Search

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

Global change exposes forest ecosystems to many risks including novel climatic conditions, increased frequency of climatic extremes and sudden emergence and spread of pests and pathogens. At the same time, forest landscape restoration has regained global attention as an integral strategy for climate change mitigation. Owing to unpredictable future risks and the need for new forests that provide multiple ecosystem services, mixed-species forests have been advocated for this purpose. However, the successful establishment of mixed forests requires intrinsic knowledge of biodiversity's role for forest ecosystem functioning. In this respect, a better understanding of tree-tree interactions and how they contribute to observed positive tree species richness effects on key ecosystem functions is critical. Here, we review the current knowledge of the underlying mechanisms of tree-tree interactions and argue that positive net biodiversity effects at the community scale may emerge from the dominance of positive over negative interactions at the local neighbourhood scale. In a second step, we demonstrate how tree-tree interactions and the immediate tree neighbourhood's role can be systematically assessed in a tree diversity experiment. The expected results will improve predictions about the effects of tree interactions on ecosystem functioning based on general principles. We argue that this knowledge is urgently required to guide the design of tree species mixtures for the successful establishment of newly planted forests.

Nitrogen (N) and phosphorus (P) additions are the widely used restoration management for degraded grasslands. However, soil microbial community responses to N and P additions are still not well understood, especially multiple levels or/and combined N and P additions in high-altitude grasslands. In this study, the single and interactive effects of N and P additions on the soil microbes were explored. A field experiment with two factors (N 0, 7.5, and 15 g N m−2 year−1; P 0, 3.27, and 6.55 g P m−2 year−1) was conducted by a completely randomized block design in a Tibetan plateau steppe. Total and specific microbial biomass (bacteria, fungi, G+/G− bacteria, saprotrophs, actinomycetes, and arbuscular mycorrhizal fungi) were analyzed by using phospholipid fatty acids (PLFAs). Additionally, based on their fixed weight composites, we used a composite variable (phenotype) to represent the microbial community. The results showed that N addition decreased the microbe phenotype, total microbial biomass, and specific microbial biomass (total bacteria, G+, and G− bacteria). P application and its interaction with N application all increased the microbial phenotype, total microbial biomass, and specific microbial biomass (total bacteria, G+, G−, saprotrophs, and actinomycetes). Besides, the microbial phenotype was positively correlated with P application rates and not correlated with N application rates. In conclusion, P was the key limiting factor for soil microbes, and its interaction with N is also important for some specific microbes. In addition, long-term responses of soil microbes after N and P amendments need further investigation in the Tibetan alpine steppe.

Drylands cover about 41% of the Earth's land surface and are inhabited by more than two billion people, who rely on the diversified ecosystem services provided by drylands for their livelihoods. Achieving sustainable livelihoods (SLs) is a key component of achieving the sustainable development goals set by the agenda in 2030. Although it has aroused extensive interest, research on SLs in drylands at a regional scale is still limited. This paper aims to address this research gap by evaluating SLs through a geographic gradient of aridity in Inner Mongolia. A sustainable livelihood index (SLI) was developed using a wide range of indicators in a sustainable livelihood framework (SLF). The weight of the indicators was determined by the entropy weight method, and the characteristics of the spatial distribution of the SLI were analyzed. The results showed that the SLI varies greatly across aridity zones. In terms of livelihood assets, the SLI in the dry sub-humid zone was 15% higher than in the arid zones, while, surprisingly, semi-arid zones were found to have the most vulnerable livelihoods (rather than the arid zones). The reason for this is that land management and planning approaches are necessary in drylands. In further detail, Moran's I index illustrated that the overall performance of the SLI of each league or city has a positive spatial correlation, while through local spatial correlation it was found that Hinggan and Chifeng are hot spot areas and Hohhot is a cold spot area. The lack of physical and social capital is an important obstacle for SLs. Based on the analysis of SLs in Inner Mongolia, the characteristics of the sustainable development of local residents were revealed. In this paper, we call for an integrated (i.e., focusing on natural and human capital) land management and planning approach for drylands to reflect the nature of the tightly coupled socio-ecological systems.

