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Purpose: The escalation of nitrogen (N) deposition has resulted in phosphorus (P) limitation in alpine grasslands on the Qinghai–Tibetan Plateau (QTP). However, the impact of N deposition affects soil P transformations in alpine grasslands, and whether there is a universal pattern of N-induced soil P fraction change in terrestrial ecosystems is still not well understood. Methods: We performed field experiments in two alpine grasslands on the QTP and a meta-analysis including 1033 records worldwide to analyze the responses of soil P fractions to N addition. Results: We found that N addition significantly altered soil P fractions in alpine meadow, whereas it induced a minor response in alpine steppe. The N addition induced a decrease in soil inorganic phosphorus (Pi) in the alpine meadow, resulting from occluded P (i.e., C.HCl-Pt and residual-Pt). Though N addition did not change total organic P (Po) concentration, there were remarkable changes among soil organic P fractions (C.HCl-Po, NaOH-Po, and NaHCO3-Po) in the alpine meadow, with an increase in NaOH-Po but a decrease in C.HCl-Po. Soil inorganic P in the alpine meadow was associated with Ca2+ and soil pH that was also reduced by N addition. By contrast, meta-analysis results showed that N addition significantly increased the lnRR of NaOH-Pi, but decreased lnRR of C.HCl-Pt and marginally reduced lnRR of NaHCO3-Po across all terrestrial ecosystems. Among multiple environmental and experimental variables, soil pH, mean annual temperature (MAT), mean annual precipitation (MAP), N forms, and soil phosphatase activity mainly drove the response of NaHCO3-Po to N addition at the large scale. Structural equation model (SEM) further showed that soil phosphatase activity was the main direct factor controlling NaHCO3-Po response. Conclusions: Our results suggest that soil P fractions are more sensitive to N addition in alpine meadow than in alpine steppe. The reduction of inorganic P fractions and uneven changes of organic P fractions in alpine meadow suggested that N addition may accelerate inorganic P dissolution but depress organic P mineralization. Environmental factor (e.g., MAP) and experimental variables (N rate) affected soil P fractions in response to N addition mediated by soil pH and enzymatic activities. Collectively, these findings improved our understanding of the consequences of N addition on soil organic and inorganic P transformations and predicted the trajectory of soil phosphorus fraction change under increasing N deposition. [ABSTRACT FROM AUTHOR]

Soil nematodes play pivotal roles in the soil food web. However, the trophic regulation on nematodes in the soil micro‐food web and its temporal dynamics are less explored. Here, we investigated the seasonal dynamics of soil nematode community in an alpine grassland on the Qinghai–Tibet Plateau at non‐degraded (ND), moderately degraded (MD) and severely degraded (SD) stages, either under open‐top chamber warming (W) or not (NW). Nematode richness was higher in SD than in MD and ND that did not differ, accompanied by a clear shift from the bottom‐up regulation via belowground productivity in ND to the top‐down regulation via omnivore and predator nematodes in SD. The intensified top‐down regulation in SD was explained by increased proportions of predator and omnivore nematodes, likely induced by the expanded soil pore size in SD. As a result, accelerated temporal turnover of nematode community was observed under grassland degradation, which were 0.006 (slope, p = 0.78), 0.045 (p = 0.07), and 0.077 (p = 0.001) in ND, MD, and SD, respectively. Moreover, grassland degradation strengthened the association between nematode density and ecosystem respiration, whose slope was higher in SD (0.0025, p < 0.05) than in MD (0.0008, p < 0.01) or ND (insignificant). In contrast, warming did not affect the temporal turnover of the nematode community, richness, nor its role in ecosystem respiration. Overall, we revealed the accelerated temporal turnover of nematode community and its strengthened role in determining ecosystem respiration under alpine grassland degradation, suggesting that trophic cascade changes under ecosystem degradation may alter the ecosystem functioning. [ABSTRACT FROM AUTHOR]

Resources can affect plant productivity and biodiversity simultaneously and thus are key drivers of their relationships in addition to plant–plant interactions. However, most previous studies only focused on a single resource while neglecting the nature of resource multidimensionality. Here we integrated four essential resources for plant growth into a single metric of resource diversity (RD) to investigate its effects on the productivity–biodiversity relationship (PBR) across Chinese grasslands. Results showed that habitats differing in RD have different PBRs—positive in low-resource habitats, but neutral in medium- and high-resource ones—while collectively, a weak positive PBR was observed. However, when excluding direct effects of RD on productivity and biodiversity, the PBR in high-resource habitats became negative, which leads to a unimodal instead of a positive PBR along the RD gradient. By integrating resource effects and changing plant–plant interactions into a unified framework with the RD gradient, our work contributes to uncovering underlying mechanisms for inconsistent PBRs at large scales. [ABSTRACT FROM AUTHOR]

Extreme climatic events are likely to intensify under climate change and can have different effects on ecosystems depending on their timing and magnitude. Understanding how productivity responds to extreme precipitation patterns requires assessing stability and vulnerability during critical growing periods at the plant community level. In this study, we experimentally imposed two contrasting types of extreme precipitation patterns, including extreme drought (excluding all rainfall for 30 consecutive days) during early-, mid-, and late-stages of the growing season, and heavy rainfall (adding 14.1 mm of rainfall every day for 20 consecutive days) during mid- and late-stages of the growing season over four years (2013–2016) in a steppe community in Inner Mongolia, China. We found that extreme drought and heavy rainfall had no effect on community aboveground net primary productivity (ANPP), species richness, and dominance at any stage of the growing season. Community stability in response to extreme drought was mainly driven by compensation among species and the stability of dominant species, while the compensatory effect among species and functional groups, and the stability of dominant species contributed to the community stability in response to heavy rainfall. Overall, our findings indicate that the responses of the ecosystem to intra-seasonal contrasting extreme precipitation patterns can be driven by similar stability mechanisms and suggest that semiarid temperate steppe communities may have strong initial resistance to more frequent extreme climatic events in the future. [ABSTRACT FROM AUTHOR]

