Aims Soil nitrogen (N) plays a vital role in regulating the structure and function of ecosystems and is affected by N deposition. Most previous studies focus on the responses of the N content in bulk soil to N deposition, but the responses of the N content in different soil organic matter (SOM) fractions remain unclear. We aimed to investigate how long-term N addition influenced soil N of different SOM fractions in a semi-arid grassland.

Methods A manipulated N addition experiment with 4 levels of N addition (0, 8, 32 and 64 g·m^-2·a^-1) has been conducted for 13 years in Duolun country, Nei Mongol. SOM was separated to particulate organic matter (POM) and mineral associated organic matter (MAOM) by density fractionation. The plant and soil properties were also measured.

Important findings The results showed that N addition had no significant effect on the carbon (C) content in bulk soil, POM, or MAOM. With increasing levels of N addition, the N content in bulk soil and in POM increased significantly. Furthermore, we found that the increased N content of POM was mainly associated with greater aboveground biomass following N addition. The N content of MAOM is mainly correlated with soil texture, but was not affected by N addition. These results suggest that continuous N addition can increase the soil N in bulk soil, but the increased N is mostly distributed in labile POM pools, which can be vulnerable to land use and climate change.

Aims It is very important to investigate the relationships between litter decomposition characteristics and plant functional traits in understanding the maintenance mechanism of ecosystem functions.

Methods In order to study the main driving factors that affect the leaf litters and root decomposition of different species, this study took the leaf litters and roots of six main plant species Stipa grandis, Cleistogenes squarrosa, Anemarrhena asphodeloides, Leymus chinensis, Convolvulus ammannii and Carex korshinskyi in S. grandis steppe. The litter bag method was used to study the decomposition rate constant of both leaf litters and root through 501 days of field incubation. Plant functional traits including leaf dry matter content, root specific surface area, root tissue density, contents of C, N and different cellulose components of the leaf and root litters were determined and the relationships between decomposition characteristics and functional traits of leaf litters and root across six plant species were examined.

Important findingsThe results showed that there were significant interspecific differences in leaf and root traits of six plant species. The ratios of maximum to minimum values for most traits were between 1 and 2, while the difference in some traits, such as C:N and specific surface area of roots between species was nearly 4 times. For the six plant species, the overall trend of the mass residue and decomposition rate constant of the leaf litter and root during 501 days of decomposition all showed the rapid decomposition in the early stage, relatively slow decomposition in the middle stage and the slowest decomposition in the later stage. During the decomposition process of leaf litters and roots, Cleistogenes squarrosa showed the slowest one, while the leaf litter decomposition of Anemarrhena asphodeloides was the fastest, and the root decomposition of Convolvulus ammannii was the fastest. Through the correlation analysis and stepwise regression analysis, it was found that the decomposition process of leaf litters and roots was affected by different traits in different decomposition periods. The structural carbohydrate content was the main factor affecting the early and late decomposition of litters and the early decomposition of roots, while the non-structural carbohydrate content was the main factor affecting the middle and late decomposition of roots. In addition, the decomposition rate of leaf litters in the middle stage of decomposition was mainly affected by leaf dry matter content, while the decomposition rates of roots in the middle and late stages of decomposition were also significantly affected by C:N and N content, respectively. Our results present the important guide for the prediction of carbon and nutrient cycling process in the S. grandis steppe.

Aims The balance between soil organic carbon (SOC) input and output processes determines SOC content. However, it is not clear which of the two processes dominantly affect SOC content during the degradation of alpine meadows in Zoigê Wetland. In this study, the changes in SOC contents of alpine meadows and their causes at different degradation stages (alpine meadow (AM), slightly degraded alpine meadow (SD), and heavily degraded alpine meadow (HD)) in the Zoigê Wetland were investigated using the method of spatial sequence instead of temporal successional sequence.
Methods First, the changes in C input to soil and their causes along the degradation gradient were analyzed by investigating main soil physicochemical properties, microbial biomass, plant biomass and community composition of plant functional groups at different degradation stages. Secondly, the changes in the C output from soil were estimated based on lab incubation experiments of soil C mineralization and the temperature sensitivity of soil respiration (Q₁₀) and monthly average air temperature of the Zoigê Wetland. Finally, the main causes and processes leading to changes in SOC content along the degradation gradient were analyzed.
Important findings The results showed that soil water content (SWC), SOC content, total nitrogen (TN) content, microbial biomass C and N content decreased with the increase of degradation. Plant community composition gradually changed from sedges and grasses dominated community to forbs dominated community. Plant biomass and SOC mineralization rate decreased during the degradation of alpine meadows. The potential accumulation of organic C reduced during the degradation (compared with AM, the potential input, output and accumulation of organic C in SD and HD decreased by 16%, 18%, 15% and 59%, 63%, 41%, respectively). The decrease in SWC changed soil physical and chemical properties, including bulk density, SOC content, TN content, total phosphorus content, and C:N, which led to the shifts in the distribution pattern of plant functional groups and in soil microorganisms, consequently reducing the inputs and outputs of SOC. The decrease in potential plant-derived C input to soil caused by decreased SWC was the main reason for the decline in SOC content along the degradation gradient of alpine meadows in Zoigê Wetland.

Aims Livestock grazing is one of the most important factors affecting grassland plant diversity. However, the information on the effects of different livestock types and their grazing behaviors on grassland plant diversity and community composition are less available. A better understanding of the changes in plant diversity and community composition in response to the grazing of various livestock types is essential to the management and preservation of grassland biodiversity.

Methods We conducted a grazing experiment in a typical steppe of Nei Mongol to examine the effects of different livestock species (cattle, goat, sheep) and their behaviors (forage selection and grazing aggregation) on plant diversity (i.e., α, β and γ diversity) and community composition.

Important findings Our results showed that: (1) cattle, goat, and sheep grazing all increased plant α, β, and γ diversity at moderate grazing intensity, and the increase was the largest and significant under cattle grazing. (2) Three livestock species all changed community structure; sheep grazing reduced the relative abundance of dominant short grass Cleistogenes squarrosa, which is in contrary to the changes in community structure induced by cattle and goat grazing. (3) Cattle and goat grazing significantly reduced the aboveground biomass of dominant species, including tall grasses Leymus chinensis and Stipa grandis and short grass C. squarrosa, while sheep grazing only decreased that of short grass C. squarrosa. Cattle grazing also had a lower spatial aggregation than that of goat and sheep. (4) Plant diversity decreased with the increase of the aboveground biomass of either tall or short dominant species, indicating that livestock grazing promoted plant species diversity by reducing plant aboveground biomass of dominant species. (5) Plant diversity decreased with the increase in spatial aggregation of livestock grazing, indicating a lower aggregation benefiting plant diversity maintenance. Overall, our study suggests that grazing animal types should be considered along with grazing intensity in the development of grazing management regime for better conservation and sustainable use of the grassland resources.

Aims Grazing exclusion is an important intervention for restoring degraded grasslands. Understanding the changes of grassland productivity and plant diversity during the long-term grazing exclusion succession, as well as the relationship between diversity and productivity, are helpful for grassland restoration management and utilization.

Methods This study was conducted in a typical grassland system with a long-term grazing exclusion gradient, located at Yunwu Mountain National Nature Reserve in Ningxia Huizu Autonomous Region. The grassland communities under continuous grazing, 9 years of grazing exclusion, 26 years of grazing exclusion, and 34 years of grazing exclusion were chosen as the study objects. We tracked changes in aboveground net primary productivity (ANPP), species diversity and functional diversity, and quantified the relationships between diversity and productivity.

Important findings The results showed that grazing exclusion significantly increased ANPP, litter biomass, functional richness, and functional dispersion of typical grassland, but did not alter species richness, Shannon- Wiener index and functional evenness. In contrast, Simpson dominance index and Pielou evenness index significantly decreased after long-term grazing exclusion (34 years). In addition, grazing exclusion showed diverse effects on community-weighted means of different functional traits. The results of random forest model and variance partition analysis indicated that community-weighted mean functional traits accounted for 70.70% of ANPP variation, and plant height was identified as the most important trait. Functional diversity explained 36.86% of ANPP variation, mainly contributed by functional richness. In contrast, species diversity only explained 14.72% of ANPP variation. Therefore, the contribution of mean trait values and functional diversity to ANPP was much higher than that of species diversity in grassland after grazing exclusion. We suggest that community mean trait values and functional diversity should be incorporated into the studies of plant community dynamics during restoration succession, which will contribute to a comprehensive understanding of the relationship between plant diversity and ecosystem function, and provide basis for better realization of ecological restoration goals.

