than that of combined N supply. We also demonstrated PP-NPs addition alone and combined N supply reduced the niche breadth of the plant community and affected the niche overlap of dominant species. In the assembly of plant communities, stochastic processes played a dominant role. We conclude that N fertilization can amend the terrestrial nanoplastics pollution, thus mitigating the effects of PP-NPs on the plant community.

Contrasting responses of α- and β-multifunctionality to aboveground plant community in the Qinghai-Tibet Plateau

The aboveground plant communities are crucial in driving ecosystem functioning, particularly being the primary producers in terrestrial ecosystems. Numerous studies have investigated the impacts of aboveground plant communities on multiple ecosystem functions at α-scale. However, such critical effects have been unexplored at β-scale and the comparative assessment of the effects and underlying mechanisms of aboveground plant communities on α- and β-multifunctionality has been lacking. In this study, we examined the effects of aboveground plant communities on soil multifunctionality both at α- and β-scale in the alpine meadow of the Tibetan Plateau. Additionally, we quantified the direct effects of aboveground plant communities, as well as the indirect effects mediated by changes in biotic and abiotic factors, on soil multifunctionality at both scales. Our findings revealed that: 1) Aboveground plant communities had significantly positive effects on α-multifunctionality whereas, β-multifunctionality was not affected significantly. 2) Aboveground plant communities directly influence α- and β-multifunctionality in contrasting ways, with positive and negative effects, respectively. Apart from the direct effects of plant community, we found that soil water content and bacterial β-diversity serving as the primary predictors for the responses of α- and β-multifunctionality to the presence of aboveground plant communities, respectively. And β-soil biodiversity appeared to be a stronger predictor of multifunctionality relative to α-soil biodiversity. Our findings provide novel insights into the drivers of ecosystem multifunctionality at different scales, highlight the importance of maintaining biodiversity at multiple scales and offer valuable knowledge for the maintenance of ecosystem functioning and the restoration of alpine meadow ecosystems.

High-resolution maps of vegetation nitrogen density on the Tibetan Plateau: An intensive field-investigation

Nitrogen (N) is a vital macronutrient in plant growth and development that plays a crucial role in the regulation of numerous physiological processes. The Tibetan Plateau is among the most species-diverse vegetation zones in the world, and is sensitive to climate change; however, research on vegetation N in the region remains limited. This study used field grid-sampling of 2040 plant communities to investigate the spatial variation and driving factors of vegetation N on the Tibetan Plateau. The results yielded an average N content, density and storage in vegetation of 8.48 mg g-1, 27.02 g m-2, and 29.84Tg, respectively. The ratio-based optimal partitioning hypothesis appears to be more suitable than the isometric allocation hypothesis to explain variation in vegetation N on the Tibetan Plateau. Variation in vegetation N density, was influenced by several environmental factors of which the most significant was radiation. Based on these results, a Random Forest model was used to predict a N density distribution map at 1 km resolution, achieving an accuracy (R2) of 0.72 (aboveground N density), 0.61 (belowground N density), and 0.69 (total vegetation N density). Trends for high densities were predicted in the southeast and low densities in the northwest of the region. Our findings and maps could be used to provide key N cycle parameters, contributing to future remote sensing, radar analyses, modeling and ecological management.

Grassland stability decreases with increasing number of global change factors: A meta-analysis

Experiments manipulating a single global change factor (GCF) have provided increasing evidence that global environmental changes, such as eutrophication, precipitation change, and warming, generally affect the temporal stability of grassland productivity. Whether the combined impact of global changes on grassland stability increases as the number of global changes increases remains unknown. Using a meta-analysis of 673 observations from 143 sites worldwide, including 7 different GCFs, we examined the responses of grassland temporal stability of productivity to increasing numbers of GCFs. We quantified the links between community stability, biotic factors (i.e., species richness, species stability, and species asynchrony), and abiotic factors (i.e., aridity index, experimental duration, and experimental intensity). Although inconsistent responses of community stability were found with different GCF types and combinations, when integrating existing GCFs studies and ignoring the identity of GCFs, we found a general decrease in community stability as the number of GCFs increases, but the main drivers of community stability varied with the numbers of GCFs. Specifically, one GCF mainly reduced species stability through species richness and thus weakened community stability. Two GCFs weakened community stability via independently weakening species stability and species asynchrony. Three GCFs reduce community stability mainly via independently weakening species asynchrony. Moreover, for single factor, the impact of GCFs on community stability was weaker under dryer conditions, but stronger when two or three factors were manipulated. In addition, the negative effect of GCFs on community stability was weaker with increasing experimental duration. Our study reveals that reduced community stability with increasing numbers of GCFs is caused by a shift from reduced species stability to reduced species asynchrony, suggesting that persistent global changes will destabilize grassland productivity by reducing asynchronous dynamics among species in response to natural environmental fluctuations.