Sustainable Livelihoods (SLs) is now a high-priority field in global environmental change research, becoming one of the key research paradigms in sustainability science increasingly and an important component of the Sustainable Development Goals. A Scientometrics analysis based on Science Citation Index-Expanded was performed to understand the research trends and areas in SLs studies. A total of 6441 papers related to SLs studies and 265, 759 references published from 1991 to 2020 were selected as the research objects. Using advanced quantitative analysis tools such as CiteSpace and VOSviewer, we quantify and visualize the characteristics and evolution of the literature in the field of SLs research, to clarify the historical research characteristics, knowledge base, and future research trends. The results show that the annual number of documents increased exponentially since 1991. Ecological sciences and ecology were the most popular Web of Science research areas. The institution with the greatest research documents and most citations was the CGIAR. The most influential journal is Word Development. Singh R.K., Shackleton C.M., was the most productive author. Six clusters of research areas were determined by keyword co-occurrence analysis. The results of the evolution of research hotspots show that the four tags currently still active. We also detected 11 directions of SLs research studies based on the keywords’ score relevance, which has allowed the iden-tification of future lines of research with more importance. These results can help related researchers better understand the past current and future of SLs research studies, which is significant for achieving livelihoods sustainability.

Nitrous oxide (N2 O) emissions from soil contribute to global warming and are in turn substantially affected by climate change. However, climate change impacts on N2 O production across terrestrial ecosystems remain poorly understood. Here, we synthesized 46 published studies of N2 O fluxes and relevant soil functional genes (SFGs, that is, archaeal amoA, bacterial amoA, nosZ, narG, nirK and nirS) to assess their responses to increased temperature, increased or decreased precipitation amounts, and prolonged drought (no change in total precipitation but increase in precipitation intervals) in terrestrial ecosystem (i.e. grasslands, forests, shrublands, tundra and croplands). Across the data set, temperature increased N2 O emissions by 33%. However, the effects were highly variable across biomes, with strongest temperature responses in shrublands, variable responses in forests and negative responses in tundra. The warming methods employed also influenced the effects of temperature on N2 O emissions (most effectively induced by open-top chambers). Whole-day or whole-year warming treatment significantly enhanced N2 O emissions, but daytime, nighttime or short-season warming did not have significant effects. Regardless of biome, treatment method and season, increased precipitation promoted N2 O emission by an average of 55%, while decreased precipitation suppressed N2 O emission by 31%, predominantly driven by changes in soil moisture. The effect size of precipitation changes on nirS and nosZ showed a U-shape relationship with soil moisture; further insight into biotic mechanisms underlying N2 O emission response to climate change remain limited by data availability, underlying a need for studies that report SFG. Our findings indicate that climate change substantially affects N2 O emission and highlights the urgent need to incorporate this strong feedback into most climate models for convincing projection of future climate change.

As an indicator and regulator of climate and environmental change, the Tibet Plateau is an important barrier for ecological security. However, despite the importance of soil microbial communities in almost all soil biochemical processes and ecosystem functions, the biogeography of soil microbial communities on the Tibet Plateau is poorly understood, especially at large scales over different ecosystem types. In this study, we collected samples from 64 sampling sites representing different grassland ecosystem types and spanning 2121 km across on the Tibet Plateau. We then used next generation high-throughput sequencing to investigate the soil prokaryote community (i.e. bacteria and archaea) diversity and spatial patterns and to explore their relationship with biotic (e.g. plant functional group diversity and biomass) and abiotic (e.g. aridity index, soil carbon and nitrogen levels) factors. Among the four alpine grassland types (i.e. alpine meadow, alpine steppe, alpine shrub and alpine desert) sampled in this study, alpine meadow had the highest soil microbial biomass and alpine desert had the lowest soil microbial richness and Shannon diversity. The soil microbial diversity in the alpine grassland correlated with plant diversity and climate factors. Soil microbial diversity negatively correlated with the annual average air temperature, but was not correlated with the annual average precipitation, indicating that temperature, rather than precipitation, may be more important in controlling the soil microbial diversity in alpine grassland ecosystems at cold temperatures. Higher air temperature likely led to an intensified aridity under limited precipitation, and thus decreased microbial diversity. As a result, the aridity index combined with temperature and precipitation explained more of the variance in the soil microbial diversity than air temperature or precipitation did individually. Moreover, after separating plant species into four functional groups (grass, forb, legume and sedge), microbial diversity positively correlated with plant functional group diversity, explaining more of the variance in microbial diversity than plant species diversity did. Results of structural equation modeling revealed that the aridity index and annual air temperature affected soil microbial diversity, directly or indirectly, through influencing plant functional group diversity and aboveground biomass; while aboveground biomass changed the soil carbon to nitrogen ratio in the upper soil layers and thus impacted soil microbial diversity. However, in contrast to microbial diversity, soil microbial biomass carbon was not correlated with plant functional group diversity, plant species diversity, or the climate factors annual average air temperature, annual precipitation and aridity index, but were linked to soil nutrient status (e.g. soil dissolved organic carbon, ammonia, available phosphorus, and carbon to nitrogen ratio) and plant biomass of sedges and forbs, demonstrating that microbial biomass and diversity were likely controlled by different factors. In summary, this study investigated the spatial patterns of soil microbial communities across different alpine grassland ecosystem types on the Tibet Plateau and enhanced our understanding of biotic and abiotic factors controlling microbial biomass and diversity, which will be important in predicting microbial changes on the Tibet Plateau under future climate change. Under future warming and wetting scenarios on the Tibet Plateau, it is possible that the aridity index would decrease, leading to increased soil microbial diversity. Results of this study also suggest a focus on the aridity index and plant functional group diversity in future microbial biogeography studies in order to further determine their roles in controlling or mediating soil microbial biomass and diversity.