Understanding the response patterns and potential mechanisms of structure and function in grassland ecosystems to nitrogen (N) enrichment is essential to evaluate ecological impacts of external N input. The muti-level N manipulative experiment offers the possibility to explore the nonlinear response patterns and associated mechanisms of structure and function in grassland ecosystems to additional N input. In this review, we summarized the impacts of additional N inputs on community diversity, carbon (C) and N cycling in grassland ecosystems around the world. Numerous studies illustrated that N enrichment induced the decline of plant species diversity, plant functional diversity and soil bacteria richness in grassland ecosystems, yet the change of fungal diversity was not significant. Above- and below-ground plant productivity showed different responses to N input: aboveground plant productivity exhibited initial increasing and subsequent saturation trends, but root productivity and root:shoot ratio showed downward patterns, and root turnover rate appeared a single-peak pattern of first increasing and then decreasing with the continuous increase of N addition rate. Meanwhile, different C decomposition processes responded variously to N enrichment. Specifically, litter decomposition rates exhibited multiple response of “exponential decrease, liner increase or insignificant change with N addition level”. However, the relationship of soil respiration and CH₄ consumption with N addition was dominated by a single peak trend of increasing at low to medium N levels but declining at high N levels. Likewise, different soil C fractions showed multiple response patterns to N input. N addition generally stimulated soil C storage and particulate organic C accumulation, while the mineral-associated organic C exhibited divergent responses of “increase, unaltered, and decrease” along the N addition gradient. In addition, plant N uptake exhibited initial increasing and subsequent situation trends along N addition gradients, while different soil N transformation processes showed differentiated responses along N addition gradients and the relationship between N₂O emission and N addition rate varied among various grassland ecosystems. An exponential increase of N₂O fluxes with N addition rate was observed in temperate grasslands, while the patterns of first increase and then saturation or linear increase of the N-induced changes in N₂O emissions had been discovered in alpine grasslands. Future studies should focus on the nonlinear responses of rhizosphere processes and phosphorus (P) cycle to external N input, and also explore potential mechanisms from the aspect of multi-dimensional biodiversity changes.

Aims The objectives were to reveal the response patterns of plant species diversity and above-ground biomass to nutrients addition and to clarify their relationships in alpine grasslands of Tianshan Mountains.
Methods The nitrogen (N), phosphorus (P) and potassium (K) addition experiments were conducted in Tianshan alpine grasslands. The single factor effects and the interaction effects on plant species diversity and above-ground biomass were studied from 2019 to 2020.
Important findings 1) Nutrient addition reduced plant species diversity of local community. Especially, the addition of N + P, N + K and N + P + K showed significant effects, suggesting that the decrease of soil niche dimension caused by multiple-nutrient addition was an important reason for local species loss. 2) Nutrient addition significantly increased above-ground biomass of local plant communities, with the highest above-ground biomass being found under N + P + K treatment, indicating that N was the first limiting resource, P and K became the limiting resources after N limiting was alleviated. 3) There was a negative linear regression between species diversity and above-ground biomass following two years of nutrient addition, which indicated that the dominant species but not species diversity determined above-ground biomass at our study site.

Aims The relationship between biodiversity and ecosystem function is one of the hotspots in ecological research. In the past, the research on the relationship between biodiversity and ecosystem function only focused on the experimental or observational investigation of single ecosystem function (SEF), ignoring the most essential value that ecosystem can provide multiple functions and services at the same time. Identifying the relationship between plant functional diversity and ecosystem multifunctionality (EMF) can provide a clear understanding of changes in ecosystem function.

Methods In this study, Bayanbulak alpine meadow was taken as the study area, and five altitude sites were set at an interval of 200 m from 2 194 to 3 062 m above sea level. Soil total nitrogen content, nitrate nitrogen content, ammonium nitrogen content, total phosphorus content, available phosphorus content, total potassium content, available potassium content, soil density, aboveground and underground biomass of plant community were selected to characterize EMF, which were closely related to nutrient cycling, soil organic carbon accumulation and plant growth.

Important findings (1) The species composition of the plant community varied greatly along the altitude gradient, and the species richness at the altitude of 2 600 m was significantly higher than that at the other altitudes. Functional evenness index (FEve), functional richness index (FRic) and functional dispersion index (FDis) all showed a “single peak” trend with the rise of altitude, and the highest values were found at 2 600, 2 800 and 2 800 m, respectively. Rao’ quadratic entropy (Rao’Q) showed a monotonically decreasing trend. (2) FRic and FDis at each altitude were positively correlated with soil EMF, which accounted for 47% and 43% of the variation in EMF, respectively. FEve was significantly correlated with nutrient cycling index and soil organic carbon storage index at the altitude of 2 600 m. Rao’Q at 3 000 m was significantly correlated with soil nutrient cycling index, organic carbon storage and EMF. The relationship between plant functional diversity and EMF along the altitude gradient was analyzed by constructing a structural equation model, which showed that altitude could exert impacts on EMF through changing functional diversity, with the greatest effect of functional richness on EMF. In conclusion, with the alteration of altitude, the functional diversity may result in changes, thereby affect the SEF and EMF, and the functional diversity is important to maintain the EMF.

Aims Plant diversity is the basis for plant communities to maintain ecosystem stability. Despite the scarcity of vegetation, desert steppes play an irreplaceable ecological service function in terms of wind-break and sand- fixation, etc. However, how plant diversity in desert steppes responds to long-term extreme precipitation changes still remains poorly understood.

Methods Based on a long-term field experiment involving five precipitation treatments (50% reduction, 30% reduction, natural, 30% increase, and 50% increase) conducted in a desert steppe in Ningxia since 2014, the changing characteristics of plant biomass, species diversity and their relationships with soil properties were studied from May to October in 2020.

Important findings During the growing season, plant community biomass, Patrick richness index and Shannon-Wiener diversity index tended to increase first and then decrease, whereas no obvious regularities in Pielou evenness index and Simpson dominance index. Compared with the natural precipitation, the decreased precipitation had less effect on plant biomass and diversity, especially the 30% reduction in precipitation. In most cases, the increased precipitation stimulated the growth of Sophora alopecuroides, Stipa brevifloraand Pennisetum centrasiaticum,and thus increasing plant biomass. However, it did not significantly change plant diversity when precipitation increased, especially the 30% increase of precipitation. Plant biomass was significantly affected by soil urease activity, temperature, water content, pH, phosphatase activity and sucrase activity, while plant diversity was significantly affected by soil water content, electrical conductivity, and urease activity. In general, the results indicated that plants have high adaptability to moderate or even extreme drought in the research area under seven consecutive years of changing precipitation; moderately increasing precipitation increased soil water availability, enhanced exchangeable ion mobility, and stimulated enzyme activity, thereby promoting plant growth. However, the continuous increase of precipitation leaded to the increase of plant biomass and plant water consumption, resulting in the lack of soil water in the late growth season and then the early completion of the life cycle of some plants.

Aims Grazing, one of the primary ways of grassland utilization in Nei Mongol, has essential influences on plant community properties of grasslands. However, the comprehensive response patterns of Nei Mongol grassland plant community properties to grazing remain unclear.
Methods Based on a dataset derived from 76 studies, the plant community characteristics and soil physicochemical properties of Nei Mongol grasslands under different grazing intensities, different grassland types and different grazing years were integrated and analyzed in order to comprehensively evaluate the response patterns of Nei Mongol grasslands to grazing.
Important findings Our results showed that grazing significantly reduced plant above/below ground biomass, cover, height, density, species richness, Shannon-Wiener diversity index, Pielou evenness index, Simpson diversity index, and soil water content. The negative effects of grazing were strengthened with increasing of grazing intensity and duration. Moreover, grazing had a greater negative effect on the grasslands with sparse vegetation and low environmental carrying capacity (e.g., desert grasslands, sandy areas, etc.). This study shows that the responses of plant community characteristics to grazing in Nei Mongol grasslands are regulated by multiple factors, and appropriate grazing intensity and grazing time should be set according to different types of grasslands to achieve sustainable utilization of grasslands. The standards of grazing intensity in current grazing studies are not uniform, making it difficult to compare different studies, and the results from some studies do not have statistical significance due to a lack of replications in the experiment. The exploration of uniform quantitative standards for grazing intensity will be an important and challenging issue in future grazing studies, and the rationality of experimental design should also be emphasized.