The two sides of resistance-resilience relationship in both aboveground and belowground communities in the Eurasian steppe

The ongoing nitrogen (N) deposition has led to profound changes in aboveground and belowground ecosystems. However, the stability of plant and soil microbial community toward N addition in terms of resistance and resilience is less understood. We established a long-running field trial (2008-2018) in a series of N applications in combination with a mowing and fencing (unmown) treatment in a semiarid steppe. We assessed the resistance via ongoing N treatment of one subplot and the resilience via discontinuing N treatment in another to promote natural recovery since 2014. Plant resistance was negatively correlated with N application rate, while microbial resistance was independent of N rate. Mowing significantly reduced plant resistance and resilience, reduced soil microbial resistance but improved its resilience. Generally, plants are more resilient but less resistant to N than soil microbes. The two sides of resistance-resilience relationship were revealed: trade-offs exist between resistance and resilience for both plants and microbes at the community level; and trade-offs between resistance and resilience cannot be scaled down to species/group level. This study provided an important theoretical basis for the recovery and conservation of semiarid steppe and new insight into resistance-resilience relationship.

Nitrogen addition and mowing alter drought resistance and recovery of grassland communities

Nitrogen enrichment and land use are known to influence various ecosystems, but how these anthropogenic changes influence community and ecosystem responses to disturbance remains poorly understood. Here we investigated the effects of increased nitrogen input and mowing on the resistance and recovery of temperate semiarid grassland experiencing a three-year drought. Nitrogen addition increased grassland biomass recovery but decreased structural recovery after drought, whereas annual mowing increased grassland biomass recovery and structural recovery but reduced structural resistance to drought. The treatment effects on community biomass/structural resistance and recovery were largely modulated by the stability of the dominant species and asynchronous dynamics among species, and the community biomass resistance and recovery were also greatly driven by the stability of grasses. Community biomass resistance/recovery in response to drought was positively associated with its corresponding structural stability. Our study provides important experimental evidence that both nitrogen addition and mowing could substantially change grassland stability in both functional and structural aspects. Our findings emphasize the need to study changes across levels of ecological organization for a more complete understanding of ecosystem responses to disturbances under widespread environmental changes.

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.

Functional structure mediates the responses of productivity to addition of three nitrogen compounds in a meadow steppe

Atmospheric nitrogen (N) deposition is altering grassland productivity and community structure worldwide. Deposited N comes in different forms, which can have different consequences for productivity due to differences in their fertilization and acidification effects. We hypothesize that these effects may be mediated by changes in plant functional traits. We investigated the responses of aboveground primary productivity and community functional composition to addition of three nitrogen compounds (NH₄NO₃, [NH₄]₂SO₄, and CO[NH₂]₂) at the rates of 0, 5, 10, 20 g N m^-2 yr^-1. We used structural equation modeling (SEM) to evaluate how functional structure influences the responses of productivity to the three N compounds. Nitrogen addition increased community-level leaf chlorophyll content but decreased leaf dry matter content and phosphorus concentration. These changes were mainly due to intra-specific variation. Functional dispersion of traits was reduced by N addition through changes in species composition. SEM revealed that fertilization effects were more important than soil acidification for the responses of productivity to CO(NH₂)₂ addition, which enhanced productivity by decreasing functional trait dispersion. In contrast, the effects of (NH₄)₂SO₄ and NH₄NO₃ were primarily due to soil acidification, influencing productivity via community-weighted means of functional traits. Our results suggest that N forms with different fertilizing and acidifying effects influence productivity via different functional traits pathways. Our study also emphasizes the need for in situ experiments with the relevant N compounds to accurately understand and predict the ecological effects of atmospheric N deposition on ecosystems.

Winter warming alleviates the severely negative effects of nitrogen addition on ecosystem stability in a Tibetan alpine grassland

Many recent studies have explored how global warming and increased nitrogen (N) deposition affect the structure and function of natural ecosystems. However, how ecosystems respond to the combination of warming and N enrichment remains unexplored, especially under asymmetric seasonal warming scenarios. We conducted a decade-long field experiment in an alpine grassland to investigate the effects of warming (ambient condition (NW), winter-only (WW), and year-round (YW) warming) and N addition on the temporal stability of communities. Although N addition significantly reduced community temporal stability in NW, WW, and YW, WW relieved the severely negative effects of N addition compared to NW and YW (from 47.7 % in NW and 76.1 % in YW to 18.6 % in WW under 80 kg N hm^-2 year^-1). The most remarkable finding is that the main factors driving community stability shifted with warming patterns. The increase in community dominance under NW was a significant driver of the decreased temporal stability in the community. However, the decrease in community stability caused by N addition was ascribed to the decreased stability of both dominant and common species under WW. In contrast, N addition decreased community temporal stability mainly via a decrease in species asynchrony under YW. Our results suggested that warming patterns can modulate the effects of N enhancement on community stability. To predict the effects of climate change on alpine grasslands accurately, the idiosyncratic effects of asymmetric seasonal warming under future climate change scenarios should be considered.

Identifying thresholds of nitrogen enrichment for substantial shifts in arbuscular mycorrhizal fungal community metrics in a temperate grassland of northern China

Nitrogen (N) enrichment poses threats to biodiversity and ecosystem stability, while arbuscular mycorrhizal (AM) fungi play important roles in ecosystem stability and functioning. However, the ecological impacts, especially thresholds of N enrichment potentially causing AM fungal community shifts have not been adequately characterized. Based on a long-term field experiment with nine N addition levels ranging from 0 to 50 g N m^-2  yr^-1 in a temperate grassland, we characterized the community response patterns of AM fungi to N enrichment. Arbuscular mycorrhizal fungal biomass continuously decreased with increasing N addition levels. However, AM fungal diversity did not significantly change below 20 g N m^-2  yr^-1 , but dramatically decreased at higher N levels, which drove the AM fungal community to a potentially unstable state. Structural equation modeling showed that the decline in AM fungal biomass could be well explained by soil acidification, whereas key driving factors for AM fungal diversity shifted from soil nitrogen : phosphorus (N : P) ratio to soil pH with increasing N levels. Different aspects of AM fungal communities (biomass, diversity and community composition) respond differently to increasing N addition levels. Thresholds for substantial community shifts in response to N enrichment in this grassland ecosystem are identified.