Droughts and heat waves are increasing in magnitude and frequency, altering the carbon cycle. However, understanding of the underlying response mechanisms remains poor, especially for the combination (hot drought). We conducted a 4‐year field experiment to examine both individual and interactive effects of drought and heat wave on carbon cycling of a semiarid grassland across individual, functional group, community and ecosystem levels. Drought did not change below‐ground biomass (BGB) or above‐ground biomass (AGB) due to compensation effects between grass and non‐grass functional groups. However, consistently decreased BGB under heat waves limited such compensation effects, resulting in reduced AGB. Ecosystem CO2 fluxes were suppressed by droughts, attributed to stomatal closure‐induced reductions in leaf photosynthesis and decreased AGB of grasses, while CO2 fluxes were little affected by heat waves. Overall the hot drought produced the lowest leaf photosynthesis, AGB and ecosystem CO2 fluxes although the interactions between heat wave and drought were usually not significant. Our results highlight that the functional group compensatory effects that maintain community‐level AGB rely on feedback of root system responses, and that plant adjustments at the individual level, together with shifts in composition at the functional group level, co‐regulate ecosystem carbon sink strength under climate extremes. • It is indispensable to explore the mechanisms and impacts of climate extremes on the carbon cycle given they are increasing in magnitude and frequency.• Aboveground biomass responses to climate extremes rely on the functional group compensatory effects and feedbacks of root system responses.• Plant physiological adjustments and shifts in community composition co‐regulate ecosystem CO2 fluxes under climate extremes. [ABSTRACT FROM AUTHOR]

The spatial variability of microorganisms in grasslands can provide important insights regarding the biogeographic patterns of microbial communities. However, information regarding the degree of overlap and partitions of microbial communities across different habitats in grasslands is limited. This study investigated the microbial communities in three distinct habitats from Xilingol steppe grassland, i.e. animal excrement, phyllosphere, and soil samples, by Illumina MiSeq sequencing. All microbial community structures, i.e. for bacteria, archaea, and fungi, were significantly distinguished according to habitat. A high number of unique microorganisms but few coexisting microorganisms were detected, suggesting that the structure of microbial communities was mainly regulated by species selection and niche differentiation. However, the sequences of those limited coexisting microorganisms among the three different habitats accounted for over 60% of the total sequences, indicating their ability to adapt to variable environments. In addition, the biotic interactions among microorganisms based on a co-occurrence network analysis highlighted the importance of Microvirga, Blastococcus, RB41, Nitrospira, and four norank members of bacteria in connecting the different microbiomes. Collectively, the microbial communities in the Xilingol steppe grassland presented strong habitat preferences with a certain degree of dispersal and colonization potential to new habitats along the animal excrement- phyllosphere-soil gradient. This study provides the first detailed comparison of microbial communities in different habitats in a single grassland, and offers new insights into the biogeographic patterns of the microbial assemblages in grasslands. [ABSTRACT FROM AUTHOR]

Purpose: Understanding the biogeographic distribution of soil fungal communities is crucial for assessing the impacts of environmental factors on terrestrial biodiversity and ecosystem functioning. Here, we investigated spatial variations of fungal communities across three different types of temperate grasslands along a transect in the Inner Mongolia, China. The aims were to understand the biogeographic patterns of fungi and key drivers shaping soil fungal communities in temperate grasslands. Materials and methods: The composition and diversity of soil fungal community across 30 sites of the meadow steppe, typical steppe, and desert steppe along a 1200-km transect were studied through pyrosequencing. The relationships between fungal communities and environmental and biotic factors were assessed. Results and discussion: The results showed that the fungal community along this transect exhibited strong dependence on soil moisture content and nitrate concentration, while the fungal alpha diversity was positively correlated with precipitation and below-ground biomass. Drier environment has resulted in a shift towards an Ascomycota-dominating fungal community. Conclusions: Our findings suggest that the distribution and community structure of soil fungal communities are primarily driven by precipitation. Plant biomass and soil nutrient status which are also influenced by precipitation are also predictors of fungal community. Our results provide important implications for understanding the linkages among environmental factors and soil fungal communities in Eurasian steppe ecosystems. [ABSTRACT FROM AUTHOR]

Purpose: In this study, a process-based denitrification-decomposition (DNDC) model was parameterized and validated with CO fluxes data measured using eddy covariance (EC) technique in the meadow and desert steppes of Inner Mongolia grasslands, China. The validated model was applied to predict the influence of moderate climate change on carbon (C) budget of the two steppes over the next 30 years. Materials and methods: A series of parameters associated with plant growth including maximum biomass production, C/N ratio of plant tissue, and accumulating thermal degree days based on local observations were adjusted to meet the requirements of adapting DNDC for the two targeted steppes. Daily weather data for the studied year together with soil properties for the study sites were employed as inputs to simulate the grass growth and soil C dynamics. A baseline and five alternative climate scenarios were utilized to estimate the effect of climate change on the ecosystem C dynamics for the next 30 years. The coefficient of determination ( R ), root mean square error (RMSE), relative mean deviation (RMD), and modeling efficiency (EF) were used to assess the DNDC model based on the EC measurements. Results and discussion: The DNDC model successfully captured the maximum and minimum soil surface temperature and the effects of rainfall pulse on soil water content (SWC). Validation of ecosystem daily CO fluxes against the EC observed values resulted in satisfactory results in both of the meadow and the desert steppe. Additionally, the comparison of monthly simulated results is better than that of daily results. Between 2011 and 2040, if local climate becomes wetter, both steppes would become C sinks, leading to a great net primary productivity (NPP) of 821 and 828 kg C ha year sequestered in soil organic C (SOC), respectively. If local climate becomes drier, they would become C-sources, having the lowest NPP with 166 and 121 kg C ha year lost from the soil C pool, respectively. Conclusions: The validated DNDC model can be used to predict C dynamics in two semiarid grassland ecosystems. For the upcoming 30 years, if the local climate became wetter, the meadow and desert steppes would act as C sink, absorbing CO from the atmosphere and increase the C sequestration potential. However, both steppes would act as C source, releasing CO to the atmosphere, and decrease the C sequestration when the local climate becomes drier. [ABSTRACT FROM AUTHOR]