Aims When the external environment changes, plants can change their own functional traits and adjust adaptation strategies in a timely manner. Therefore, plant functional traits can effectively reflect the response of plants to the change of grassland use. However, there are few studies on the effects of grassland use patterns from the perspective of plant functional traits in the Nei Mongol grassland. Therefore, from the perspective of functional traits, this paper reveals the adaptation strategies of plants after external disturbances, aiming to provide basic data support and scientific basis for the sustainable management of natural grasslands.
Methods In this study, four important dominant species of Stipa grandis, Leymus chinensis, Cleistogenes squarrosa and Artemisia frigida in the typical grasslands of Nei Mongol were selected as the research objects to explore the differences in the functional traits of dominant species under the influence of long-term free grazing, mowing, short-term enclosed and long-term reservation.
Important findings We found that: 1) Vegetation height, root length and carbon and nitrogen concentration of dominant plants in Nei Mongol typical grasslands are reduced under long-term grazing. Changes in these traits can miniaturize individual plants and reduce their palatability, indicating that plants may adopt avoiding grazing strategy to adjust to the interference of long-term free grazing. Under the mowing management, the vegetation height and specific leaf area of the dominant species tend to increase. Among them, the nitrogen content of A. frigida is the most sensitive to the response of mowing. The nitrogen concentration in its roots, stems, leaves are the lowest in the mowing sites; the carbon and nitrogen concentration of plants increases under enclosure and long-term non-disturbance treatments, indicating that the plant transforms from resource acquisition strategy to resource storage strategy through changes in functional traits when the disturbance intensity is reduced. 2) The analysis of the assemblage of functional traits of dominant species shows that C. squarrosa has a small plant height and a high specific leaf area, and A. frigida has a high lignin and nitrogen concentration. The two species thus can adopt some avoidance (escaping from grazers) and tolerance (regrowth capacity after defoliation) strategies to improve their grazing resistance; S. grandis has the highest plant height, the largest leaf dry matter concentration, the highest stem and leaf cellulose concentration, which indicates that S. grandis is a very typical competitive species. Under management conditions with low interference intensity, the competitive pressure of S. grandis (Competitive strategy) against other species may be an important reason for its high dominance.

Aims Vegetation traits are one of the research hotspots in plant ecology and they reflect the strategies of plant survival. According to the CSR survival strategy model, plant species may be classified into three categories: C strategy that can maximize biomass in resource-rich environments (competitive species); R strategy that can rapidly intake resources and reproduce in environments with a high frequency of disturbance (opportunistic species); and S strategy that can maintain individual survival in resource-poor environments (tolerant species). Leaf traits have adaptive changes in response to environmental gradients, which have impacts on plant adaptation. The mechanisms of how leaf traits affect CSR survival strategies in the alpine grassland of Qingzang Plateau remain unclear. The objective of this paper is to investigate the spatial patterns of CSR survival strategies of alpine plants and the mechanisms by which environmental factors influence plant survival.
Methods We surveyed a field transect which consists of 53 sample sites in an alpine grassland on the Qingzang Plateau from July to August 2020. Vegetation traits of leaf area, leaf fresh mass, and leaf dry mass were measured and C, S, R values were calculated. Finally, we analyzed the critical drivers and mechanism of plant CSR strategies in response to geographical elements.
Important findings Our results showed that: (1) In the alpine grassland of the Qingzang Plateau, 41.6%-96.7% of plants are identified as S strategy. (2) With the increase of the longitude, the proportion of C strategy plants increased gradually from the west to the east, whereas along the altitude gradient, the proportion of C strategy plants decreased with the increasing altitude. (3) Random forest analysis showed that the contribution of precipitation to C strategy is the highest (25.74%), and the contribution of altitude to S strategy is the highest (27.34%). Additionally, both precipitation and temperature had significant effects on leaf area, and leaf water content significantly affects plant CSR strategies. In summary, results of our study highlighted that precipitation is the most critical factor that governs plant CSR survival strategies. This finding has important implications for studying the ecological adaptation along environmental gradients in alpine grasslands.

Aims This study aimed to understand whether the coupling between environmental factors and chlorophyll contents is related to the dominance of plant life forms in the grasslands.

Methods In July 2020, we measured chlorophyll contents in 185 plant samples from 50 forage species of 11 sampling plots of a typical steppe in Xilinhot. Through correlation analysis, single factor analysis of variance, redundancy analysis (RDA), stepwise regression analysis and path analysis, we analyzed the effects of multiple environmental factors on indices of plant chlorophyll and the mechanism underlying the competition advantage of different plant species.

Important findings 1) The contents of chlorophyll a and chlorophyll b were significantly positively correlated with that of total chlorophyll, with the correlation coefficients of 0.807 and 0.936, respectively. The contents of total chlorophyll were more affected by that of chlorophyll b. 2) The contents of chlorophyll a, chlorophyll b, and total chlorophyll of the four life forms followed an order of semi-shrubs > perennial grasses > annual or biennial plants > perennial forbs. 3) The variations of chlorophyll contents among plant life forms were differently explained by environmental factors. The RDA results showed that the explanatory degree of semi-shrubs was the highest (28.0%), followed by annual or biennial plants (18.3%) and perennial forbs (17.7%), and that of perennial grasses was the lowest (12.7%). 4) The chlorophyll index of plant life forms were affected by various environmental factors. The chlorophyll b contents in semi-shrubs were affected by soil organic carbon content and relative air humidity, while their total chlorophyll contents were mainly influenced by relative air humidity. Chlorophyll b contents of perennial forbs were mainly restricted by surface temperature. The contents of chlorophyll a, chlorophyll b, and total chlorophyll of annual or biennial species were mainly affected by soil pH. 5) Path analysis results showed that chlorophyll a contents of grasses were mainly affected by soil factors, chlorophyll b and total chlorophyll contents were mainly impacted by atmospheric factors in the grassland habitats. Comparatively, perennial grasses were not susceptible to environmental factors so that they become more tolerant to environmental changes than the other life forms. Therefore, these plants gradually became dominant during community succession and evolution.

Aims Grassland is an important component of the terrestrial ecosystems in China, and plays a vital role in ecosystem productivity and functioning. During the past decades, 90% of natural grasslands have been degraded as a result of climate change and anthropogenic activities. Grassland degradation altered soil nutrient balance, exerting substantial impacts on ecosystem structure and functions. Our objective was to explore the responses of soil and microbial carbon (C), nitrogen (N) and phosphorus (P) stoichiometry to grassland degradation across the Qingzang Plateau alpine grasslands.

Methods We collected soil samples (0-10 cm) along the degradation sequence (i.e., non-degradation, moderate degradation and heavy degradation) from five sites across the “Three-River Source” region. By determination of soil and microbial C, N and P, we examined the changes in their contents and stoichiometric ratios with grassland degradation. We further synthesized data from the whole Qingzang Plateau alpine grasslands to validate the measured results using a meta-analytical approach.

Important findings Grassland degradation significantly reduced soil organic C, total N and total P contents and their stoichiometric ratios. Although microbial C and N content declined with degradation, change in microbial P content was limited along the degradation gradient. The microbial C:N:P ratios showed minimal responses to degradation. No obvious relationships were observed among soil and microbial C:N:P ratios. The above results indicate that soil microbes have the ability to maintain a given elemental composition despite variation in soil elemental composition following grassland degradation. From a long-term perspective, the nutrient-balance based soil quality promotion technology is able to effectively enhance grassland restoration and improve ecosystem service.

Aims To further understand how community functional diversity drives biomass change following nitrogen (N) addition, a nitrogen addition experiment was conducted in an alpine grassland.

Methods Species composition of community and six functional traits of common species were measured in a short-term N addition experiment in Bayanbulak alpine grassland of Tianshan Mountains. We compared the response patterns of species diversity, functional diversity, and community level traits, and quantified the relative contribution of those factors to community biomass variation.

Important findings Both aboveground and belowground biomass increased following short-term N addition, with higher proportional enhancement of aboveground biomass. N addition reduced functional diversity, but did not affect species diversity. At the community level, height and leaf carbon content increased following N addition, whereas specific leaf area, seed mass, and leaf phosphorus content decreased. The variations of species diversity contributed less to the variations of community biomass change, whereas functional diversity and community level traits explained most of the variation of community biomass. Our results support the mass ratio hypothesis. In conclusion, community level functional traits and functional diversity were sensitive to short-term N addition, and played a key role in driving community biomass.