Responses of grassland productivity to mowing intensity and precipitation variability in a temperate steppe

Mowing for hay is an important land use in grasslands that is affected by precipitation variability, due to the water-limited nature of these ecosystems. Past land use and precipitation conditions can have legacy effects on ecosystem functions, potentially altering responses to both mowing and precipitation. Nonetheless, it is still unclear how natural variation in precipitation will affect plant responses to changes in mowing intensity. We conducted a seven-year field experiment with three mowing intensity treatments compared to the traditional mowing intensity (5 cm stubble height) as a control: increased mowing (2 cm stubble), decreased mowing (8 cm stubble) and ceased mowing. Decreased mowing increased both plant aboveground net primary productivity [ANPP] and forage yield across the whole community, driven by increases in graminoids, mainly owing to the positive response of plants to precipitation. Both mowing disturbance and precipitation variability had legacy effects on plant ANPP; however, these responses differed among the whole community, graminoid, and forb levels. Current-year community-wide ANPP [ANPP_n] was positively associated with current-year precipitation [PPT_n] in all mowing treatments, driven by positive precipitation responses of the dominant graminoids. For forbs, however, ANPP_n was negatively associated with prior-year growing season precipitation [PPT_n-1] across mowing treatments, potentially due to lagged competition with the dominant graminoids. Our results suggest that the response of the dominant graminoids is the primary factor determining the response of ANPP to mowing and precipitation variability in these grassland ecosystems, and highlight that decreasing mowing intensity may maximize both herder's income and grassland sustainability.

Mowing mitigated the sensitivity of ecosystem carbon fluxes responses to heat waves in a Eurasian meadow steppe

The heat waves (HW) will be more frequent and intense in the future with increased human activity and uncertain implications for ecosystem carbon fluxes. The semi-arid Eurasian grassland is sensitive to climate change and under frequent HWs attacks. Mowing as one of the most common human practices in this region, combining with HW can have comprehensive effects on plant communities, biomass, and nutrient cycling. Hence, a 3-year (2019-2021) field manipulation experiment was conducted to assess how mowing influenced the carbon cycling under HWs, and the interactions between HWs and mowing on carbon fluxes at the community and ecosystem levels in a Eurasian meadow steppe. Over the three years, HW significantly reduced net ecosystem CO₂ exchange (NEE) and gross ecosystem production (GEP) by 28 % and 8 % (P < 0.05), respectively, whereas ecosystem respiration (Re) did not show significant changes. Moderate mowing (stubble height was set at 6-8 cm) for harvest effectively mitigated ecosystem sensitivity to HWs and significantly increased ecosystem carbon fluxes (NEE, Re, and GEP), biomass and the number of species. Mowing reduced the negative impact of HWs on ecosystem carbon fluxes by about 15 % compared to HWs alone, contributing to the invasion of species such as Thalictrum squarrosum and Vicia amoena, and increased the indirect effect of HW on NEE in the structural equation model. In addition, the higher soil water content (SWC) was another effective way to reduce the impact of HWs. Therefore, mowing and higher SWC would be effective ways to counteract the negative effects of HWs on carbon fluxes in future grassland management.

Long-term drought aggravates instability of alpine grassland productivity to extreme climatic event

The frequency and severity of extreme weather events are increasing and expected to increase more in the future together with global change. However, how extreme events and global change factors interactively influence community structures and ecosystem processes is largely unknown. Here, we investigated responses of temporal stability and resilience of aboveground net primary productivity (ANPP) of an alpine meadow to an extreme flooding event under different treatments of experimental drought and clipping. We found that ecosystems that were exposed to drought treatments for three years significantly decreased temporal stability of community productivity but increased resilience to flooding, whereas their resistance to or recovery from flooding did not change. Neither clipping nor its interaction with drought altered responses of these community stability metrics to flooding. Drought treatments significantly decreased plant species richness, asynchrony and dominant species stability, leading to the decrease in temporal stability and the increase in resilience in response to the extreme flooding event. We also revealed that the change in species asynchrony was the dominant impact pathway determining the responses of resilience and temporal stability to flooding. Our results highlight that the alpine grassland experiencing multi-year drought may aggravate instability of community productivity to extreme climatic events by reducing species asynchrony.

Plant C and N Pools Improved by N Addition Levels but Not Frequencies in a Typical Grassland of Northern China

The pools of plant community carbon (C) and nitrogen (N) are important sources of soil organic matter in terrestrial ecosystems and directly affect soil C and N cycling. A large amount of studies were manipulated with multiple N levels on soil C and N pools. However, how and whether the frequency of N addition can affect the plant C and N pools is still unclear. In order to comprehensively understand the N addition effects (including frequencies and levels) on C and N pools of the plant community, we executed a randomized complete block experiment with the addition of five levels of N, including 0, 2, 10, 20 and 50 g N m−2 yr−1 (designated as N-0, N-2, N-10, N-20 and N-50) and two N addition frequencies (twice a year vs. monthly, F2, F12) in August of 2008. After 5 years of treatment, the physical-chemical properties of the plants and soil were measured in 2013. The results indicated that with increasing N addition levels, the C and N pools of the plant community significantly increased, while N addition frequency had no significant effects. Moreover, significant interactions between N addition levels and the frequencies on the C and N pools of the plant community were also found in this typical grassland. Under different frequencies of N addition treatment, the plant community C and N pools showed different response patterns along with N addition levels in plants aboveground and belowground, respectively. Under different frequencies of N addition, the changes in the C and N pools of the plant community caused by N addition were regulated by different environmental factors. We highlight that long-term N deposition could affect the plant community C and N pools and would influence C and N cycling of terrestrial ecosystems based on global climate change in the future.