In the alpine steppe zone on the Central Tibetan Plateau, a large amount of area has been degraded due to natural and artificial factors. N & P fertilization is widely accepted to recover degraded pastures in other regions all over the world. However, it is not clear how alpine steppe communities respond to N & P fertilization, and what is the optimal application rate, in the perspective of forage production. To attempt to explore these questions, in July 2013, two fencing sites were designed in Baingoin County with 12 treatments of different levels of nitrogen (N0: 0; N1: 7.5 g m-2 yr-1; N2: 15 g m-2 yr-1) & phosphate (P0: 0; P1: 7.5 gP2O5 m-2 yr-1; P2: 15 gP2O5 m-2 yr-1; P3: 30 gP2O5 m-2 yr-1). The results indicated N&P addition was capable to ameliorate the quality of the two sites in the Tibetan Plateau steppe. Increasing N application level resulted in significant increment in Gramineae and total biomass in the two sites. P addition significantly improved the quantity of Compositae, total biomass and the biomasss of other species in site II, while it only significantly improved the total biomass in site I. Gramineae was much more sensitive to N-induced changes than P-induced changes, and this indicated N addition was better to ameliorate the quality of plateau steppe than P-induced changes. No strong evidence was found for critical threshold within 15 g N m-2 yr-1, and there was decreasing tendency when P addition rate was above 15 g m-2 yr-1. N&P has the potential to accelerate soil acidification, which improved the content of available K, likely as a result of nonsignificant correlation between biomass and soil moisture. This work highlights the the tradeoffs that exist in N and P addition in recovering degraded steppe. [ABSTRACT FROM AUTHOR]

Purpose: Carbon (C) dynamics in grassland ecosystem contributes to regional and global fluxes in carbon dioxide (CO) concentrations. Grazing is one of the main structuring factors in grassland, but the impact of grazing on the C budget is still under debate. In this study, in situ net ecosystem CO exchange (NEE) observations by the eddy covariance technique were integrated with a modified process-oriented biogeochemistry model (denitrification-decomposition) to investigate the impacts of grazing on the long-term C budget of semiarid grasslands. Materials and methods: NEE measurements were conducted in two adjacent grassland sites, non-grazing (NG) and moderate grazing (MG), during 2006-2007. We then used daily weather data for 1978-2007 in conjunction with soil properties and grazing scenarios as model inputs to simulate grassland productivity and C dynamics. The observed and simulated CO fluxes under moderate grazing intensity were compared with those without grazing. Results and discussion: NEE data from 2-year observations showed that moderate grazing significantly decreased grassland ecosystem CO release and shifted the ecosystem from a negative CO balance (releasing 34.00 g C m) at the NG site to a positive CO balance (absorbing −43.02 g C m) at the MG site. Supporting our experimental findings, the 30-year simulation also showed that moderate grazing significantly enhances the CO uptake potential of the targeted grassland, shifting the ecosystem from a negative CO balance (57.08 ± 16.45 g C m year) without grazing to a positive CO balance (−28.58 ± 14.60 g C m year) under moderate grazing. The positive effects of grazing on CO balance could primarily be attributed to an increase in productivity combined with a significant decrease of soil heterotrophic respiration and total ecosystem respiration. Conclusions: We conclude that moderate grazing prevails over no-management practices in maintaining CO balance in semiarid grasslands, moderating and mitigating the negative effects of global climate change on the CO balance in grassland ecosystems. [ABSTRACT FROM AUTHOR]

Abstract: Precipitation quantity and timing have been shown to fundamentally influence grassland ecosystem CO2 exchange by both model simulation and field observation. Carbon dioxide exchange and patterns of variation (magnitude and timing) in precipitation were observed to determine their relationship in typical steppe in Inner Mongolia Plateau during four years (2003–2006) with different rainfall patterns. The response of ecosystem-level CO2 fluxes to rainfall pulse events was also examined. The four years studied were classified into three levels of rainfall amount (100%, 2004; reduced by 70% in 2005 and by 45% in 2006 compared with 2004) and two temporal patterns of rain events (average temporal patterns in 2004 and altered precipitation timing in 2003, a year with precipitation fell during the first half of the growing season, compared with 2004). The grassland ecosystem switched from a carbon sink (2004) to carbon sources (2005 and 2006) under reduced rainfall amounts. Carbon sink reduced by 34% under variational rainfall timing conditions (2003). There was a significant positive correlation between annual rainfall and above-ground NPP across the study (P =0.05, r 2 =0.9). Antecedent soil water before rainfall pulse and rainfall pulse size are important in controlling the response of CO2 exchange to rainfall in this steppe. After rainfall pulse events, CO2 fluxes had 4 to 5days of system-level hysteresis and the increase degree of CO2 exchange was different among the four years. Precipitation pulse size not only affected the duration of CO2 fluxes, but also instant flux rate. Our results suggest that CO2 exchange in temperate steppe will be more sensitive to altered rainfall timing and more affected by changes in rainfall quantity, with a longer dry season resulting in significant C losing from Eurasia grassland in Inner Mongolia Plateau. [Copyright &y& Elsevier]