Aims Soil enzymes, which are mainly produced by plant roots and soil microbes, involve in the organic matter degradation and element cycling and other key processes in plant-soil systems. Study on the relationships between soil enzyme activity and plant community composition and microbial activity under changing precipitation pattern and increasing nitrogen (N) deposition can provide a new insight for evaluating the influencing mechanism of global change on the biogeochemical cycling in plant-soil systems.
Methods Based on a field experiment involving five precipitation treatments (50% reduction, 30% reduction, natural precipitation, 30% increase, and 50% increase) and two N addition treatments (0 and 5 g·m^-2·a^-1) conducted in a desert steppe of Ningxia since 2017, the changes of soil enzyme activities (sucrase, urease, and phosphatase) were studied and their relationships with plant community composition and microbial ecological stoichiometry were analyzed in 2018 and 2019.
Important findings Compared with decreasing precipitation, increasing precipitation had greater impacts on the three enzyme activities, but its effects were interacted with N addition and sampling year. Increasing precipitation had no significant impacts on the three enzyme activities in 2018, but enhanced them in 2019. By contrast, N addition had less influences on the three enzyme activities, especially in 2019. The biomass of Astragalus melilotoides was negatively correlated with urease and phosphatase activities, while the biomass of Cleistogenes squarrosa had positive correlation with the three enzyme activities. Except the Patrick richness index, plant community diversity indices were generally negatively correlated with the three enzyme activities. Soil enzyme activities were more greatly affected by soil pH, soil total phosphorus (P), and microbial biomass carbon (C):N:P. Therefore, short-term precipitation change and N addition have little effects on the soil enzymes in the studied desert steppe (especially under reducing precipitation); increasing precipitation and N addition could pose direct influences on soil enzyme activities by increasing plant biomass, changing plant diversity, regulating microbial biomass ecological stoichiometry, and enhancing soil P availability. Given the diversity and functional complexity of soil enzymes, it is necessary to deeply analyze the influencing mechanism of global change on enzyme activities by measuring the long-term responses of various enzyme activities.

Aims Responses of soil carbon (C), nitrogen (N), and phosphorus (P) contents and their stoichiometric ratios to climatic variables (mean annual precipitation (MAP) and mean annual air temperature (MAT)) along soil depths are important for understanding the effects of climate change on terrestrial ecosystem functions.
Methods To explore the responses of soil C, N, and P contents and their stoichiometric ratios along soil profile to MAP and MAT at a regional scale, we investigated these variables for four soil layers (0-20, 20-40, 40-60, and 60-80 cm) at 44 sites in grasslands on the Mongolia Plateau.
Important findings (1) Soil C and N contents decreased while soil P did not change with increasing soil depth. Soil C:P and N:P decreased while soil C:N was relatively stable with increasing soil depth. (2) Soil C, N, and P contents, as well as C:P and N:P, were positively correlated with MAP, but negatively correlated with MAT. Soil C:N was negatively correlated with MAP but did not correlate with MAT. The correlations between climate variables and soil C, N, and P contents and their stoichiometric ratios were weakened with increasing soil depth. (3) The effect of MAP or MAT on soil C, N, and P contents and their stoichiometric ratios were different among four soil depths. The total interpretation of the variations in soil C, N, and P contents and their stoichiometric ratios explained by MAP or MAT decreased with increasing soil depth. These results indicate that climatic variables had a top-down regulation on soil C, N, P contents and their stoichiometric ratios, and the effect of MAP was more important than that of MAT on soil C, N, P contents and their stoichiometric ratios in grasslands on the Mongolia Plateau.

Aims The relationship between biodiversity and ecosystem function is an important ecological issue that is increasingly receiving global attention. Plant functional diversity, as one of the most important components of biodiversity, is directly linked to ecosystem functions. Traditional in-situ monitoring of grassland plant functional diversity is not only time-consuming and laborious, but also difficult to expand to large-scale research due to the limitations of time and space. The development of remote sensing technology provides an economical and effective means for assessing the grassland functional diversity over large areas. We estimated functional diversity and aboveground biomass based on Sentinel-2 satellite images and field data across the meadow steppe in the Ulgai Management Area of Xilin Gol League in Nei Mongol.

Methods We selected 46 spectral feature variables from the Sentinel-2 satellite imagery in the study area. Next, three methods, including stepwise regression, partial least squares regression (PLSR), and random forest regression (RFR) were applied to retrieve the grassland functional richness (FRic), functional evenness (FEve) and functional divergence (FDiv). Finally, the grassland aboveground biomass was also estimated using PLSR method, and the relationships between remotely sensed grassland functional diversity and grassland aboveground biomass were analyzed.

Important findings Our results showed that: (1) Band 11, optimized soil adjusted vegetation index (OSAVI), water band index (WBI) were the most important predictor of FRic; Band 6, Band 10, Band 12, carotenoid reflectance index 1 (CRI1), double-peak optical index (D), normalized difference index 45 (NDI45) were significantly related to FEve; and Band 5, Band 9, Band10, Band11, weighted difference vegetation index (WDVI), convex hull area played a critical role in predicting FDiv. (2) Based on 10-fold cross-validation, the retrieval accuracies of FRic and FEve estimated by stepwise regression were much higher than that of the other two regression methods, with R² of 0.52 and 0.44, respectively. However, the FDiv was best estimated by PLSR (R² = 0.61). (3) Grassland aboveground biomass was estimated with an accuracy of R² = 0.61, and FRic was the best indicator of aboveground biomass (R² = 0.40), followed by FDiv (R² = 0.28) and FEve (R² = 0.27). Our findings indicated the ability of Sentinel-2 satellite images to estimate grassland plant functional diversity, providing reference and basis for grassland plant functional diversity estimation at a large regional scale.

Aims Variations in temperature and snow accumulations in winter will change the structure and function of the soil-microbial system. As a key biological factor in the terrestrial ecosystem, microorganisms play an important role in regulating soil nutrient cycles. However, they are very sensitive to environmental disturbances, especially to winter climate changes. It is in great need to study the response of soil nutrients and microbial properties of typical semi-arid grasslands to climate change in winter, in order to predict the ecological process and functional changes of grassland ecosystem in the long term.
Methods In the present study, the semi-arid grassland in the Yunwushan National Nature Reserve in Ningxia Province was taken as the research object. The four treatments including warming (W), snow reduction (S), interaction of warming and snow reduction (WS), and control (CK) were set to explore the responses of soil nutrients, enzyme activities and soil bacterial communities in the 0-5 cm soil layer of the typical grassland of the Loess Plateau to variations in winter temperature and snow cover.
Important findings Our results indicated that: (1) Warming, snow reduction and their interaction in winter increased the 0-5 cm soil temperature, lowered the relative humidity of the soil, but significantly increased the number of soil freeze-thaw cycles. (2) Compared with the control, other different treatments generally reduced the microbial biomass and bacterial diversity, which led to reduced activity of soil β-1,4-glucosidase (BG), β-1,4-N-acetylglucosaminidase (NAG) and alkaline phosphatase (AKP). The content of soil organic carbon, total nitrogen, available phosphorus, and nitrate nitrogen in the soil increased, while the content of nitrate nitrogen decreased. (3) The soil bacterial species in the study area were mainly Acidobacteria, Proteobacteria, Actinobacteria and Gemmatimonadetes. The dominant bacteria at the class level included Acidobacteria, γ-Proteobacteria, Thermophiles and σ-Proteobacteria. Redundancy analysis (RDA) results showed that available phosphorus (AP) content had the most significant impact on the bacterial community composition, with an explanation rate of 21.3% for the community variation. In conclusion, winter climate change can significantly affect soil temperature and humidity, especially the freezing and thawing cycles, which might further influence soil nutrients cycles, enzyme activities, and soil bacterial diversity. These results are of great significance for enriching and expanding the understanding of the process and mechanism of climate change on grassland ecosystem, as well as predicting the mid and long-term dynamic changes of typical grassland ecosystems.