Niche complementarity theory explains how species coexist by using different resources. Two pathways to partition resource have been demonstrated: classical niche differentiation and plasticity in resource use. We aimed to determine N-uptake patterns in three Tibetan Plateau grassland species, and to examine how N-partitioning is driven by neighbor interactions. We conducted a transplantation experiment using ten plant communities, each comprising a different combination of Kobresia humilis, Stipa aliena, and Saussurea superba. Soil was sprayed uniformly with a mixture of (NH4)2SO4, KNO3, and glycine (C2H5NO2) (1:1:1 by mass of N, each containing one form of 15N) after growing for 45 days. Across three species, the N-uptake pattern was NO3− > NH4+ > glycine (NO3−: 58.47%; NH4+: 26.91%; glycine: 14.62%). Neighbor presence had species-specific effects on 15N recovery. Kobresia humilis took up more 15N-NO3− when it was in competition with other species, whereas Stipa aliena and Saussurea superba took up more 15N-NH4+ and 15N-glycine, respectively. Plasticity in N resource utilization of the three species was limited. The species competed for N resources proportionally to the availability of these sources, and tended to partition niches. These findings provide important insights into how plant species grow together in alpine grasslands.

Aerobic methanotrophs in upland soils consume atmospheric methane, serving as a critical counterbalance to global warming; however, the biogeographic distribution patterns of their abundance and community composition are poorly understood, especial at a large scale. In this study, soils were sampled from 30 grasslands across >2000 km on the Qinghai-Tibetan Plateau to determine the distribution patterns of methanotrophs and their driving factors at a regional scale. Methanotroph abundance and community composition were analyzed using quantitative PCR and Illumina Miseq sequencing of pmoA genes, respectively. The pmoA gene copies ranged from 8.2 × 10 5 to 1.1 × 10 8 per gram dry soil. Among the 30 grassland soil samples, Upland Soil Cluster Gamma (USCγ) dominated the methanotroph communities in 26 samples. Jasper Ridge Cluster (JR3) was the most dominant methanotrophic cluster in two samples; while Methylocystis, cluster FWs, and Methylobacter were abundant in other two wet soil samples. Interestingly, reanalyzing the pmoA genes sequencing data from existing publications suggested that USCγ was also the main methanotrophic cluster in grassland soils in other regions, especially when their mean annual precipitation was

