Examining the drivers and regulating mechanisms of multiple ecosystem services has emerged as a central issue in ecology in recent years. In this paper, we start with the definition of ecosystem services, recent progresses and research priorities in the field. Then, we propose an experimental network to examine the key drivers of ecosystem services and relationships among multiple ecosystem services across grassland and desert ecosystems in northern China. The research network include conceptual diagram, research questions and objectives, field experimental design, ecosystem properties and processes observed across four grassland and desert sites. We review the major findings from the experimental network and future research directions in grassland and desert ecosystem services.

Aims The accelerated atmospheric nitrogen (N) deposition due to human activity and climate change greatly increases the availability of reactive N in terrestrial ecosystems, leading to limitations of other nutrient elements such as phosphorus (P). The effects of N and P additions on grassland ecosystems across different organizational levels, however, have rarely been studied; particularly the underpinning mechanisms remain unclear. Our objective is to examine the effects of N and P additions on aboveground net primary productivity and functional traits of Leymus chinensis, which is a dominant species in the typical steppe of Inner Mongolia, and to study nutrient limitations at different organizational levels in a L. chinensis steppe induced by N and P additions.
Methods We conducted a field manipulation experiment with additions of N (10.5 g N·m^-2·a^-1) and P (32 g P₂O₅·m^-2·a^-1) in a L. chinensis steppe ecosystem in Inner Mongolia in 2011 and 2012. The aboveground primary productivity, population biomass and density, whole-plant traits (e.g., individual biomass and stem-leaf biomass ratio), leaf physiological (e.g., photosynthetic rate, water use efficiency, and leaf N content) and morphological traits (e.g., leaf area and specific leaf area) of L. chinensis were investigated.
Important findings Our results showed that N and P additions had different effects on the aboveground biomass at different organizational levels. The aboveground net primary productivity was greatly enhanced by the combined N and P additions in 2011 (normal precipitation) and 2012 (above average precipitation), indicating the co-limitation of N and P at the community level. At the species level, N and P additions had no significant effects on population biomass, density, and relative biomass of L. chinensis in both years, indicating that this species could maintain population stability. The individual biomass of L. chinensis was increased by N addition in 2011; whereas it was not affected by either N or P addition in 2012. It is suggested that the growth of L. chinensis may be subjected to N limitation in the dry year, but not to any nutrient limitation in the wet year due to water mediation. Leymus chinensis exhibited high specific leaf area, leaf size and leaf N content, and high photosynthetic yield in response to N enrichment. In conclusion, our results indicate that the primary productivity of typical steppe in Inner Mongolia is co-limited by N and P availability. The functional traits of L. chinensis in response to N and P additions are dependent on the organizational level and mediated by interannual precipitation fluctuations.

Aims Resource-use differentiation among species, which can reduce species competition for the same resources, is the main mechanism to maintain species diversity. Changes in soil temperature and moisture conditions, in the context of global change, may affect nitrogen (N) nutrition of plants of alpine meadow ecosystems. Our objective is to compare the characteristics of N uptake and resource allocation of dominant species of alpine meadow with changes in soil N and water.
Methods An alpine meadow was treated with N and water addition for three years using the method of ¹⁵N isotope injection. We determined the growth responses of dominant species to the N and water additions, as well as the features of N uptake capacity, N allocation and root to shoot ratio.
Important findings The species showed significantly different responses to the N and water treatments, with respect to functional traits of species in N absorption capacity, root N content and root to shoot ratio. There was no significant relationship between N absorption capacity and root N content, whereas N absorption capacity was negatively correlated with root to shoot ratio across all plant species. These results indicated there was ecological niche differentiation in N uptake and a trade-off between the N absorption capacity and resource allocation strategies among species.

Aims We examine how resources influence plant productivity and plant biomass allocation in dry sandy grasslands. Our specific objective is to determine whether 1) annual species and perennial species differ in biomass allocation in response to nitrogen and water addition and 2) if differentiation exists, how nitrogen- and water-induced changes in biomass allocation for species affect ecosystem functioning at the community scale.
Methods This experiment was established in 2010 in the central region of the Horqin sandy grassland in Inner Mongolia. Two treatments, nitrogen and water addition, were started in May, and we measured leaf traits, leaf biomass, stem biomass, stem:leaf ratio, reproductive biomass and vegetative height of the dominant species.
Important findings Biomass allocation in response to nitrogen and water differed between annual species and perennial species. With nitrogen addition the reproductive biomass of annual species decreased 75.6% and the leaf biomass of annual species increased 86.7%. Water addition did not alter biomass allocation, but the interaction of nitrogen and water did. The relative stem biomass values increased 6.2% with nitrogen addition × increased snow in winter treatment, and relative leaf biomass values increased 15.3%. In nitrogen addition × increased precipitation in summer treatments, relative stem biomass and relative leaf biomass increased 28.7% and 19.4%, respectively. Nitrogen and water addition did not alter the biomass allocation pattern of perennial species, but the interaction of nitrogen and water increased reproductive biomass 40%. Nitrogen addition altered biomass allocation at the community scale. Relative reproduction biomass decreased 39.4%, and relative leaf values increased 40.1% with nitrogen addition treatments. On average, relative stem biomass decreased 23.4% and relative leaf biomass increased 57.1% with water addition. The interaction of nitrogen and water addition also significantly altered biomass allocation. Community reproductive biomass decreased 48.3%, community stem biomass decreased 31% and community leaf biomass increased 57.5% in nitrogen × precipitation treatments. We also found that community plant height increased with added nitrogen or water, resulting in a change of community vertical structure.