Aims β diversity reflects species turnover rate across environmental gradients, and this study attempts to use β diversity to reveal relevant ecological processes underlying the changes in plant community composition along a disturbance gradient induced by small burrowing herbivores.
Methods This study conducted a field survey at Gangcha County in the Qingzang Plateau to determine the effect of disturbance intensities of plateau pika (Ochotona curzoniae) on the β diversity of an alpine meadow measured by Whittaker’s index. Then a variance decomposition was used to measure the contribution of individual species to overall β diversity (SCBD) and single interference sites to overall β diversity (LCBD) according presence- absence community matrix.
Important findings The turnover rate of species in the plant community first increased and then decreased with the increase of the disturbance intensities induced by plateau pika. Species with intermediate site occupancy had a greater contribution to β diversity, in which Agropyron cristatum, Artemisia hedinii and Anemone rivular var. flore-minore were the single-species plants that contributed the most to the β diversity of the study regions. The disturbance plot T₀ with the absence of plateau pika had the greatest contribution to the β diversity of whole study region. LCBD of individual site was negetively related to species richness of that individual site, and had no significant correlation with the disturbed intensity of plateau pika. These results indicate that alpine meadows distributed in high LCBD and Agropyron cristatum, Artemisia hedinii and Anemone rivularis var. flore-minore with high SCBD should be protected to conserve plant diversity when the plateau pikas are present.

Aims Litter decomposition plays a vital role in material cycling of ecosystems. However, the responses of litter decomposition to changing precipitation in alpine meadows and the mechanisms underlying these responses are still not clear. Thus this study was designed to address the effect of changing precipitation on litter decomposition of different plant functional groups in alpine meadows.

Methods We used the litter bag method to investigate changes of initial nutrient content, mass loss and nutrient release in the litter of three plant functional groups (grass, sedge and forb) and in communities in an alpine meadow of Eastern Qingzang Plateau, in response to five precipitation treatments, including 90%, 50% and 30% decrease (Pr-90, Pr-50, Pr-30), ambient control (CK) and 50% increase (Pr+50).

Important findings The results showed that: 1) Precipitation decrease (Pr-90, Pr-50, Pr-30) significantly increased the initial nitrogen (N) content, carbon (C):N and lignin:N ratios of grass litters, while precipitation increase (Pr+50) significantly increased the initial phosphorus (P) content of all litter types. 2) According to the Olson negative exponential model, under different precipitation, the forbs decomposed the fastest, with the 95% decomposition time of 3.49-7.45 a; the decomposition of the communities and of the sedge species were the second fastest, with the 95% decomposition time of 4.07-8.05 and 4.65-7.74 a, respectively; grasses decomposed most slowly, 5.84-11.18 a. 3) Extreme precipitation decrease (Pr-90) inhibited the decomposition of all litter types, while moderate precipitation change (Pr-50, Pr-30, Pr+50) inhibited the decomposition of grass litter, but had no significant effects on sedge, forb and community litter, only precipitation increase (Pr+50) promoted the decomposition of forb litter. 4) C release was inhibited under precipitation decrease (Pr-90, Pr-30) in all litter types. N and P release of grass litters were promoted under both precipitation increase and decrease. Pr-30 promoted N release, Pr-90 inhibited P release and Pr+50 promoted P release in sedge, forb and community litters. 5) Structural equation models (SEM) showed that the mass and nutrient remaining rate were directly negatively affected by precipitation, and indirectly affected by litter types through initial C, N, P, lignin, cellulose and hemicellulose content. In conclusion, both litter types and precipitation can affect the mass loss and nutrient release of litters in an alpine meadow. Decomposition was slower and the response to precipitation was more sensitive in grass than that in other litter types. In the future, we should pay attention to the effects of mass loss and nutrient release of grass litter under climate change, especially extreme precipitation decrease, on organic matter input and C, N and P cycling in an alpine meadow.

Aims Nutrient resorption in different organs of plants is important for plant nutrient use strategies and elemental biogeochemical cycles. Previous researches on nutrient resorption have focused on leaves, but neglected culms. Additionally, leaves of different species have been collected at the same time during the peak growth stage in previous studies, ignoring the different peak times of nutrient content of different species, which leads to the underestimation of nutrient resorption efficiency.

Methods In order to explore the seasonal variation of nutrients and nutrient resorption efficiency in the culms and leaves of herbaceous plants, 22 common plants in Hulun Buir grassland were chosen as the research objects to determine the temporal dynamics of nutrient content during the growing season, and the resorption efficiency of nitrogen (N) and phosphorus (P) in plant culms and leaves.

Important findings The content of N, P in plants had obvious temporal dynamics during the growing season, showing an increasing trend first and then a decreasing trend. For most of the 22 species, the maximum content occurred in the middle and late August, but the peak time differed among different species. The resorption efficiency of N in leaves was higher than in culms, but that of P did not differ between leaves and culms. Nutrient resorption efficiency of plants was closely related to the nutrient content at the senescence stage, but not to that at the growth stage. In previous studies, different plants were sampled at the same time during the growth stage, which led to underestimation of N and P resorption efficiency of culms and leaves. This study re-examined the sampling strategy in nutrient resorption studies, and showed that the sampling time of mature tissues in the growth stage could be determined according to the peak nutrient content time of different species.

Aims The response of plant phenology to climate warming is an important element of global change research. At present, studies on plant phenology response to climate warming are in severe shortage for high-altitude ecosystems, especially regarding responses to multiple-level warming.

Methods We conducted a multiple-level warming experiment in an alpine meadow on Qingzang Plateau, and monitored plant phenology of two dominant species, including the timing of green up, budding and flowering in 2015, 2017, 2018 and 2021.

Important findings The results showed that plant phenology of different species exhibited various trends under warming. For Kobresia pygmaea, delay in phenological development, including green up, budding and flowering, was positively correlated with temperature increases. However, the timing of phenological stages of Potentilla saundersiana showed advancing first, and then delay with increasing temperature. These results suggest that plant phenology of alpine meadow asynchronously responds to increased temperature. In addition, temperature increase exerts delayed effects on plant phenology over long-term. The structural equation modeling showed that temperature increase consistently delayed the green up of K. pygmaea, and low-level warming advanced phenological development of P. saundersiana, but this advancing trend reversed under high-level warming. Importantly, soil moisture plays a key role in determining the magnitude and direction of phenological response to climate warming in our study. Our findings indicate the asynchronous characteristics of plant phenology response to climate warming in alpine meadow ecosystems, and provide basis to predict responses of high-altitude ecosystems to climate change in the future.

Aims Nitrogen (N) availability is an important limiting factor for grassland ecosystem productivity, and soil microorganisms are the main driving factor on soil N transformation. With the increase of atmospheric N deposition, the response of soil microbial characteristics to different nitrogen input levels is still unclear especially in saline-alkaline grassland.

Methods The experiment was conducted in Youyu Loess Plateau Grassland Ecosystem Research Station, Shanxi Province. Eight different nitrogen addition levels were set, which were 0, 1, 2, 4, 8, 16, 24 and 32 g·m^-2·a^-1, respectively. The Ammonia-oxidizing microorganisms (i.e. ammonia-oxidizing bacteria (AOB) and ammonia- oxidizing archaea (AOA)) abundance, soil bacterial and fungal abundance, as well as soil microbial biomass carbon (MBC) and nitrogen (MBN) content were measured in the growing season (May to September) in 2020 to explore the effects of different levels of N addition on soil microbial characteristics.

Important findings Our results showed that: (1) Sampling month had a significant effect on soil AOB, bacteria, fungal abundance and MBC, MBN content due to the variation in soil temperature and soil water content in the growing season. (2) N addition had a significant effect on soil AOB abundance, while had no effects on soil MBC, MBN content, and bacterial and fungal composition. (3) Higher N addition (24 and 32 g·m^-2·a^-1) significantly increased the abundance of ammonia-oxidizing bacteria (AOB) on the early growth stage (May to August), while having no effect on late growth period (September). (4) Soil microorganisms were mainly regulated by soil cations concentrations and soil pH values, which explained the variation of soil microorganisms by 21.8% and 17.2%, respectively. We found that soil microorganisms were not sensitive to N addition in saline-alkaline grassland, while AOB showed a significant increase under higher N addition, indicating that higher N addition might promote soil N transformation.

Aims Seed size and number variation is the core issue in the study of plant fitness. Exploring the grazing effects of different large herbivore assemblage on the relationship between seed size and number of dominant species in Kobresia grasslands, is helpful to understand the reproductive strategy and population regeneration mechanism.

Methods This research was conducted on the technical platform of adaptive management of alpine grassland- livestock system on the Qingzang Plateau. There were six grazing treatments: no grazing (control), single yak grazing, single Xizang sheep grazing, yak:Xizang sheep grazing ratios of 1:2, 1:4 and 1:6. And we analyzed the reproductive characteristics of K. humilis, the relationship between seed size, seed number and its tradeoff.