Summary Nitrogen (N) uptake by plant roots from soil is the largest flux within the terrestrial N cycle. Despite its significance, a comprehensive analysis of plant uptake for inorganic and organic N forms across grasslands is lacking. Here we measured in situ plant uptake of 13 inorganic and organic N forms by dominant species along a 3000 km transect spanning temperate and alpine grasslands. To generalize our experimental findings, we synthesized data on N uptake from 60 studies encompassing 148 plant species world‐wide. Our analysis revealed that alpine grasslands had faster NH4+ uptake than temperate grasslands. Most plants preferred NO3− (65%) over NH4+ (24%), followed by amino acids (11%). The uptake preferences and uptake rates were modulated by soil N availability that was defined by climate, soil properties, and intrinsic characteristics of the N form. These findings pave the way toward more fully understanding of N cycling in terrestrial ecosystems, provide novel insights into the N form‐specific mechanisms of plant N uptake, and highlight ecological consequences of chemical niche differentiation to reduce competition between coexisting plant species. [ABSTRACT FROM AUTHOR]

Grassland remote sensing (GRS) is an important research topic that applies remote sensing technology to grassland ecosystems, reflects the number of grassland resources and grassland health promptly, and provides inversion information used in sustainable development management. A scientometrics analysis based on Science Citation Index-Expanded (SCI-E) was performed to understand the research trends and areas of focus in GRS research studies. A total of 2692 papers related to GRS research studies and 82,208 references published from 1980 to 2020 were selected as the research objects. A comprehensive overview of the field based on the annual documents, research areas, institutions, influential journals, core authors, and temporal trends in keywords were presented in this study. The results showed that the annual number of documents increased exponentially, and more than 100 papers were published each year since 2010. Remote sensing, environmental sciences, and ecology were the most popular Web of Science research areas. The journal Remote Sensing was one of the most popular for researchers to publish documents and shows high development and publishing potential in GRS research studies. The institution with the greatest research documents and most citations was the Chinese Academy of Sciences. Guo X.L., Hill M.J., and Zhang L. were the most productive authors across the 40-year study period in terms of the number of articles published. Seven clusters of research areas were identified that generated contributions to this topic by keyword co-occurrence analysis. We also detected 17 main future directions of GRS research studies by document co-citation analysis. Emerging or underutilized methodologies and technologies, such as unmanned aerial systems (UASs), cloud computing, and deep learning, will continue to further enhance GRS research in the process of achieving sustainable development goals. These results can help related researchers better understand the past and future of GRS research studies. [ABSTRACT FROM AUTHOR]

Soils δ13C and δ15N are now regarded as useful indicators of nitrogen (N) status and dynamics of soil organic carbon (SOC). Numerous studies have explored the effects of various factors on soils δ13C and δ15N in terrestrial ecosystems on different scales, but it remains unclear how co-varying climatic, edaphic and biotic factors independently contribute to the variation in soil δ13C and δ15N in temperate grasslands on a large scale. To answer the above question, a large-scale soil collection was carried out along a vegetation transect across the temperate grasslands of Inner Mongolia. We found that mean annual precipitation (MAP) and mean annual temperature (MAT) do not correlate with soil δ15N along the transect, while soil δ13C linearly decreased with MAP and MAT. Soil δ15N logarithmically increased with concentrations of SOC, total N and total P. By comparison, soil δ13C linearly decreased with SOC, total N and total P. Soil δ15N logarithmically increased with microbial biomass C and microbial biomass N, while soil δ13C linearly decreased with microbial biomass C and microbial biomass N. Plant belowground biomass linearly increased with soil δ15N but decreased with soil δ13C. Soil δ15N decreased with soil δ13C along the transect. Multiple linear regressions showed that biotic and edaphic factors such as microbial biomass C and total N exert more effect on soil δ15N, whereas climatic and edaphic factors such as MAT and total P have more impact on soil δ13C. These findings show that soil C and N cycles in temperate grasslands are, to some extent, decoupled and dominantly controlled by different factors. Further investigations should focus on those ecological processes leading to decoupling of C and N cycles in temperate grassland soils. [ABSTRACT FROM AUTHOR]

The alpine grassland on the Qinghai-Tibet Plateau covers an area of about 1/3 of China's total grassland area and plays a crucial role in regulating grassland ecological functions. Both environmental changes and irrational use of the grassland can result in severe grassland degradation in some areas of the Qinghai-Tibet Plateau. However, the magnitude and patterns of the physical and anthropogenic factors in driving grassland variation over northern Tibet remain debatable, and the interactive influences among those factors are still unclear. In this study, we employed a geographical detector model to quantify the primary and interactive impacts of both the physical factors (precipitation, temperature, sunshine duration, soil type, elevation, slope, and aspect) and the anthropogenic factors (population density, road density, residential density, grazing density, per capita GDP, and land use type) on vegetation variation from 2000 to 2015 in northern Tibet. Our results show that the vegetation index in northern Tibet significantly decreased from 2000 to 2015. Overall, the stability of vegetation types was sorted as follows: the alpine scrub > the alpine steppe > the alpine meadow. The physical factors, rather than the anthropogenic factors, have been the primary driving factors for vegetation dynamics in northern Tibet. Specifically, meteorological factors best explained the alpine meadow and alpine steppe variation. Precipitation was the key factor that influenced the alpine meadow variation, whereas temperature was the key factor that contributed to the alpine steppe variation. The anthropogenic factors, such as population density, grazing density and per capita GDP, influenced the alpine scrub variation most. The influence of population density is highly similar to that of grazing density, which may provide convenient access to simplify the study of the anthropogenic activities in the Tibet plateau. The interactions between the driving factors had larger effects on vegetation than any single factor. In the alpine meadow, the interaction between precipitation and temperature can explain 44.6% of the vegetation variation. In the alpine scrub, the interaction between temperature and GDP was the highest, accounting for 27.5% of vegetation variation. For the alpine steppe, the interaction between soil type and population density can explain 29.4% of the vegetation variation. The highest value of vegetation degradation occurred in the range of 448–469 mm rainfall in the alpine meadow, 0.61–1.23 people/km2 in the alpine scrub and –0.83–0.15 °C in the alpine steppe, respectively. These findings could contribute to a better understanding of degradation prevention and sustainable development of the alpine grassland ecosystem in northern Tibet. [ABSTRACT FROM AUTHOR]