Aims Our objective was to better understand the response of plant functional traits of Salsola nitraria to environmental change, such as global climate change and nutrient addition, in desert grassland.
Methods We conducted field experiments of moisture and nutrient additions for three years (2009-2011) in a desert grassland of northwest Xinjiang in China. We investigated plant functional traits of S. nitraria and in the fourth year (2012) measured plant height, stem fresh mass, stem dry mass, leaf fresh mass, leaf dry mass, etc. under different treatments of nutrient and water additions.
Important findings The combination of fertilizer and moisture had a significant influence on stem fresh mass, leaf fresh mass, leaf area, specific leaf area, leaf saturated water content, and leaf dry matter content (two-way ANOVA, p < 0.05), while the effect of either fertilizer or moisture alone was not significant. None of the treatment effects on plant height were significant. The maximum values of plant height, stem fresh weight, stem dry weight, leaf saturated fresh weight, leaf dry weight, leaf area, and specific leaf area appeared in the snow and fertilizer additions (N₁W₂), and the second highest was in the control (N₀W₀). Maximum leaf saturated water content was observed in the control (N₀W₀), and the second highest appeared in the snow and fertilizer additions (N₁W₂). The fertilizer addition (N₁W₀) could significantly decrease the level of leaf saturated water content. In all of the treatments, leaf dry matter content was higher than that under the control, and among them, the rain addition (N₀W₂) significantly increased leaf dry matter content. In the control, there were many significant negative correlations between leaf dry matter content and other functional traits and many significant positive correlations between different functional traits. After the fertilizer and/or moisture addition, the correlation coefficient was reduced. In desert grassland of the Junggar Basin, the different performance of S. nitraria under different treatments was the result of plant adaptations to environmental changes.

Aims Phenology is a sensitive, integrated indicator of environmental changes. It is important to research the responses of phenological characteristics to environmental changes for understanding interactions between plants and environment, adaptive mechanisms and survival strategies. Our objective is to examine how phenological characteristics of plants respond to nutrient and water conditions in the eastern Qinghai-Tibet Plateau.
Methods Plant phenological characteristics in alpine Kobresia humilis meadow following nutrient and water additions from 2009 to 2011 were quantitatively analyzed by the methods of nested analysis of variance, phonological index and cluster analysis.
Important findings There were no significant differences in either green-up date or senescence date of plants with nutrient addition, but these dates in several dominant species of grasses, sedges and forbs were postponed following N and P addition. The treatments of nutrient and winter moisture addition moved up flowering dates of dominant species (p < 0.01) and green-up dates of forbs (p < 0.05). Effects of nutrient and summer moisture addition were inconsistent. The senescence dates of Elymus nutans and Scirpus distigmaticus were significantly postponed (p < 0.05) and forbs were postponed. Phenological characteristics of different species had significant differences with nutrient addition (p < 0.01): the senescence dates of Potentilla nivea were significantly postponed (p < 0.05), the senescence dates of Scirpus distigmaticus were significantly moved up (p < 0.05), but the different phenology responses of plants to the nutrient addition were based mainly on plant groups, with the green-up dates of grasses mostly postponed but the green-up dates of sedges moved up. Continuance of vegetative growth and vegetative period after fruiting were negative correlated. Phenological characteristics of plants were divided into three groups by cluster analysis. Phenological characteristics of the treatment of nitrogen-phosphorus-potassium mixed fertilizer, potassium and nitrogen-potassium mixed fertilizer addition varied considerably among plant groups. In summary, phenological characteristics of species in K. humilis meadow displayed large differences after moisture addition, but displayed smaller differences after nutrient addition.