Important findings The results showed that: 1) Grazing treatment increased the seed size and seed number of K. humilis by more than 15% and 30%, respectively. The variation coefficient of seed size decreased by at least 15% under other grazing treatments except for yak:Xizang grazing ratio of 1:2. While the variation coefficient of seed number decreased by more than 25%. 2) Pearson correlation analysis showed that there were positive correlation between seed size, seed number and reproductive traits under grazing treatments. 3) Grazing treatments increased the tradeoff between seed size and seed number which was largely affected by the mass of single reproductive branch. The results suggest that even at moderate grazing, livestocks are still the limiting factors for the acquisition of resources. Long-term grazing changes the potential relationship and tradeoff among the traits of K. humilis, stabilizes the features of seed size and seed number. The reproductive strategy of K. humilis is optimized by increasing seed size and seed number, which improves the near-ground competitiveness and fitness of offsprings.

Aims Overgrazing induced reduction in supply of soil nutrients is a major mechanism leading to extensive grassland degradation in China. The capacity of carbon (C) sequestration was demoted in the degraded grassland ecosystems. However, it remains unclear whether the capacity of carbon sequestration in a degraded grassland can be restored by the supplement of nutrients to the soil. Net ecosystem CO₂ exchange (NEE), ecosystem respiration (ER) and gross ecosystem productivity (GEP) are important parameters describing the processes of ecosystem carbon cycle. Nitrogen (N) and phosphorus (P) are two limiting nutrients in typical steppe in China. To date, how these two nutrients alone or in combination affect the three parameters of carbon cycle (i.e., NEE, ER and GEP) in a degraded steppe community, especially their interactive effect, is poorly understood.

Methods To address these scientific questions, we conducted a field experiment in a degraded typical steppe community. Four treatments of nutrient addition were implemented: no nutrient addition (CK, control), adding N alone (10.5 g·m^-2, NH₄NO₃), adding P alone (7 g·m^-2, KH₂PO₄), and adding two nutrients in combination. Two stages were selected for fertilization: early-spring (April 21) and middle-summer (July 15).

Important findings Neither N nor P alone had significant effect on NEE, ER and GEP when nutrients applied at early-spring (April 21) or middle-summer (July 15), while their combination significantly increased the values ofNEE and GEP. 2) N and P exhibited strong synergistic effect on NEE, GEPand ER when applied in combination at early-spring (April 21), while a consistent additive effect between the two nutrients on the three carbon exchange parameters was observed when applied at middle-summer (July 15). Our findings have implications for the restoration of degraded grasslands. To restore the capacity of carbon sequestration of the degraded typical steppe ecosystem, supplying N and P in combination is better than a single nutrient alone, and appling these nutrients at spring is better than at summer.

For the universal grassland degradation and associated human utilization in the world, authors expound ecosystem restoration, climate climax in ecological succession, environment change and grassland state transition, grazing and disturbance climax, restoration by human intervention, thereby put forward the restoration path and state model of the degraded grassland ecosystem. This paper emphasizes that the restoration of degraded grassland should be carried out from the perspective of ecosystem, rather than only vegetation or soil processes, because there will be multiple alternative restoration states for grasslands in the context of environmental change or human disturbance. Three basic restoration modes of degraded grassland and possible restoration states are described as the followings: (1) Gradual restoration according to natural succession: based on the theory of ecological succession, grasslands from light to moderate degradation under favorable environments might reach the climax or near climax state for a long time by the systematical self-organization. (2) Intervention restoration by human activity: for those severely or extremely degraded grasslands, it needs to break through a series of abiotic (soil structure, nutrients, etc.) and biotic (plant colonization, species interaction, etc.) restrictions, and restore to a certain equilibrium or stable state, even climax state by using engineering, physical, chemical or biological-ecological methods or practices. It will take a long-time. (3) Restoration by grazing disturbance: grassland ecosystem structure (species composition and diversity), productivity and nutrient processes could be regulated through light to moderate livestock grazing, thereby maintaining and promoting grassland ecosystem multifunctionality and stability. This restoration method can be selected for medium-mild degraded grasslands. In conclusion, the holistic goal of grassland restoration is to achieve its long-term stable ecosystem multifunctionality.

Aims Aboveground biomass (AGB) is one of the most important factors affecting grassland ecosystem function and is commonly measured in grassland research. AGB is often measured using the harvest method, which can cause great disturbance to plant communities, especially for those long-term monitoring plots. A non-destructive method for AGB estimation is thus needed.

Methods Here, we conducted field measurements at a land-use manipulation experiment in a typical steppe in Nei Mongol, China. We obtained the fractional vegetation cover (FVC) using digital photographs. We also measured leaf area index (LAI), vegetation height, and plant species richness. Three different models were used to estimate AGB: univariate regression model, stepwise regression model, and random forest model.

Important findings We found that FVC, LAI, mean vegetation height, maximum vegetation height and richness were highly correlated with AGB variation. AGB can be accurately predicted by a stepwise regression model developed based on the local plant community. The determination coefficient (R²) and root-mean-square error (RMSE) of the stepwise regression model can reach 0.91 and 35.60 g·m^-2, respectively. Overall, our study provides a rapid and non-destructive method for AGB measurement that can be used as an alternative to the traditional harvest method.

Aims Plant phenology is an important indicator of ecosystem response to climate change, and it is also a central parameter for modelling plant productivity and vegetation dynamics. However, it remains unclear whether inner-annual, intra-annual, inter-species or inter-habitat variabilities exist in the response of plant phenology to environmental changes.
Methods Here we investigated the effects of long-term (>10 years) warming and nitrogen (N) addition on plant phenology in a temperate desert steppe. We used the phenological scoring observation method and Richards growth curve fitting method to monitor phenological shifts of three dominant species, Stipa breviflora, Artemisia frigida and Kochia prostrata, in the 11th, 12th and 13th treatment year.
Important findings We found that the flowering time ranged from the 129th to the 145th days of a year for S. breviflora, from the 230th to the 248th days for A. frigida, and from the 194th to the 222th days for K. prostrata. Warming and N addition tended to advance the flowering time of S. breviflora and K. prostrata, but tended to delay the flowering time of A. frigida. The fruiting time ranged from the 134th to the 148th days for S. breviflora, from the 241th to the 260th days for A. frigida, and from the 207th to the 231th days for K. prostrata. Warming and N addition tended to advance the fruiting time of S. breviflora and K. prostrata, but tended to delay that of A. frigida. The reproductive growth period lasted for 12 to 25 days for S. breviflora, 48 to 55 days for A. frigida, and 45 to 77 days for K. prostrata. Warming and N addition shortened the reproductive growth period for S. breviflora, but prolonged that period for A. frigida and K. prostrata.

Aims Soil respiration is one of the most critical components of carbon cycle in terrestrial ecosystems. The study on temporal dynamics of soil respiration and its linkage with environmental factors in desert steppes under changing precipitation can provide data supports for a deep understanding of the regulatory mechanisms of key carbon cycling processes in fragile ecosystems.
Methods A field experiment involving five precipitation treatments (50% reduction, 30% reduction, natural, 30% increase, 50% increase) was set up in 2014 in a desert steppe in Ningxia. The temporal dynamics of soil respiration rate were explored during the growing season (from June to October) in 2019, and the relationships between soil respiration rate and soil properties and plant characteristics were analyzed.
Important findings Soil respiration rate showed a seasonal variation of an increasing and a decreasing trend across the growing season, with the maximum values (2.79-5.35 μmol·m^-2·s^-1) occurring in late July or early August. Compared with the natural condition, 30% reduction in precipitation did not result in a significant effect on soil respiration rate, reflecting the adaptability of soil respiration to moderate drought. Overall, 50% reduction in precipitation reduced soil respiration rate, whereas increased precipitation (especially the 30% increase) enhanced soil respiration rate, and this positive effect was especially obvious in the early growing season (June to July). Soil respiration rate had a significantly exponential relationship with soil temperature and a significantly linear relationship with soil water content. Soil physicochemical property had a highly independent explanatory power for soil respiration rate (R² = 0.36), and its effect was highly correlated with soil biological property and plant diversity (R² = 0.31). Precipitation could affect soil respiration rate either directly or indirectly through the influences on soil biological property and plant biomass. The results indicated that a moderate increase in precipitation could accelerate soil respiration by alleviating soil water limitation, stimulating soil enzyme activity, promoting microbial activity and plant growth in the desert steppe, and that an extreme increase in precipitation would lead to a decrease in soil permeability and a hindrance to microbial metabolic activity, thus inhibiting soil respiration.