[Display omitted] Structural information of grassland changes on the Tibetan Plateau is essential for understanding alterations in critical ecosystem functioning and their underlying drivers that may reflect environmental changes. However, such information at the regional scale is still lacking due to methodological limitations. Beyond remote sensing indicators only recognizing vegetation productivity, we utilized multivariate data fusion and deep learning to characterize formation-based plant community structure in alpine grasslands at the regional scale of the Tibetan Plateau for the first time and compared it with the earlier version of Vegetation Map of China for historical changes. Over the past 40 years, we revealed that (1) the proportion of alpine meadows in alpine grasslands increased from 50% to 69%, well-reflecting the warming and wetting trend; (2) dominances of Kobresia pygmaea and Stipa purpurea formations in alpine meadows and steppes were strengthened to 76% and 92%, respectively; (3) the climate factor mainly drove the distribution of Stipa purpurea formation, but not the recent distribution of Kobresia pygmaea formation that was likely shaped by human activities. Therefore, the underlying mechanisms of grassland changes over the past 40 years were considered to be formation dependent. Overall, the first exploration for structural information of plant community changes in this study not only provides a new perspective to understand drivers of grassland changes and their spatial heterogeneity at the regional scale of the Tibetan Plateau, but also innovates large-scale vegetation study paradigm. [ABSTRACT FROM AUTHOR]

Climate change and grassland degradation have become great challenges for ecosystem multifunctionality, which is mediated by microbial communities and their interactions. However, influences of warming and grassland degradation on soil microbial interactions and their relationship with ecosystem multifunctionality still remain vague, and it is unclear whether warming could exacerbate or mitigate degradation from these prospects. In this study, we conducted a field study in alpine grasslands on the Qinghai-Tibet Plateau to investigate the effects of warming and grassland degradation (i.e., non-degraded, ND; moderately degraded, MD; and severely degraded, SD) on soil microbial network properties and their association with ecosystem multifunctionality comprising nutrient provision, biomass production, and ecosystem processes indices. Results showed that microbial network complexity, characterized by edge number, node degree, graph diameter, and graph density, was 109.5∼238.8 % higher in MD than in ND or SD, likely explained by the intermediate disturbance hypothesis. Warming stimulated the network complexity by 78.7 ∼ 92.8 % in MD due to the prevalence of prokaryote and the escalation of their negative interactions, likely implying the fiercer resource competition that may restrain the grassland restoration; while warming weakened it by 35.4 ∼ 59.9 % and 35.0 ∼ 47.4 % in ND and SD, respectively, due to declines in all interaction types, indicating undermined structure of microbial interactions that may prevent the restoration of degradation grassland. Consistently, the ecosystem multifunctionality was not stimulated by warming significantly in various degradation stages. Interestingly, the microbial network complexity, rather than diversity, was positively correlated with the ecosystem's multifunctionality under control, which was eliminated by warming. Overall, warming changed the microbial network complexity by making the strong stronger in MD while the weak weaker in ND and SD. These divergent changes by warming decouple associations of microbial network complexity with ecosystem multifunctionality. Altered microbial network complexity under future warming scenarios may tend to hamper the multifunctionality restoration of alpine grasslands, though with different mechanisms in various degradation stages. • The network is dominated by positive intra-fungi's interactions despite its lower richness. • Warming interacts with grassland degradation to alter microbial network complexity. • Ecosystem multifunctionality declines in the severely-degraded alpine grassland. • Network complexity correlated with multifunctionality, but its decoupled under warming. [ABSTRACT FROM AUTHOR]

• Precipitation was repackaged to simulate 4-level of precipitation variability. • Extra-extreme precipitation variability scenario potentially reduced N 2 O emissions. • Effects of precipitation variability on N 2 O fluxes had high interannual variations. • Soil, microbes, functional genes, and plants explained 61% variation in N 2 O fluxes. High temporal precipitation variability, characterized by less frequent but larger-magnitude precipitation events, is increasing. However, how precipitation variability affects N 2 O emissions and the underlying mechanisms remain unclear, especially at multiple-year scales. We conducted a 3-year manipulative experiment in which the same long-term mean growing season precipitation total was repackaged into events of inversely varying magnitude and frequency to simulate four levels of precipitation variability (extra-extreme, extreme, medium, and normal) in a semiarid grassland. Cumulative N 2 O emissions was the smallest under the extra-extreme precipitation variability scenario (6 very large rainfall events), 26% less than emission under the other three treatments (10, 16, and 24 rainfall events) over the three growing seasons. However, this difference was almost entirely due to three sampling events in the third year. Plant community (biomass and biodiversity), soil abiotic properties (water, dissolved organic carbon and pH), soil microbial biomass (carbon, nitrogen and the ratio), and soil functional genes (archaeal and bacterial amoA, nirS, nirK, narG , and nosZ) explained 61% of the variation in N 2 O emissions in response to the precipitation variability. Structural equation modelling indicated that the precipitation variability had direct positive effects, and indirect negative effects via soil abiotic properties, on soil functional genes that ultimately had positive effects on N 2 O emissions. The reduction in N 2 O emission in the extra-extreme precipitation variability scenario in the third year was due to low levels of soil water content, soil pH, aboveground biomass, and microbial biomass carbon simultaneously. Our results suggest that at the multi-year timescale, semiarid grasslands may have negative feedbacks to future precipitation regimes with higher variability. [Display omitted] [ABSTRACT FROM AUTHOR]