Heavy rainfall events are expected to increase in frequency and severity in the future. However, their effects on natural ecosystems are largely unknown, in particular with different seasonal timing of the events and recurrence over multiple years. We conducted a 4 yr manipulative experiment to explore grassland response to heavy rainfall imposed in either the middle of, or late in, the growing season in Inner Mongolia, China. We measured hierarchical responses at individual, community and ecosystem levels. Surprisingly, above-ground biomass remained stable in the face of heavy rainfall, regardless of seasonal timing, whereas heavy rainfall late in the growing season had consistent negative impacts on below-ground and total biomass. However, such negative biomass effects were not significant for heavy rainfall in the middle of the growing season. By contrast, heavy rainfall in the middle of the growing season had greater positive effects on ecosystem CO2 exchanges, mainly reflected in the latter 2 yr of the 4 yr experiment. This two-stage response of CO2 fluxes was regulated by increased community-level leaf area and leaf-level photosynthesis and interannual variability of natural precipitation. Overall, our study demonstrates that ecosystem impacts of heavy rainfall events crucially depend on the seasonal timing and multiannual recurrence. Plant physiological and morphological adjustment appeared to improve the capacity of the ecosystem to respond positively to heavy rainfall.

Long-duration drought can alter ecosystem plant species composition with subsequent effects on carbon cycling. We conducted a rainfall manipulation field experiment to address the question: how does drought-induced vegetation change, specifically shrub encroachment into grasslands, regulate impacts of subsequent drought on soil CO2 efflux (Rs) and its components (autotrophic and heterotrophic, Ra and Rh)? We conducted a two-year experiment in Inner Mongolia plateau, China, using constructed steppe communities including graminoids, shrubs and their mixture (graminoid + shrub) to test the effects of extreme-duration drought (60-yr return time) on Rs, Rh and Ra. Our results indicated that extreme-duration drought reduced net primary production, with subsequent effects on Rs, Rh and Ra in all three vegetation communities. There was a larger relative decline in Ra (35–54%) than Rs (30–37%) and Rh (28–35%). Interestingly, we found Rs in graminoids is higher than in shrubs under extreme drought. Meanwhile, Rh declines were largest in the shrub community. Although Ra and Rh both decreased rapidly during drought treatment, Rh recovered quickly after the drought, while Ra did not, limiting the Rs recovery. This study suggests that plant species composition regulates several aspects of soil CO2 efflux response to climate extremes. This regulation may be limited by above- and below-ground net primary production depending on soil water availability. The results of this experiment address a critical knowledge gap in the relationship between soil respiration and plant species composition. With shrub encroachment into grasslands, total soil respiration is reduced and can partly offset the effect of reduction in productivity under drought stress.

The Hindu Kush Himalayan (HKH) region is one of the most ecologically vulnerable regions in the world. Several studies have been conducted on the dynamic changes of grassland in the HKH region, but few have considered grassland net ecosystem productivity (NEP). In this study, we quantitatively analyzed the temporal and spatial changes of NEP magnitude and the influence of climate factors on the HKH region from 2001 to 2018. The NEP magnitude was obtained by calculating the difference between the net primary production (NPP) estimated by the Carnegie–Ames Stanford Approach (CASA) model and the heterotrophic respiration (Rh) estimated by the geostatistical model. The results showed that the grassland ecosystem in the HKH region exhibited weak net carbon uptake with NEP values of 42.03 gC∙m−2∙yr−1, and the total net carbon sequestration was 0.077 Pg C. The distribution of NEP gradually increased from west to east, and in the Qinghai–Tibet Plateau, it gradually increased from northwest to southeast. The grassland carbon sources and sinks differed at different altitudes. The grassland was a carbon sink at 3000–5000 m, while grasslands below 3000 m and above 5000 m were carbon sources. Grassland NEP exhibited the strongest correlation with precipitation, and it had a lagging effect on precipitation. The correlation between NEP and the precipitation of the previous year was stronger than that of the current year. NEP was negatively correlated with temperature but not with solar radiation. The study of the temporal and spatial dynamics of NEP in the HKH region can provide a theoretical basis to help herders balance grazing and forage.

Aims Grasslands are globally threatened by climate changes and unsustainable land-use, which often cause transitions among alternative stable states, and even catastrophic transition to desertification. Spatial vegetation patch configurations have been shown to signify such transitions at large spatial scale. Here, we demonstrate how small-scale patch configurations can also indicate state transitions. Methods The whole spatial series of degradation successions were chosen in alpine grasslands characterized as seven typical communities. Patch numbers, and perimeter and cover of each patch were recorded using adjacent quadrats along transects in each type of the communities. Species abundance within each patch was measured. Important Findings Across seven grazing-induced degradation stages in the world’s largest expanse of grassland, from dense ungrazed turf to bare black-soil crust, patch numbers and perimeters first increased as patch cover decreased. Numbers and perimeters then decreased rapidly beyond an intersection point at 68% of initial continuous vegetation cover. Around this point, the vegetation fluctuated back and forth between the sedge-dominated grassland breaking-up phase and the forb-dominated phase, suggesting impending shift of grassland state. This study thus demonstrates how ground-based small-scale vegetation surveys can provide a quantitative, easy-to-use signals for vegetation degradation, with promise for detecting the catastrophic transition to desertification.