Aims Grassland is an important component of terrestrial ecosystems around the world and plays an important role in terrestrial carbon cycling. However, large uncertainties still exist in predictions of soil organic carbon (SOC) dynamics in grassland ecosystems using earth system models, partly due to an inadequate understanding of the spatial patterns and drivers of soil carbon components and the rate of decomposition. In this study, we explored the determinants of the contents of total SOC and its components as well as the rate of soil carbon decomposition in the topsoil of temperate grasslands of Nei Mongol.
Methods Soil samples at depths of 0-10 cm were collected during July to August 2015 from field sites on the Nei Mongol Plateau. We measured the contents of total SOC and its partitioning in three soil aggregate size-classes, and the decomposition rate based on laboratory incubation. In addition, we acquired a suite of explanatory factors including climatic, edaphic, vegetation, and mineral variables. Variance partitioning analyses were then used to investigate the relative importance of the four factors in affecting the contents of total SOC, aggregate-classified carbon fractions and soil carbon decomposition rate.
Important findings The contents of total SOC and three carbon fractions displayed an increasing trend from southwest to northeast of the study area, while soil carbon decomposition rate (standardized by SOC) showed a reverse trend. The carbon contents in bulk soil and different aggregate fractions are highest in the meadow steppe, followed by the typical steppe and the desert steppe; whereas soil carbon decomposition rate (standardized by SOC) was highest in the desert steppe, followed by the typical steppe and the meadow steppe. The spatial variations of carbon contents in the three soil aggregate fractions were mainly driven by climatic and mineral factors, with finer soil particles attaching greater relative importance in the effect by the mineral factor. The soil carbon decomposition rate (standardized by SOC) was affected by mineral, edaphic, and climatic factors. These findings highlight the importance of considering the differential influences by minerals in different soil aggregate carbon fractions, particularly the silt- and clay-associated carbon in the Earth system models, so as to improve the accuracy in the prediction of SOC dynamics in grassland ecosystems under changing environment.

Aims Along with intensified climatic warming and human activities, global arid areas have expanded in an unprecedented rate during the past decades. Dryland ecosystems have witnessed increased vulnerability and sensitivity to climate change. Exploring the time lag effect of climate change on dryland vegetation growth is becoming an important research highlight in current global change related studies.

Methods In this study, we synthesized the normalized difference vegetation index (NDVI) from Moderate- resolution Imaging Spectroradiometer (MODIS), the monthly gridded CRU TS4.05 (Climatic Research Unit Time-Series version 4.05) climate and drought information developed by the University of East Anglia, solar radiation information from ERA5 (ECMWF’s Fifth Generation Atmospheric Reanalysis of the Global Climate) and soil moisture information from the European Space Agency (ESA) Climate Change Initiative program (CCI). These data were designed to investigate the effects of climatic factors and their time-lag on grassland NDVI in Asian grasslands from 2001 to 2020. This analysis was conducted based on the window cross-correlation and one-dimensional linear regression.

Important findings Our study revealed that: 1) The grassland NDVI responded strongly to average temperature and total precipitation when there was no lag, but expressed a lag response to solar radiation and soil moisture (1-month). 2) The spatial distributions of the lag response of grassland NDVI to climate change were nonuniform, with significant differences observed between the western and eastern Asian grasslands. 3) We did not detect any apparent time-lag effects on interactions between grassland NDVI and self-calibrating Palmer Drought Index. 4) We argue that altitude could partly modulate the response of grassland NDVI to climatic variables in the grassland of Asian drylands.

Aims Populations are the basis for the formation and development of the structure and function of grassland ecosystems. However, long-term grazing and global climate change like nitrogen addition profoundly impact the growth and reproduction of populations, such as Stipa bungeana, a dominant species in typical steppe of the Loess Plateau that has a high ecological and economic value. This study investigated how grazing and nitrogen addition affect the growth of S. bungeana.

Methods The study was based on a long-term rotational sheep grazing experiment in the typical steppe of the Loess Plateau. A completely randomized split-plot experimental design was employed, with stocking rate (0, 2.7, 5.3, 8.7 sheep·hm^-2) as the main factor and nitrogen addition levels (0, 5, 10, 20 g·m^-2) as the secondary factor. Morphological traits, aboveground biomass, the proportion of population biomass to total community aboveground biomass (PPB) and the relationship between them in S. bungeana were examined to investigate the effects of stocking rate, nitrogen addition and their interaction.

Important findings As the stocking rate increased, the plant height, canopy diameter, tiller density, seedlings, aboveground biomass, and PPB all followed a “single peak” curve trend, while the population density decreased. Nitrogen addition increased the plant height, canopy diameter, reproductive branch density, tiller density, aboveground biomass and PPB, while density of seedlings initially increased and then decreased as the nitrogen addition levels rose. The total effect of grazing on population aboveground biomass and the PPB was small compared with that of nitrogen addition. Specifically, grazing had a direct negative effect on aboveground biomass and affected PPB by regulating tiller density, population density and aboveground biomass. Nitrogen addition had a positive effect on aboveground biomass, both directly and indirectly through increasing plant height, reproductive branch density. It also impacted the PPB through regulating population density, canopy diameter, tiller density, and reproductive branch density. Overall, nitrogen addition increased canopy diameter and reproductive branch density and grazing increased density of seedings. The interaction of grazing and nitrogen addition significantly affected reproductive branch density. Stipa bungeana had maximum aboveground biomass or community status at a stocking rate of 4.10 or 5.29 sheep·hm^-2. These results indicated that grazing and nitrogen addition regulated the aboveground biomass and community status of S. bungeana through affecting its morphological characteristics, providing a basis for the scientific management and sustainable development of grassland populations.

Aims Soil available nitrogen (N), generated from a series of soil mineralization processes, is a major limiting factor of terrestrial ecosystem productivity. Soil N availability depends on soil microorganisms, vegetation types, and soil physical and chemical properties. Soil microorganisms are very sensitive to environmental changes, especially the temperature change, which is closely related with microbial growth and reproduction. Therefore, it is important to understand the temperature sensitivity (Q₁₀) of microbial regulation of N mineralization rates in a large spatial scale for predicting the impacts of global climate changes on terrestrial ecosystem productivity.
Methods Three types of grasslands (namely meadow steppe, typical steppe, and desert steppe) were selected in Nei Mongol Plateau, Loess Plateau, and Qingzang Plateau, respectively. Soil net N mineralization rates were measured at different temperatures in the laboratory, and then Q₁₀ of N mineralization rates were calculated across different grassland types. Relative parameters, including soil microbe, soil physical and chemical properties, were also analyzed.
Important findings (1) The highest Q₁₀ of soil net N mineralization rates was found in all of three grassland types of Loess Plateau than those of Nei Mongol and Qingzang Plateaus. (2) The Q₁₀ values of soil net N mineralization rates in the meadow steppes and typical steppes on the Loess Plateau and Nei Mongol Plateau were significantly higher than those in the desert steppes, while on the Qingzang Plateau, the values in the alpine meadow steppes were significantly lower than that in the alpine typical steppes and alpine desert steppes. (3) Q₁₀ values of soil net N mineralization rates was closely correlated with soil microbial biomass carbon content across different grassland types. (4) The spatial pattern of Q₁₀ is jointly regulated by microorganisms, soil texture and substrate. The results of this study provide important data for understanding of the response of soil N cycle to global change in different grassland types in China, which is valuable for optimization of N cycle models of terrestrial ecosystems in the future.

Aims Eddy covariance (EC) systems are widely used for measuring the fluxes of carbon, water, and energy, as well as meteorological factors. As one important reference of independently evaluating scalar flux by EC technique, energy balance closure is widely used for evaluating data quality of carbon, water, and energy fluxes.

Methods Using the data of energy fluxes and meteorological variables retrieved from 56 site-year, the energy balance closure of six sites across three ecosystems (i.e. desert steppe, typical steppe, and meadow steppe) was analyzed by two widely used methods: linear regression from the ordinary least squares (OLS) and the energy balance ratio (EBR). The overall evaluation of energy balance closure, the seasonal and interannual variations and the related influencing factors were investigated.