[Display omitted] • Devised a unique Sustainable Livelihood Index (SLI) using a top-down data collation method. • Spatio-temporal nuances of the SLI revealed through spatial visualization and cold hotspot analysis in TRHR. • Yushu county showed significant growth; Jiuzhi identified as a primary cold spot in TRHR. • Grassland carrying pressure and income noted to be as hindrances for sustainable livelihoods. • There is a need for innovative solutions to evolve regional scale livelihood resilience. Applying novel approaches to assess the sustainable livelihood potentially contribute to learning and managing social-ecological systems. This research demonstrates the trajectory of sustainable livelihoods in the Three Rivers Headwater Region (TRHR) of Western China for a period of 2003 to 2017, utilizing a bespoke Sustainable Livelihood Index (SLI). Devised through a top-down data collation method, the SLI integrates comprehensive official statistics, encompassing socio-economic metrics alongside climatic and other environmental conditions. We employed spatial visualization and cold hotspot analysis to discern the spatio-temporal nuances of the SLI. Furthermore, the Obstacle Factor Analysis (OFA) was implemented to identify salient barriers impacting the SLI. Results indicated a steady rise in the SLI over the examined period, indicating progressive enhancements in the livelihood sustainability. However, marked disparities were observed across the TRHR counties, reflecting heterogeneity in socioeconomic progress and resource allocation. While Yushu county displayed commendable growth, Jiuzhi county stood out as a significant cold spot. Notably, natural and financial resources surfaced as paramount stresses to SLI progression, emphasizing the necessity for robust resource management and economic support. Key challenges, such as the grassland carrying pressure index, deteriorated grassland territories, and per capita disposable income were identified as critical obstacles to the sustainable livelihood of pastoral communities. These factors collectively influence economic viability, grassland resource stewardship, and human capital development, which are intertwined with local economic gains and future potentials. In nutshell, this study highlights the multifaceted challenges faced by the TRHR in charting sustainable livelihood pathways and champions recalibrated policies to protect natural assets, amplify economic support, reinforce human capital, and nurture regional synergies for sustainable evolution. The key insights suggest the pressing need for innovative, nature-based solutions that amplify regional-scale livelihood resilience, especially considering the challenges related to natural assets, financial support, and human capital development. [ABSTRACT FROM AUTHOR]

Highlights • Grazing interacted extreme drought timing to regulate steppe ecosystem CO 2 exchange. • Extreme drought reduced CO 2 uptake in a fenced steppe regardless of seasonal timing. • Early-season drought caused largest declines in CO 2 uptake in a grazed steppe. Abstract Climate models predict a substantial increase in the frequency of extreme drought, suggesting subsequent impacts on the carbon (C) and water cycles. Although many studies have investigated the impacts of extreme drought on ecosystem functioning, it remains unknown how the timing of extreme drought within a growing season may affect carbon and water cycling. Here we conducted a 3-year field experiment to investigate the influence of seasonal drought timing on ecosystem carbon and water exchange by excluding rainfall (for consecutive 30 days) during three periods of the growing season (May–June, July–August and August–September) in fenced and grazed sites of a semiarid temperate steppe in Inner Mongolia, China. In the fenced steppe, extreme drought reduced growing-season net CO 2 uptake regardless of drought timing, while in the grazed steppe, early-growing season drought caused relatively larger reductions to net CO 2 uptake than drought imposed later in the season. The effect of extreme drought on evapotranspiration (ET) was similar to that of CO 2 exchange at the fenced site, with consistent reductions of seasonally-integrated ET for all treatments compared with the ambient condition. In contrast, at the grazed site, the response of ET to extreme drought was more variable, possibly due to the absence of litter and greater bare ground. Surprisingly, both gross and net carbon uptake declined with increasing ET at the grazed site, while the fenced site showed the positive water-carbon linkage typically seen in semiarid ecosystems. The different responses of CO 2 and water exchanges for the fenced and grazed sites were regulated predominately by soil temperature and soil water content. Together, our results show that drought timing within the growing season can significantly alter drought impacts on ecosystem water and CO 2 exchanges, and that grazing management may further mediate the response. [ABSTRACT FROM AUTHOR]

Biological nitrogen fixation, conducted by soil diazotrophs, is the primary nitrogen source for natural grasslands. However, the diazotrophs in grassland soils are still far from fully investigated. Particularly, their regional-scale distribution patterns have never been systematically examined. Here, soils (0–5 cm) were sampled from 54 grasslands on the Tibetan Plateau to examine the diazotroph abundance, diversity, and community composition, as well as their distribution patterns and driving factors. The diazotroph abundance was expressed as nifH gene copies, measured using real-time PCR. The diversity and community composition of diazotrophs were analyzed through MiSeq sequencing of nifH genes. The results showed that Cyanobacteria (47.94%) and Proteobacteria (45.20%) dominated the soil diazotroph communities. Most Cyanobacteria were classified as Nostocales which are main components of biological crusts. Rhizobiales, most of which were identified as potential symbiotic diazotrophs, were also abundant in approximately half of the soil samples. The soil diazotroph abundance, diversity, and community composition followed the distribution patterns in line with mean annual precipitation. Moreover, they also showed significant correlations with prokaryotic abundance, plant biomass, vegetation cover, soil pH values, and soil nutrient contents. Among these environmental factors, the soil moisture, organic carbon, available phosphorus, and inorganic nitrogen contents could be the main drivers of diazotroph distribution due to their strong correlations with diazotroph indices. These findings suggest that autotrophic and symbiotic diazotrophs are the predominant nitrogen fixers in Tibetan grassland soils, and highlight the key roles of water and nutrient availability in determining the soil diazotroph distribution on the Tibetan Plateau. [ABSTRACT FROM AUTHOR]