Increasing heavy rainfalls can strongly affect ecosystem nitrogen (N) cycling processes and thereby alter soil N fluxes and pools. However, the effects of heavy rainfalls on soil N fluxes and pools are poorly understood, particularly with regards to high rainfall timing under field conditions. We conducted a 3-year (2014–2016) manipulative experiment in which heavy rainfall was imposed in middle (plant peak growing stage) or late (plant senescent growth stage) growing season in a semiarid grassland of Inner Mongolia, China to explore the responses of N2O fluxes and soil total N contents and the underlying microbial mechanisms. Mid-season heavy rainfall promoted soil N2O emissions by 65% on average across the three years, attributable mainly to increases in denitrifying nirK and nirS abundances induced large denitrification at higher soil water contents. However, archaeal and bacterial amoA and narG genes did not change significantly due probably to counteracting effects of increased soil water content (positive) and soil pH (negative). Mid-season heavy rainfall led to 17% reduction in soil total N by the end of the last year of the study (2016), partly due to the enhanced accumulated N2O emissions over the three years. In contrast, late-season heavy rainfall did not change N2O emissions and soil total N contents even though soil water content, soil pH and nirK and nirS abundance were significantly increased, perhaps due to limitation by low temperature. Timing of the heavy rainfall events during the plant growing season strongly influenced their impacts on soil N fluxes and pools and heavy rainfalls in the peak stage of plant growth may potentially cause a positive feedback to global warming and exacerbate N limitation in terrestrial ecosystems.

Grazing is expected to exert a substantial influence on antibiotic resistance genes (ARGs) in grassland ecosystems. However, the precise effects of grazing on the composition of ARGs in grassland soils remain unclear. This is especially the case for grassland soils subject to long-term grazing. Here, we investigated ARGs and bacterial community composition in soils subject to long-term historic grazing (13–39 years) and corresponding ungrazed samples. Using a combination of shotgun metagenomics, amplicon analyses and associated soil physicochemical data, we provide novel insights regarding the structure of ARGs in grassland soils. Interestingly, our analysis revealed that long-term historic grazing had no impacts on the composition of ARGs in grassland soils. An average of 378 ARGs, conferring resistance to 14 major categories of antibiotics (80%), were identified in both grazing and ungrazed sites. Actinobacteria, Proteobacteria and Acidobacteria were the most prevalent predicted hosts in these soils and were also shown to harbour genetic capacity for multiple-resistant ARGs. Our results suggested that positive effects of bacterial community composition on ARGs could potentially be controlled by affecting MGEs. Soil properties had direct effects on the composition of ARGs through affecting the frequency of horizontal gene transfer among bacteria. Twelve novel ARGs were found in S. grandis steppe grasslands, indicating that different vegetation types might induce shifts in soil ARGs. Collectively, these findings suggest that soil properties, plants and microorganisms play critical roles in shaping ARG patterns in grasslands. Together, these data establish a solid baseline for understanding environmental antibiotic resistance in grasslands.