Contrasting Effects of Nitrogen Addition on Vegetative Phenology in Dry and Wet Years in a Temperate Steppe on the Mongolian Plateau

Changes in spring and autumn phenology and thus growing season length (GSL) pose great challenges in accurately predicting terrestrial primary productivity. However, how spring and autumn phenology in response to land-use change and nitrogen deposition and underlying mechanisms remain unclear. This study was conducted to explore the GSL and its components [i.e., the beginning of growing season and ending of growing season (EGS)] in response to mowing and nitrogen addition in a temperate steppe on the Mongolia Plateau during 2 years with hydrologically contrasting condition [dry (2014) vs. wet (2015)]. Our results demonstrated that mowing advanced the BGS only by 3.83 days, while nitrogen addition advanced and delayed the BGS and EGS by 2.85 and 3.31 days, respectively, and thus prolonged the GSL by 6.16 days across the two growing seasons from 2014 to 2015. When analyzed by each year, nitrogen addition lengthened the GSL in the dry year (2014), whereas it shortened the GSL in the wet year (2015). Further analyses revealed that the contrasting impacts of nitrogen on the GSL were attributed to monthly precipitation regimes and plant growth rate indicated by the maximum of normalized difference vegetation index (NDV_max). Moreover, changes in the GSL and its two components had divergent impacts on community productivity. The findings highlight the critical role of precipitation regimes in regulating the responses of spring and autumn phenology to nutrient enrichment and suggest that the relationships of ecosystem productivity with spring and autumn phenology largely depend on interannual precipitation fluctuations under future increased nitrogen deposition scenarios.

Linking leaf traits to the temporal stability of above- and belowground productivity under global change and land use scenarios in a semi-arid grassland of Inner Mongolia

The biotic drivers for the temporal stability of aboveground net productivity (ANPP) in natural ecosystems are well understood. However, knowledge gaps still exist regarding the relative importance of biotic and abiotic drivers regulating the temporal stability of aboveground productivity (ANPP), belowground net productivity (BNPP), and community net productivity (NPP) under global change and land use scenarios. Thus, in this study, we aimed to study the effects of increased water and nitrogen availability on temporal stability of ANPP, BNPP, and NPP and underlying mechanisms at sites with different long-term grazing histories in typical grasslands of the Inner Mongolia. The results suggested that resource addition affected the ANPP stability, but it did not change the stability of BNPP and NPP, which were all mediated by grazing histories. Most importantly, our study further indicated that species asynchrony, primarily contributed to the stability of ANPP and NPP by weakening their variation, and species asynchrony was regulated directly by plant diversity-related variables and indirectly by soil variables which were affected by resource addition and grazing history. In addition, an increase of ANPP stimulated under resource addition was a secondary contributor to ANPP stability. Specifically, the community-weighted mean of specific leaf area (CWM SLA) regulated the ANPP stability indirectly by promoting species asynchrony, while functional diversity of leaf area and SLA both directly controlled the BNPP stability. Findings of our study demonstrate that different mechanisms drove temporal stability of above- and belowground productivity. Our study has important implications for maintaining the temporal stability of community productivity and for establishing sustainable management practices of semi-arid grasslands under global change and land use scenarios.

Leaf trait network architecture shifts with species-richness and climate across forests at continental scale

Variation in the architecture of trait networks among ecosystems has been rarely quantified, but can provide high resolution of the contrasting adaptation of the whole phenotype. We constructed leaf trait networks (LTNs) from 35 structural, anatomical and compositional leaf traits for 394 tree species in nine forests from tropical to cold-temperate zones in China. Our analyses supported the hypothesis that LTNs would increase in modular complexity across forests in parallel with species-richness and climatic warmth and moisture, due to reduced phenotypic constraints and greater opportunities for niche differentiation. Additionally, we found that within LTNs, leaf economics traits including leaf thickness would have central importance, acting as hub traits with high connectivity due to their contributions to multiple functions. Across the continent, the greater species richness and trait diversity observed in forests under resource-rich climates enable greater complexity in whole phenotype structure and function as indicated by the trait network architecture.

Decoupled responses of above- and below-ground stability of productivity to nitrogen addition at the local and larger spatial scale

Temporal stability of net primary productivity (NPP) is important for predicting the reliable provisioning of ecosystem services under global changes. Although nitrogen (N) addition is known to affect the temporal stability of aboveground net primary productivity (ANPP), it is unclear how it impacts that of belowground net primary productivity (BNPP) and NPP, and whether such effects are scale dependent. Here, using experimental N addition in a grassland, we found different responses of ANPP and BNPP stability to N addition at the local scale and that these responses propagated to the larger spatial scale. That is, N addition significantly decreased the stability of ANPP but did not affect the stability of BNPP and NPP at the two scales investigated. Additionally, spatial asynchrony of both ANPP and BNPP among communities provided greater stability at the larger scale and was not affected by N addition. Our findings challenge the traditional view that N addition would reduce ecosystem stability based on results from aboveground dynamics, thus highlighting the importance of viewing ecosystem stability from a whole system perspective.