• Seasonal timing strongly regulates the response of N 2 O emissions to extreme droughts. • Early drought affected N 2 O emission with high interannual variability. • Middle drought consistently suppressed N 2 O emissions. • Late drought had little effect on N 2 O fluxes. Terrestrial ecosystems are important sources of nitrous oxide (N 2 O), a powerful greenhouse gas which can be strongly impacted by increasing droughts in association with climate change. However, detailed information on whether and how drought timing regulates N 2 O fluxes is still lacking. Here, we conducted a 3-year field experiment on a semiarid grassland in which extreme drought was imposed in either early-, mid-, or late-growing seasons repeatedly from 2014 to 2016. We found that early drought affected N 2 O emission with high interannual variability (increased, decreased and unchanged N 2 O emission in 2014, 2015, and 2016, respectively), coincident with changes in inorganic nitrogen (SIN), dissolve organic carbon (DOC), microbial biomass carbon (MBC), and soil functional genes (bacterial amoA , nirK , nirS , and nosZ). However, middle drought consistently suppressed N 2 O emissions due to simultaneous decreases in MBC, DOC and the abundances of archaeal amoA , nirK , and narG genes , causing the largest reduction in N 2 O emissions across the three years. In contrast, late drought had little effect on N 2 O fluxes, even though DOC and SIN decreased and the abundance of nirK , nirS , and nosZ increased. As a result, soil organic C and mineral N availability and functional gene abundances were not always robust factors for predicting N 2 O emissions under droughts across all treatments, except for abundance of AOA and nosZ. Our results highlight the vital role of seasonal timing in regulating the response of N 2 O emissions to extreme droughts. [ABSTRACT FROM AUTHOR]

The plant productivity and species richness (P-SR) relationship has been a central topic in ecology for decades. It is increasingly recognized that climate resources (CR, temperature and precipitation) might affect the P-SR relationship as (1) the response of plant productivity and species richness to CR are different; (2) plant-plant interactions change along the CR gradient. However, little is known about how these effects mediate the shape and strength of the P-SR relationship from local-site to regional scales. Here, we proposed a cross-scale framework to disentangle the climatic control over the P-SR relationship in grasslands using the HerbDivNet dataset, which covers 28 natural grasslands in 19 countries and 6 continents with wide precipitation and temperature gradients. Our results showed that, at the local-site scale, a humped-back P-SR relationship is common at sites under poor and rich CR conditions, whereas both the shape and strength of the P-SR relationship strongly vary among sites under moderate CR condition. However, at the regional scale, the humped-back P-SR relationship is only observed under poor CR condition. We suggested that the asynchronous responses of productivity and species richness to increasing CR is an abiotic driver of the inconsistent P-SR relationships along the climatic gradient in addition to the changing plant-plant interactions suggested by the stress-gradient hypothesis. These findings help to reconcile the long-standing debate over the P-SR relationship and highlight the importance of CR conditions in developing or comparing grassland ecosystem theories and management at small scales. • Climate effects cause inconsistent P-SR relationships from local to regional scales. • Asynchronous response of P and SR to CR is an abiotic driver of P-SR relationship. • Unimodal P-SR relationship at regional scale only exists with poor CR. • None unimodal P-SR relationships at site scale are mostly observed with moderate CR. • Shape and strength of P-SR relationship strongly vary among sites with moderate CR. [ABSTRACT FROM AUTHOR]

• Belowground biomass was positively sensitive to early and middle droughts. • Belowground biomass was negatively sensitive to late drought. • Aboveground biomass was insensitive to all droughts. • CO 2 fluxes had the largest sensitivity to early drought. • CO 2 fluxes had the lowest sensitivity to middle drought. Quantifying the sensitivity of ecosystems to droughts, particularly with different seasonal timing, could improve our predictions of ecosystem-climate feedbacks, but few experiments have explicitly addressed seasonal timing per se effects on ecosystem sensitivity to droughts. Here, we present a seasonal timing × drought manipulation experiment to examine sensitivity (relative change in response parameters to the relative change in precipitation) of key ecosystem processes (community biomass and ecosystem CO 2 fluxes) to pulse-drought with different seasonal timing (early, middle or late) on a temperate semiarid grassland. We found belowground and total biomass were positively sensitive (i.e. ecological processes promoted by droughts and vice versa) to early and middle droughts but negatively sensitive to late drought while aboveground biomass was insensitive to all droughts. Ecosystem CO 2 fluxes had the largest negative sensitivity to early drought and smallest negative sensitivity to middle drought, although gross ecosystem production showed larger negative response to droughts than ecosystem respiration, leading to reduction in net ecosystem production, regardless of seasonal timing. Our results highlight the crucial role of seasonal drought timing in regulating sensitivity of key carbon cycle processes to droughts and suggest that droughts at plant peak stage cause the least detrimental ecological consequences. [ABSTRACT FROM AUTHOR]