Aims Rising soil temperature under the warming process stimulates microbial activity in soils on the Qinghai- Xizang Plateau. Moreover, the eastern edge of Qinghai-Xizang Plateau has been experiencing distinct atmospheric nitrogen deposition with an increasing trend. All of these have led to an increase in the available nutrients in soils. This study was aimed to determine the responses of carbon fixation in the alpine meadow to nitrogen and phosphorus additions on the Qinghai-Xizang Plateau.
Methods The study was conducted in an alpine meadow ecosystem at the Haibei National Field Research Station of Alpine Grassland Ecosystem, Northwest Institute of Plateau Biology, Chinese Academy of Sciences. Four treatments were set up in 2009, including control, nitrogen addition only (N), phosphorus addition only (P), and combined nitrogen and phosphorus additions (NP). Nutrients were added in June or July each year. The aboveground biomass of functional groups and the above- and belowground biomass of plant communities were measured by harvesting in 2012.
Important findings (1) N and P additions increased the aboveground biomass of grass, and the proportion of grass biomass in the community, but decreased the proportion of forb biomass in the community. Only P addition decreased the aboveground biomass of sedge, and the proportion of sedge biomass in the community. (2) N and P additions increased the aboveground biomass by 24% and 52%, respectively, compared with the control. (3) N addition had no effect on the belowground biomass, whereas P addition slightly increased the belowground biomass. (4) N addition had no effect on the total biomass, whereas P addition significantly increased the total biomass. Therefore, N and P additions could relieve the nutrient limitation and stimulate plant growth. Furthermore, the results suggest that the Qinghai-Xizang Plateau could be more limited by P than N on plant growth.

Aims Better understanding the response of community structure and species diversity to nutrient and water enrichment is critical for desert grassland in arid areas. Nitrogen, phosphorus and potassium are the three elements necessary for plant production, and water promotes plant absorption of nutrients. Our objective was to analyze the effects of nutrient and water addition on community structure and species diversity in typical desert grassland in northwestern China.
Methods Field experiments were conducted in Gubantunggut Desert grassland from 2009 to 2012. Nutrient and water additions were done on May 15, 2009-2011, and vegetation investigation was conducted on May 25, 2009-2012. The responses of community structure and species diversity of plants to nutrient, rainfall and snow additions were studied.
Important findings After the nutrient and water additions, the number of species decreased by 35.3%, the number of species in Poaceae family increased and species in Cruciferae family decreased. In 2009 and 2010, none of the three treatments (nutrient addition, water addition and the coupling effect of nutrient and water) significantly influenced species richness, Simpson index, Shannon-Wiener index or Pielou evenness index. In 2012, nutrient addition had a significant effect on species richness (p < 0.05), while treatments of water addition and the coupling effect of nutrient and water did not. The coupling effect of nutrient and water had significant influence on the Simpson and Shannon-Wiener indexes, while treatments of nutrient addition and water addition did not. The Simpson, Shannon-Wiener, Pielou evenness indexes increased from 2009 to 2012.

Aims Our objective is to determine: 1) how species richness and abundance vary with grazing intensity and topography (i.e., flat vs. slope) in typical steppe of Inner Mongolia grassland, and 2) how common and rare species respond to grazing intensity and what role they play in species diversity maintenance.
Methods The study was carried out at the Sino-German grazing experiment site, which was established in June 2004, and located in the typical steppe region of Inner Mongolia grassland dominated by Leymus chinensis and Stipa grandis. The experimental treatments included seven levels of stocking rates (i.e., 0, 1.5, 3.0, 4.5, 6.0, 7.5, 9.0 sheep·hm^-2) and two topographical systems (i.e., flat and slope). Three hundred quadrats (1 m × 1 m each) were investigated on each plot in August 2009, and the total number of species and the number of individuals for each species were measured within each quadrat. The log-normal model, log-series model, and power fraction model were used to fit the observational data.
Important findings Our results showed that the species richness and abundance increased at low stocking rates (1.5, 3.0 sheep·hm^-2), but decreased at high stocking rates (7.5, 9.0 sheep·hm^-2) on the flat, which partially supported the grazing optimization hypothesis. The power fraction model well fitted for entire species abundance at most of stocking rates, while the log-normal model only fitted well for entire species abundance just at the high stocking rates. The species richness and abundance decreased greatly with stocking rates on slopes. Abundance distribution of entire species at each of the stocking rates followed the log-normal function and power fraction function. The groups of common species and the entire species had similar responses in abundance to grazing at each of the stocking rates on both the flat and the slope, which were well fitted by both the power fraction model and log-normal model; whereas the groups of rare species and the entire species had similar richness responses to grazing at each of the stocking rates under both topographical features, which were well fitted by the power fraction model. It is suggested that the effect of grazing on species abundance in plant community depends on common species; whilst grazing effect on species richness depends on rare species. Our findings indicate that the niche partitioning mechanism plays an important role in species abundance maintenance in grassland ecosystems. To restore and maintain a high level of biodiversity and primary productivity in the Inner Mongolia grassland, it is necessary to reduce the excessively high stocking rate at present to a moderate level in future.