Detection of Grassland Mowing Events for Germany by Combining Sentinel-1 and Sentinel-2 Time Series

Grasslands cover one-third of the agricultural area in Germany and play an important economic role by providing fodder for livestock. In addition, they fulfill important ecosystem services, such as carbon storage, water purification, and the provision of habitats. These ecosystem services usually depend on the grassland management. In central Europe, grasslands are grazed and/or mown, whereby the management type and intensity vary in space and time. Spatial information on the mowing timing and frequency on larger scales are usually not available but would be required in order to assess the ecosystem services, species composition, and grassland yields. Time series of high-resolution satellite remote sensing data can be used to analyze the temporal and spatial dynamics of grasslands. Within this study, we aim to overcome the drawbacks identified by previous studies, such as optical data availability and the lack of comprehensive reference data, by testing the time series of various Sentinel-2 (S2) and Sentinal-1 (S1) parameters and combinations of them in order to detect mowing events in Germany in 2019. We developed a threshold-based algorithm by using information from a comprehensive reference dataset of heterogeneously managed grassland parcels in Germany, obtained by RGB cameras. The developed approach using the enhanced vegetation index (EVI) derived from S2 led to a successful mowing event detection in Germany (60.3% of mowing events detected, F1-Score = 0.64). However, events shortly before, during, or shortly after cloud gaps were missed and in regions with lower S2 orbit coverage fewer mowing events were detected. Therefore, S1-based backscatter, InSAR, and PolSAR features were investigated during S2 data gaps. From these, the PolSAR entropy detected mowing events most reliably. For a focus region, we tested an integrated approach by combining S2 and S1 parameters. This approach detected additional mowing events, but also led to many false positive events, resulting in a reduction in the F1-Score (from 0.65 of S2 to 0.61 of S2 + S1 for the focus region). According to our analysis, a majority of grasslands in Germany are only mown zero to two times (around 84%) and are probably additionally used for grazing. A small proportion is mown more often than four times (3%). Regions with a generally higher grassland mowing frequency are located in southern, south-eastern, and northern Germany.

Unused railway lines for conservation of pollinators in the intensively managed agricultural landscape

Pollinating insects are under high human pressure due to agricultural intensification and urbanization. Although many research and conservation projects have been applied worldwide, there is still a need for a comprehensive approach that meets local conditioning and capabilities. This paper investigated the composition, abundance, richness, alpha, beta, and gamma-diversity of pollinators between unused railway embankments and semi-natural grasslands. On 50 study sites (25 sites in unused railways and 25 their reference on grasslands), we collected data on the abundance, species richness and species diversity of bees, butterflies, and hoverflies. We conducted five samplings yearly (April-September) for two years, 2017-2018. To assess differences in abundance, species richness, alpha and beta diversity of pollinators between unused railways and controls, we used generalized linear mixed models (GLMM). To compare the composition of pollinator species, we applied non-metric multidimensional scaling (NMDS). Abundance, species richness, and Shannon-Wiener diversity index of all three groups of pollinators were significantly higher in unused railway lines than in control grasslands. Pollinator communities were more constant in unused railway lines than in grasslands. The NMDS analysis highlighted the importance of these structures for bee, butterfly and hoverfly communities, which were a subset of grassland species, but were more abundant. We indicated the highest total taxonomic beta-diversity for bees, butterflies, and hoverflies and species turnover for bees and butterflies in control grassland compared with unused railway lines. The taxonomical nestedness was significantly higher in unused railways lines for bees and butterflies than in control grasslands. In the case of hoverflies, we did not found any significant differences in species turnover and nestedness. Unused railway lines may act as a conservation tool for pollinator communities in intensively managed farmland and complement the declining semi-natural habitats.

Belowground Root Competition Alters the Grass Seedling Establishment Response to Light by a Nitrogen Addition and Mowing Experiment in a Temperate Steppe

Predicting species responses to climate change and land use practices requires understanding both the direct effects of environmental factors as well as the indirect effects mediated by changes in belowground and aboveground competition. Belowground root competition from surrounding vegetation and aboveground light competition are two important factors affecting seedling establishment. However, few studies have jointly examined the effect of belowground root and light competition on seedling establishment, especially under long-term nitrogen addition and mowing. Here, we examined how belowground root competition from surrounding vegetation and aboveground light competition affect seedling establishment within a long-term nitrogen addition and mowing experiment. Seedlings of two grasses (Stipa krylovii and Cleistogenes squarrosa) were grown with and without belowground root competition under control, nitrogen addition, and mowing treatments, and their growth characteristics were monitored. The seedlings of the two grasses achieved higher total biomass, height, mean shoot and root mass, but a lower root/shoot ratio in the absence than in the presence of belowground root competition. Nitrogen addition significantly decreased shoot biomass, root biomass, and the survival of the two grasses. Regression analyses revealed that the biomass of the two grass was strongly negatively correlated with net primary productivity under belowground root competition, but with the intercept photosynthetic active radiation in the absence of belowground root competition. This experiment demonstrates that belowground root competition can alter the grass seedling establishment response to light in a long-term nitrogen addition and mowing experiment.