• Grassland degradation is predictable on a large scale by Random Forest. • Specific microbial groups, mainly Solirubrobacter, were screened as credible biomarkers. • Grassland degradation exhibited a longitudinal pattern and was correlated with current Net primary productivity. Extensive grassland degradation under climate change and intensified human activities has threatened ecological security and caused a variety of environmental problems. However, it is still challenging to predict the grassland degradation status on a large scale because it is a multi-factorial phenomenon with complex changes in ecosystem structure and function, which is hard to be fully characterized through mechanism models. The emergence of machine learning algorithms provides a potential to model complex systems and mine information from multi-source data without elucidating underlying mechanisms. Here, we utilized random forest and neural network algorithms to predict the grassland degradation represented by the net primary productivity (NPP) changing rate based on multi-source data including geographic, meteorological, plant traits, land use type and microbial variables in the Chinese Northern grassland. Particularly, the microbial roles in determining the degradation status were concerned. Results show that a high prediction precision was achieved by random forest model, rather than by neural network model, with a mean relative error of 16.9% and a mean square error of 9.273e-05. Besides identified longitude, arid index and current NPP state, specific soil microbial groups, mainly Solirubrobacter, were screened as credible biomarkers. Regarding model fitting, geographic, meteorological and plant variables explained 61.8% of the total variance, which was enhanced up to 72.8% by the rest microbial markers. These findings provide a theoretical basis to establish a pre-warning system for grassland management and policy-making. [ABSTRACT FROM AUTHOR]

Increasing heavy rainfalls can strongly affect ecosystem nitrogen (N) cycling processes and thereby alter soil N fluxes and pools. However, the effects of heavy rainfalls on soil N fluxes and pools are poorly understood, particularly with regards to high rainfall timing under field conditions. We conducted a 3-year (2014–2016) manipulative experiment in which heavy rainfall was imposed in middle (plant peak growing stage) or late (plant senescent growth stage) growing season in a semiarid grassland of Inner Mongolia, China to explore the responses of N 2 O fluxes and soil total N contents and the underlying microbial mechanisms. Mid-season heavy rainfall promoted soil N 2 O emissions by 65% on average across the three years, attributable mainly to increases in denitrifying nirK and nirS abundances induced large denitrification at higher soil water contents. However, archaeal and bacterial amoA and narG genes did not change significantly due probably to counteracting effects of increased soil water content (positive) and soil pH (negative). Mid-season heavy rainfall led to 17% reduction in soil total N by the end of the last year of the study (2016), partly due to the enhanced accumulated N 2 O emissions over the three years. In contrast, late-season heavy rainfall did not change N 2 O emissions and soil total N contents even though soil water content, soil pH and nirK and nirS abundance were significantly increased, perhaps due to limitation by low temperature. Timing of the heavy rainfall events during the plant growing season strongly influenced their impacts on soil N fluxes and pools and heavy rainfalls in the peak stage of plant growth may potentially cause a positive feedback to global warming and exacerbate N limitation in terrestrial ecosystems. • N 2 O flux and soil total N responses to heavy rainfall depended on seasonal timing. • Mid-season heavy rainfall increased N 2 O flux by 65% due to higher denitrification. • Enhanced N 2 O emissions coincided with 17% of reduction in soil total N. • Late-season heavy rainfall did not change N 2 O emissions and soil total N contents. [ABSTRACT FROM AUTHOR]

• Non-linear patterns of microbial N genes occurred along precipitation gradient. • Breaking points were clearly related to certain soil types. • Attribution of different environmental factors varied by ecosystem. Despite the importance of microorganisms in soil nitrogen (N) cycling, studies on spatial patterns of microbial N-cycling gene abundances in temperate grasslands are still lacking, whose productivity is limited by N. Here, we investigated N functional genes in soil microorganisms from 60 sites in temperate grasslands across 1661 km in Inner Mongolia, China. Abundances of all N functional genes and 16S rDNA tended to decrease from northeast to southwest, consistent with the changing tend of precipitation but contrary to that of temperature. Non-linear saturation curves dominated patterns for abundances of most N functional genes and 16S rDNA along precipitation as they increased with the rising precipitation when <288–343 mm, but remained stable after these breaking points. Interestingly, these breaking points were clearly related to certain soil types, e.g. nifH did not change with precipitation in Chernozems and Gleysols, but increased with it in other soil types. Non-linear saturation patterns were also observed for most N functional gene and 16S rDNA abundances with temperature and soil total organic carbon. In contrast, no consistent spatial pattern for ratios of N functional genes: 16S rDNA was observed, which varied greatly by different N functional genes. Moreover, decay relationships were discovered for the abundance-based matrix of N functional genes over geographic distance. From the temperate meadows to typical steppes and to temperate desert steppes, the influence of relatively long-term environmental variables increased, but that of short-term ones decreased. Overall, we revealed non-linear patterns of N functional gene abundances along precipitation with a clear relationship with soil type, and discovered that attributions of geographic distance, plant community, historical-contingency and contemporary-disturbance to N functional gene community similarity decay were ecosystem–dependent. [ABSTRACT FROM AUTHOR]

• Structural stable state triggers a change in functional stable state. • Long-term ungrazed steppe had an abrupt transition in plant functional type and followed a shift in ecosystem function. • Moderate grazed activity no abrupt transitions for stable states. Theoretical models suggest that ecosystems can be found in one of several possible alternative stable states, and a shift in structural stable state (SSS) can trigger a change in functional stable state (FSS). But we still lack the empirical evidence to confirm these states and transitions, and to inform the rates of change. Here, a 30-yr dataset from long-term ungrazed and grazed temperate grasslands was analyzed to determine whether abrupt transitions of SSS and FSS can occur. We found that the long-term ungrazed grassland experienced abrupt transitions in the dominant plant functional type (shift in SSS) that was followed by a transition between carbon sink and source 1–2 year later (shift in FSS). A directional shift in precipitation and temperature accounted for 40% of the variation in the SSS transition, while the SSS transition explained 20% of the variation in the FSS transition. In contrast, no abrupt transitions for SSS and FSS were observed in the long-term moderately grazed grassland. These findings provide important insight into the interacting effects of climate change and livestock grazing on ecosystem transitions in temperate grasslands. Moderate utilization of production in ecosystems that have co-evolved with herbivores can offset structural and functional transitions induced by climate change. [ABSTRACT FROM AUTHOR]