Aims Human disturbance, such as overgrazing have resulted in widespread declines in biodiversity and ecosystem functioning and services in arid and semiarid grasslands worldwide. This study is aimed to examine the effects of grazing intensity, topography and precipitation fluctuation on plant diversity across three levels of organization (i.e., plant species, functional group, and community) in a typical steppe of the Inner Mongolia grassland.
Methods Based on a long-term grazing experiment maintained for eight years with seven levels of grazing intensity and two topographic systems in a typical steppe of the Inner Mongolia grassland, the effects of grazing intensity, topography (flat vs. slope), and precipitation fluctuations (wet vs. normal years) on plant diversity (i.e. α, β and γ diversity) and their controlling mechanisms were examined.
Important findings Our results showed that: (1) The diversity-grazing intensity relationship differed between two topographic systems and over two years (wet vs. normal years). The α, β, and γ diversity in the wet year (2012) were higher than those in the normal year (2011). The effect of topography on plant diversity was dependent on precipitation, with higher α diversity in the slope system and normal year and higher α and γ diversity in the flat system and wet year. There was no significant effect of topography on β diversity. (2) The α diversity decreased with increasing grazing intensity in both the flat and slope systems. The α diversity response to grazing differed substantially among the dominant species, common species, and rare species in the two topographic systems, with the highest negative response for rare species, intermediate negative response for common species, and the weakest response for dominant species. (3) In the flat system, γ diversity declined with increasing grazing intensity, while it firstly decreased and then increased at the intermediate level of grazing intensity in the slope system. (4) The β diversity decreased with increasing grazing intensity in the flat system due to grazing-induced species convergence, while it did not show any tendency in the slope system. Our results suggest that the topography and precipitation are two key factors governing the relationship between plant biodiversity and grazing intensity in the arid and semiarid grasslands. The loss of species in the dry year was greater in the flat system than in the slope system, while the opposite result was found in the wet year. The rare species plays an important role in maintaining species diversity. These findings provide a better understanding of the biodiversity-grazing intensity relationship in the context of different precipitation and topographic conditions in the semiarid steppe and beyond.

Aims Vegetation in Inner Mongolia grassland has changed due to overgrazing and climate change. Our objective was to explore the effects of plant species on soil microbial communities in an Inner Mongolia grassland to understand the ecological consequences of vegetation changes in this region.
Methods We investigated the sizes and community structures of bacteria and fungi in the rhizosphere and non-rhizosphere soils of several typical plant species in a degradation-restoration site in Xilin Gol Grassland using real-time PCR and terminal restriction fragment length polymorphism (T-RFLP) analysis.
Important findings Different plant species have significant effects on the sizes of bacterial communities in both rhizosphere and non-rhizosphere soils and size of fungal communities in rhizosphere soil. The abundances of bacteria and fungi in the rhizosphere soil were generally higher than those in non-rhizosphere soils, particularly for fungi. Multi-response permutation procedures and principal component analysis of T-RFLP data showed significant differences of the bacterial and fungal community structure between rhizosphere and non-rhizosphere soil for most species, and the fungal community structures of all samples can be divided into the rhizosphere and non-rhizosphere soils. In addition, the community structure clustered by plant species is more evident for bacteria. These results revealed important effects of plant species on microbial community composition as well as significant differences of microbial community between rhizosphere and non-rhizosphere soils. Findings are helpful for understanding plants-soil-microbe interactions and mechanisms involved in the Inner Mongolia grassland region.

Aims Soil respiration is a major way that CO₂ is emitted into the atmosphere, and it is important in global change research. Our objective was to examine the effects of degradation on carbon flux in alpine grassland.
Methods We measured soil respiration rates in alpine grassland under four degrees of degradation (no, light, moderate, and heavy degradation) using a LI-8100A open-circuit soil carbon flux measuring system. We analyzed the relationship between soil respiration and soil temperature, as well as between soil respiration and soil moisture.
Important findings Soil respiration under each level of degradation showed a monthly dynamic, but it varied by degree of degradation. With an increase of degradation, average soil respiration of the growing season first increased and then decreased. The highest soil respiration occurred under the moderate level ((2.46 ± 0.27) μmol·m^-2·s^-1), which was significantly higher than under no degradation ((1.92 ± 0.11) μmol·m^-2·s^-1) and heavy degradation ((1.30 ± 0.16) μmol·m^-2·s^-1) (p < 0.01). There was no significant difference between the moderate degradation and the light degradation (p > 0.05). The respiration under heavy degradation was significantly lower than under the other degradation levels (p < 0.01). There was a significant positive linear correlation between aboveground biomass and soil respiration (p = 0.004), but not between soil respiration and underground biomass (p = 0.056). There was a significant positive correlation between soil respiration and soil temperature at each level except heavy degradation. There were correlations between soil respiration and soil moisture (binomial fitting) with no degradation as well as moderate and heavy degradation (p < 0.05), and it was significantly correlated with light degradation (p < 0.01).