Effect of Nitrogen Addition on Soil Microbial Functional Gene Abundance and Community Diversity in Permafrost Peatland

Nitrogen is the limiting nutrient for plant growth in peatland ecosystems. Nitrogen addition significantly affects the plant biomass, diversity and community structure in peatlands. However, the response of belowground microbe to nitrogen addition in peatland ecosystems remains largely unknown. In this study, we performed long-term nitrogen addition experiments in a permafrost peatland in the northwest slope of the Great Xing'an Mountains. The four nitrogen addition treatments applied in this study were 0 g N·m-2·year-1 (CK), 6 g N·m-2·year-1 (N1), 12 g N·m-2·year-1 (N2), and 24 g N·m-2·year-1 (N3). Effects of nitrogen addition over a period of nine growing seasons on the soil microbial abundance and community diversity in permafrost peatland were analyzed. The results showed that the abundances of soil bacteria, fungi, archaea, nitrogen-cycling genes (nifH and b-amoA), and mcrA increased in N1, N2, and N3 treatments compared to CK. This indicated that nitrogen addition promoted microbial decomposition of soil organic matter, nitrogen fixation, ammonia oxidation, nitrification, and methane production. Moreover, nitrogen addition altered the microbial community composition. At the phylum level, the relative abundance of Proteobacteria increased significantly in the N2 treatment. However, the relative abundances of Actinobacteria and Verrucifera in the N2 treatment and Patescibacteria in the N1 treatment decreased significantly. The heatmap showed that the dominant order composition of soil bacteria in N1, N2, and N3 treatments and the CK treatment were different, and the dominant order composition of soil fungi in CK and N3 treatments were different. The N1 treatment showed a significant increase in the Ace and Chao indices of bacteria and Simpson index of fungi. The outcomes of this study suggest that nitrogen addition altered the soil microbial abundance, community structure, and diversity, affecting the soil microbial carbon and nitrogen cycling in permafrost peatland. The results are helpful to understand the microbial mediation on ecological processes in response to N addition.

Effects of nitrogen addition on plant-soil micronutrients vary with nitrogen form and mowing management in a meadow steppe

Nitrogen (N) addition and mowing can significantly influence micronutrient cycling in grassland ecosystems. It remains largely unknown about how different forms of added N affect micronutrient status in plant-soil systems. We examined the effects of different N compounds of (NH₄)₂SO₄, NH₄NO₃, and urea with and without mowing on micronutrient Fe, Mn, Cu, and Zn in soil-plant systems in a meadow steppe. The results showed that (NH₄)₂SO₄ addition had a stronger negative effect on soil pH compared with NH₄NO₃ and urea, resulting in higher increases in soil available Fe and Mn herein. Nitrogen addition decreased plant community-level biomass weighted (hereafter referred to as community-level) Fe concentration but increased Mn concentration, with a greater effect under (NH₄)₂SO₄ addition. Community-level Cu concentration increased with (NH₄)₂SO₄ and NH₄NO₃ addition only under mowing treatment. Mowing synergistically interacted with urea addition to increase community-level Mn and Zn concentrations even with decreased soil organic matter, possibly because of compensatory plant growth and thus higher plant nutrient uptake intensity under mowing treatment. Overall, responses of plant-soil micronutrients to N addition varied with mowing and different N compounds, which were mainly regulated by soil physicochemical properties and plant growth. Different magnitude of micronutrient responses in plants and soils shed light on the necessity to consider the role of various N compounds in biogeochemical models when projecting the effects of N enrichment on grassland ecosystems.

Shoot-Root Interplay Mediates Defoliation-Induced Plant Legacy Effect

Shoot defoliation by grazers or mowing can affect root traits of grassland species, which may subsequently affect its aboveground traits and ecosystem functioning (e.g., aboveground primary production). However, experimental evidence for such reciprocal feedback between shoots and roots is limited. We grew the perennial grass Leymus chinensis-common across the eastern Eurasian steppe-as model species in a controlled-hydroponics experiment, and then removed half of its shoots, half of its roots, or a combination of both. We measured a range of plant aboveground and belowground traits (e.g., phenotypic characteristics, photosynthetic traits, root architecture) in response to the shoot and/or root removal treatments. We found the regenerated biomass was less than the lost biomass under both shoot defoliation and root severance, generating a under-compensatory growth. Root biomass was reduced by 60.11% in the defoliation treatment, while root severance indirectly reduced shoot biomass by 40.49%, indicating a feedback loop between shoot and root growth. This defoliation-induced shoot-root feedback was mediated by the disproportionate response and allometry of plant traits. Further, the effect of shoot defoliation and root severance on trait plasticity of L. chinensis was sub-additive. That is, the combined effects of the two treatments were less than the sum of their independent effects, resulting in a buffering effect on the existing negative influences on plant persistence by increased photosynthesis. Our results highlight the key role of trait plasticity in driving shoot-root reciprocal feedbacks and growth persistence in grassland plants, especially perennial species. This knowledge adds to earlier findings of legacy effects and can be used to determine the resilience of grasslands.