Important findings The results show that: 1) the multiple-year EBR and OLS slope over the six sites had a mean value of 0.89 ± 0.11 and 0.96 ± 0.04, respectively, which are better than the results of the FLUXNET and ChinaFLUX. 2) There were significant differences over different sites and grassland types, with EBR of desert steppe (1.01 ± 0.09) and typical steppe (0.90 ± 0.11) both higher than meadow steppe (0.83 ± 0.05). There were seasonal variations of EBR over the six studied sites, and with better and stable results in growing season than non-growing season. The air temperature (T_a), vapor pressure deficit (VPD), soil moisture (SWC), and Albedo regulated the seasonal variation of EBR, with the low T_a and high Albedo remarkably reducing EBR during the non-growing season. 3) There were significant interannual variations of EBR across different sites and grassland types. The latent heat fraction (the ratio of latent heat flux to net radiation, LE/R_n), mean annual air temperature (MAT) and growing season Albedo significantly influenced interannual variation of EBR. The LE/R_n showed the strongest impact and explained 44% of the interannual variation of EBR. The significantly increasing in leaf area index (LAI) strongly regulated the upward of the available energy (net radiation minus ground heat flux, R_n- G₀), which contributes to the significant downward of EBR during observed years. It should be noted that EBR and OLS slope should be combined to better evaluate the energy balance closure. In conclusion, this study help improve our understanding of the potential linkage between energy balance closure and environmental factors, evaluate the quality of scalar flux estimates from EC technique, as well as improve the data processing protocol of flux data in the semi-arid and arid grassland region.

Aims Grasslands provide many kinds of ecological services, including carbon sequestration, sand fixation, and biodiversity maintenance. However, some grasslands are experiencing degradation. To provide scientific theoretical support for grassland restoration, it is necessary to understand the limiting factors for vegetation restoration in degraded grasslands. In this study, we explored nutrient limiting factors for vegetation restoration under different degradation stages in typical steppe of Nei Mongol.

Methods Plant and soil samples were collected during August in 2021-2022, from 13 sampling sites (four plant communities under different degradation stages within each site: non-degradation, light degradation, moderate degradation, and heavy degradation) in typical steppe. We examined the effects of degradations on above-ground biomass, coverage, and density of plant communities. Soil organic carbon, nitrogen and phosphorus contents were measured. Multiple statistical analyses, including least squares regression analysis, redundancy analysis, and multiple linear regression analysis, were used to clarify the nutrient limiting factors for vegetation restoration in degraded grasslands.

Important findings Plant community above-ground biomass, coverage, and density, as well as the contents of soil organic carbon, total nitrogen, and available phosphorus significantly decreased with the intensification of degradation. Under the whole degradation sequence and adjacent degradation succession stages, soil nitrogen content was the most influential factor on plant community properties, while soil phosphorus content marginally affected the overall degradation sequence. These results indicate that soil nitrogen availability is the most important nutrient factor limiting vegetation restoration. Consequently, nitrogen fertilization should be concerned in the future restoration works.

Aims Precipitation regime alteration and increasing nitrogen deposition have substantially altered the structure and function of grassland ecosystems. However, the responses of stoichiometry in soil and vegetation remain elusive, which limits the accuracy in predicting functional changes of alpine meadow.

Methods Based on a manipulation experiment platform of nitrogen addition (10 g·m^-2·a^-1) and precipitation change (precipitation reduction by 50% and increase by 50%) in an alpine meadow on the southern foot of Qilian Mountains, organic carbon (SOC), total nitrogen (SN), total phosphorus (SP) contents in topsoil (0-10 cm), and foliar carbon (LC), nitrogen (LN), phosphorus (LP) and potassium (LK) contents of dominant plant species, including Gentiana straminea, Elymus nutans, Oxytropis ochrocephalaand Kobresia humilis,were continuously surveyed from 2017 to 2020.

Important findings The soil stoichiometry varied significantly among different years, but was not affected by experimental treatments. The aboveground plant biomass showed inter-annual variations and was significantly affected by nitrogen addition. The responses of leaf stoichiometry were species-specific. Foliar stoichiometry of a resource-conservative species, E. nutans, showed limited variations, while that of the sensitive species, K. humilis, fluctuated significantly. To exclude the impacts of temporal variations, we conducted the analysis based on the relative changes (Δ) between treatment plots and the control plots from the same year and the results showed that nitrogen addition significantly increased ΔPB by 15.6%. Precipitation reduction significantly decreased ΔLC of O. ochrocephala by 6.8% while increased ΔLP of K. humilis by 19.8%. Our findings suggest that only nitrogen addition increased aboveground biomass and precipitation reduction altered LC and LP contents in some plant species. The temporal or species-specific effect, rather than experiment treatments effect, dominated the stoichiometric variations of soil and vegetation, highlighting the complex responses of alpine meadow to precipitation regime alteration and nitrogen addition.

Aims This study primarily aims to investigate the spatiotemporal evolution patterns of fractional vegetation cover (FVC) in the grasslands of Xinjiang section of Tianshan Mountains over the period 2001 to 2020. It seeks to elucidate the determinants of these patterns, emphasizing the influence of vegetation types, livestock farming practices, precipitation levels, and mean annual air temperature. Ultimately, the goal is to contribute important insights that will inform decisions concerning the sustainable management and ecological conservation of these grasslands.

Methods The investigation into the spatiotemporal evolution patterns of vegetation in Xinjiang section of Tianshan Mountains and the factors influencing these patterns leveraged MODIS NDVI remote sensing data spanning two decades. A suite of analytical techniques, including Senʼs Slope Estimator + Mann-Kendall trend analysis, coefficients of variation, and land-use dynamics assessments, were employed to analyze the spatiotemporal variations in grassland FVC.

Important findings The study revealed a general stability in FVC, with the multi-year average varying between 0.33 and 0.42. Notable FVC increases were observed in the northeastern and southwestern low-altitude areas of the range, comprising 3.14% of the analysis area. In contrast, significant FVC declines were predominantly in the Ili River Basin, representing 15.81% of the area studied. The dynamics of grassland FVC were primarily driven by vegetation types and the total value of livestock farming output, each influencing over 29.85% of the variation. When considering the interaction with precipitation and mean annual air temperature, the influence of these factors on FVC increased to above 48.70%. Furthermore, a positive association between FVC and both annual precipitation and average annual temperature was noted, with areas displaying a correlation to precipitation covering 80.84% of the total area, largely encircling the basins flanking Tianshan Mountains range. Meanwhile, a correlation with mean annual air temperature spanned 71.69% of the area, predominantly at higher elevations. These findings offer valuable reference data to support strategic planning for the sustainable use and protection of grassland ecosystems in the Tianshan Mountainous region.

Aims Both the carbon cycle and the function of grassland ecosystem as a carbon sink are impacted by the rising nitrogen deposition. Active organic carbon content is an important measure that can reveal changes in soil carbon pool. For a thorough understanding of carbon cycling and the creation of sensible ecosystem management strategies, it is essential to investigate the impacts of nitrogen addition on the active organic carbon fractions of grassland soils.

Methods Five different nitrogen addition treatments were set up in a temperate typical steppe of Nei Mongol. Soil organic carbon fractions content, soil physical and chemical properties, aggregate stability, microbial activities and extracellular enzyme activities were measured. Pearson correlation and structural equation model (SEM) were used to examine the relationships.

Important findings Nitrogen addition reduced the contents of dissolved organic carbon (DOC), microbial biomass carbon (MBC), and easily oxidizable organic carbon (EOC). The contents of DOC, MBC, and EOC all decreased with the increases of soil depth. The treatment of 5 g·m^-2·a^-1 nitrogen addition significantly promoted the decomposition of active organic carbon fractions. The effect of nitrogen addition on soil active organic carbon fractions content was regulated by biotic (microbial biomass, extracellular enzyme activity, etc.) and abiotic (soil physical and chemical properties, aggregate stability, etc.) factors. Nitrogen addition reduced soil density, increased mean mass diameter and the proportion of large aggregates, increased the contact between organic matter and substrate, promoted the decomposition of active organic carbon, and reduced the contents of DOC and EOC. Nitrogen addition inhibited the activities of polyphenol oxidase and peroxidase, reduced the decomposition of difficult-to-decompose organic matter and the contents of EOC and MBC. Nitrogen addition increased the activities of β-glucosidase and cellulose hydrolase, promoted the utilization of DOC by microorganisms, and reduced the content of DOC. Our results indicated that nitrogen addition treatment can affect the decomposition process of active organic carbon by changing soil physicochemical properties and the secretion of extracellular enzymes from microorganisms, promoting the release of carbon in grassland soils. This provided a theoretical basis for further exploration of grassland soil carbon dynamics under nutrient addition in the future.