General destabilizing effects of eutrophication on grassland productivity at multiple spatial scales

Eutrophication is a widespread environmental change that usually reduces the stabilizing effect of plant diversity on productivity in local communities. Whether this effect is scale dependent remains to be elucidated. Here, we determine the relationship between plant diversity and temporal stability of productivity for 243 plant communities from 42 grasslands across the globe and quantify the effect of chronic fertilization on these relationships. Unfertilized local communities with more plant species exhibit greater asynchronous dynamics among species in response to natural environmental fluctuations, resulting in greater local stability (alpha stability). Moreover, neighborhood communities that have greater spatial variation in plant species composition within sites (higher beta diversity) have greater spatial asynchrony of productivity among communities, resulting in greater stability at the larger scale (gamma stability). Importantly, fertilization consistently weakens the contribution of plant diversity to both of these stabilizing mechanisms, thus diminishing the positive effect of biodiversity on stability at differing spatial scales. Our findings suggest that preserving grassland functional stability requires conservation of plant diversity within and among ecological communities.

Eutrophication has been shown to weaken diversity-stability relationships in grasslands, but it is unclear whether the effect depends on scale. Analysing a globally distributed network of grassland sites, the authors show a positive role of beta diversity and spatial asynchrony as drivers of stability but find that nitrogen enrichment weakens the diversity-stability relationships at different spatial scales.

Plant community temporal stability in response to nitrogen addition among different degraded grasslands

Atmospheric nitrogen (N) deposition is generally believed to decrease plant community temporal stability. However, it remains unclear whether the responses of community temporal stability to N deposition vary with disturbance regimes, such as among different grasslands with degrees of degradation. We established a 4-year field experiment with six N addition levels on four grassland sites in northern China with varying degradation statuses (non-degraded, moderately, severely and extremely degraded grasslands). We examined the response of community temporal stability (quantified as the ratio of the mean of community biomass to its standard deviation) to N addition and important regulating factors. Asynchrony was calculated as the difference between one and synchrony (quantified as the ratio of the variance of community biomass to the square of the sum of the standard deviation of species biomass). The most interesting result we found was that community temporal stability of the moderately and severely degraded grasslands was relatively higher among the four grasslands without N addition. This was attributed to shifts in dominant species composition rather than species diversity. Community temporal stability of nearly all sites were not significantly affected by N addition except the moderately degraded grassland. Community temporal stability of the moderately degraded grassland responded to N addition non-linearly, being promoted by low N addition levels (10 and 20 g N m^-2 yr^-1) and decreased by high N addition levels (30-50 g N m^-2 yr^-1). Furthermore, community temporal stability of the non-degraded and moderately degraded grasslands was mainly driven by species asynchrony. Whereas, in the severely and extremely degraded grasslands, community temporal stability was mainly regulated by species richness and dominant species stability, respectively. These findings highlight the importance of grassland degradation in the response of community temporal stability to N deposition and provide scientific support for the management and restoration of degraded grasslands.

Asymmetry in above- and belowground productivity responses to N addition in a semi-arid temperate steppe

Nitrogen (N) enrichment often increases aboveground net primary productivity (ANPP) of the ecosystem, but it is unclear if belowground net primary productivity (BNPP) track responses of ANPP. Moreover, the frequency of N inputs may affect primary productivity but is rarely studied. To assess the response patterns of above- and belowground productivity to rates of N addition under different addition frequencies, we manipulated the rate (0-50 g N m^-2  year^-1 ) and frequency (twice vs. monthly additions per year) of NH₄ NO₃ inputs for six consecutive years in a temperate grassland in northern China and measured ANPP and BNPP from 2012 to 2014. In the low range of N addition rates, BNPP showed the greatest negative response and ANPP showed the greatest positive responses with increases in N addition (<10 g N m^-2  year^-1 ). As N addition increased beyond 10 g N m^-2  year^-1 , increases in ANPP dampened and decreases in BNPP ceased altogether. The response pattern of net primary productivity (combined above- and belowground; NPP) corresponded more closely to ANPP than to BNPP. The N effects on BNPP and BNPP/NPP (f_BNPP ) were not dependent on N addition frequency in the range of N additions typically associated with N deposition. BNPP was more sensitive to N addition frequency than ANPP, especially at low rates of N addition. Our findings provide new insights into how plants regulate carbon allocation to different organs with increasing N rates and changing addition frequencies. These root response patterns, if incorporated into Earth system models, may improve the predictive power of C dynamics in dryland ecosystems in the face of global atmospheric N deposition.

Fire Intensity Affects the Relationship between Species Diversity and the N Utilization Stability of Dominant Species

Stabilizing the local elemental stoichiometry is an important step toward restoring species diversity in a damaged ecosystem, especially those affected by wildfire. Stability of nitrogen (N) utilization is mainly affected by wildfire through restoration, which is one of the most important parts of stoichiometric utilization. However, the mechanisms underlying the relationship between N utilization stability and species diversity are not well understood in burned areas. We investigated variation in species diversity and in the stability of leaf N utilization of locally dominant tree species in a series of burned areas during early community restoration following wildfires of different intensities. This study shows that low fire intensity led to an increase in the soil N concentration, and significantly affected the utilization of leaf N. With higher fire intensity, the leaf N concentration first decreased, and then increased as fire intensity increased. The dominant trees showed more stable N utilization at a medium intensity, compared with other intensities, but the stability of N utilization was overall higher for the dominant species than for the regenerating pioneer species. We also concluded that other soil nutrients altered the stability of plant N utilization, which we found was closely related to species diversity during restoration. The Shannon index and N utilization stability in burned areas were most significantly correlated. The N utilization stability regulation between soil total nitrogen (STN) and leaf total nitrogen (LTN) (HSTN-LTN) of Betula platyphylla Suk (BPS) correlated significantly and positively with the increase of the Shannon index (H), but the HSMN-LTN of the dominant species correlated significantly and negatively with H.

Mowing mitigates the negative impacts of N addition on plant species diversity

Increasing availability of reactive nitrogen (N) threatens plant diversity in diverse ecosystems. While there is mounting evidence for the negative impacts of N deposition on one component of diversity, species richness, we know little about its effects on another one, species evenness. It is suspected that ecosystem management practice that removes nitrogen from the ecosystem, such as hay-harvesting by mowing in grasslands, would mitigate the negative impacts of N deposition on plant diversity. However, empirical evidence is scarce. Here, we reported the main and interactive effects of N deposition and mowing on plant diversity in a temperate meadow steppe with 4-year data from a field experiment within which multi-level N addition rates and multiple N compounds are considered. Across all the types of N compounds, species richness and evenness significantly decreased with the increases of N addition rate, which was mainly caused by the growth of a tall rhizomatous grass, Leymus chinensis. Such negative impacts of N addition were accumulating with time. Mowing significantly reduced the dominance of L. chinensis, and mitigated the negative impacts of N deposition on species evenness. We present robust evidence that N deposition threatened biodiversity by reducing both species richness and evenness, a process which could be alleviated by mowing. Our results highlight the changes of species evenness in driving the negative impacts of N deposition on plant diversity and the role of mowing in mediating such negative impacts of N deposition.

Nitrogen addition does not reduce the role of spatial asynchrony in stabilising grassland communities

While nitrogen (N) amendment is known to affect the stability of ecological communities, whether this effect is scale-dependent remains an open question. By conducting a field experiment in a temperate grassland, we found that both plant richness and temporal stability of community biomass increased with spatial scale, but N enrichment reduced richness and stability at the two scales considered. Reduced local-scale stability under N enrichment arose from N-induced reduction in population stability, which was partly attributable to the decline in local species richness, as well as reduction in asynchronous local population dynamics across species. Importantly, N enrichment did not alter spatial asynchrony among local communities, which provided similar spatial insurance effects at the larger scale, regardless of N enrichment levels. These results suggest that spatial variability among local communities, in addition to local diversity, may help stabilise ecosystems at larger spatial scales even in the face of anthropogenic environmental changes.

Intensity and frequency of nitrogen addition alter soil chemical properties depending on mowing management in a temperate steppe

Anthropogenic nitrogen (N) enrichment can significantly alter soil chemical properties in various ecosystems. Previous manipulative N experiments mainly focused on the intensity of N addition on soil properties by changing N input rates. It remains unclear, however, whether frequency of N addition can affect soil chemical properties. We examined the effects of frequency (2 versus 12 applications yr^-1) and rate (ranging from 0 to 50 g N m^-2 yr^-1) of N addition on soil chemical properties of pH, base cations, soil pH buffering capacity (pHBC), and soil available micronutrients in a temperate steppe with and without mowing. Mowing significantly increased the effective cation exchange capacity (ECEC), soil exchangeable Ca and Na, available Fe, and soil pHBC when N was applied at low frequency. Low frequency of N addition significantly decreased soil pH and exchangeable Na but increased soil exchangeable Mg without mowing; however, it increased soil exchangeable Na and available Zn with mowing, while available Fe and Mn increased both with and without mowing. Higher rates of N addition (≥20 g N m^-2 yr^-1) decreased soil pH, ECEC and exchangeable Ca but increased soil available Fe, Mn and Cu regardless of the mowing treatment and frequency of N addition. Changes in soil organic matter, pHBC and ECEC were the main reasons affecting soil pH across mowing and N application treatments. Our results indicate that frequency of N addition played an essential role in altering soil chemical properties. Simulating N deposition via large and infrequent N additions can underestimate (exchangeable Mg and available Fe and Mn) or overestimate (soil pH and exchangeable Na) changes in soil properties. Our results further suggest that the effects of frequency of N addition on soil chemical attributes in semi-arid grassland ecosystems can be regulated by appropriate mowing management.

Scale dependence of the diversity-stability relationship in a temperate grassland

A positive relationship between biodiversity and ecosystem stability has been reported in many ecosystems; however, it has yet to be determined whether and how spatial scale affects this relationship. Here, for the first time, we assessed the effects of alpha, beta and gamma diversity on ecosystem stability and the scale dependence of the slope of the diversity-stability relationship.By employing a long-term (33 years) dataset from a temperate grassland, northern China, we calculated the all possible spatial scales with the complete combination from the basic 1-m² plots.Species richness was positively associated with ecosystem stability through species asynchrony and overyielding at all spatial scales (1, 2, 3, 4 and 5 m²). Both alpha and beta diversity were positively associated with gamma stability.Moreover, the slope of the diversity-area relationship was significantly higher than that of the stability-area relationship, resulting in a decline of the slope of the diversity-stability relationship with increasing area.Synthesis. With the positive species diversity effect on ecosystem stability from small to large spatial scales, our findings demonstrate the need to maintain a high biodiversity and biotic heterogeneity as insurance against the risks incurred by ecosystems in the face of global environmental changes.