In the context of climate change, precipitation is predicted to become more intense at the global scale. Such change may alter soil microbial communities and the microbially mediated carbon and nitrogen dynamics. In this study, we experimentally repackaged precipitation patterns during the growing season (from June to September) of 2012 in a semi-arid temperate steppe of the Xilin River Basin in Inner Mongolia of China, based on the 60-year growing season precipitation data. Specifically, a total amount of 240 mm simulated precipitation was assigned to experimental plots by taking the following treatments: (1) P6 (6 extreme precipitation events, near the 1st percentile); (2) P10 (10 extreme precipitation events, near the 5th percentile); (3) P16 (16 moderate precipitation events, near the 50th percentile); and (4) P24 (24 events, 60-year average precipitation, near the 50th percentile). At the end of the growing season, we analyzed soil microbial community structure and biomass, bacterial abundance, fungal abundance and bacterial composition, by using phospholipid fatty acid (PLFA), real-time quantitative polymerase chain reaction (RT-qPCR) and 16S rRNA gene clone library methods. The extreme precipitation events did not change soil microbial community structure (represented by the ratio of PLFA concentration in fungi to PLFA concentration in bacteria, and the ratio of PLFA concentration in gram-positive bacterial biomass to PLFA concentration in gram-negative bacterial biomass). However, the extreme precipitation events significantly increased soil microbial activity (represented by soil microbial biomass nitrogen and soil bacterial 16S rRNA gene copy numbers). Soil fungal community showed no significant response to precipitation events. According to the redundancy analysis, both soil microbial biomass nitrogen and soil ammonium nitrogen (NH4-N) were found to be significant in shaping soil microbial community. Acidobacteria, Actinobacteria and Proteobacteria were the dominant phyla in soil bacterial composition, and responded differently to the extreme precipitation events. Based on the results, we concluded that the extreme precipitation events altered the overall soil microbial activity, but did not impact how the processes would occur, since soil microbial community structure remained unchanged.

Climate models predict a substantial increase in the frequency of extreme drought, suggesting subsequent impacts on the carbon (C) and water cycles. Although many studies have investigated the impacts of extreme drought on ecosystem functioning, it remains unknown how the timing of extreme drought within a growing season may affect carbon and water cycling. Here we conducted a 3-year field experiment to investigate the influence of seasonal drought timing on ecosystem carbon and water exchange by excluding rainfall (for consecutive 30 days) during three periods of the growing season (May–June, July–August and August–September) in fenced and grazed sites of a semiarid temperate steppe in Inner Mongolia, China. In the fenced steppe, extreme drought reduced growing-season net CO2 uptake regardless of drought timing, while in the grazed steppe, early-growing season drought caused relatively larger reductions to net CO2 uptake than drought imposed later in the season. The effect of extreme drought on evapotranspiration (ET) was similar to that of CO2 exchange at the fenced site, with consistent reductions of seasonally-integrated ET for all treatments compared with the ambient condition. In contrast, at the grazed site, the response of ET to extreme drought was more variable, possibly due to the absence of litter and greater bare ground. Surprisingly, both gross and net carbon uptake declined with increasing ET at the grazed site, while the fenced site showed the positive water-carbon linkage typically seen in semiarid ecosystems. The different responses of CO2 and water exchanges for the fenced and grazed sites were regulated predominately by soil temperature and soil water content. Together, our results show that drought timing within the growing season can significantly alter drought impacts on ecosystem water and CO2 exchanges, and that grazing management may further mediate the response.

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

General circulation models predict that precipitation will become more extreme, i.e. rainfall events of larger size but reduced frequency. Studies in North American grasslands have shown that such repackaging of precipitation into fewer, larger events enhanced above ground net primary productivity (ANPP), likely due to deeper soil moisture infiltration favoring plant water use over evaporation. However, ANPP responses in other regions remain poorly understood, and responses of carbon and water exchanges with the atmosphere remain unknown. Here we manipulated rainfall in a steppe ecosystem of northern China over 4 years to investigate how temporal packaging of precipitation impacts ANPP, evapotranspiration (ET), net ecosystem CO2 exchange (NEE) and the component fluxes gross primary productivity (GPP) and ecosystem respiration (RE). Experimental plots received precipitation equivalent to the 60-year growing-season average of 240 mm, variously packaged into 6, 10, 16, or 24 events representing extreme (P6) to historical average (P24) rainfall frequency. Extraordinarily extreme frequency (6 large events) reduced NEE, GPP, RE, ET and water use efficiency (WUE = |NEE|/ET). The average NEE, GPP and RE declined 35%, 45% and 48% respectively in the P6 treatment as compared to P16, which showed maximum ET and CO2 exchange. After peaking in the 16-event treatment, GPP and WUE in P24 were not distinguishable from P6. These peaks suggest that P16 was optimal for photosynthesis, with sufficiently frequent rain to maintain unregulated plants and adequately deep soil moisture infiltration to favour transpiration, with associated carbon uptake, over evaporation. Path analysis indicated the lower CO2 fluxes were influenced by reduced soil water content and leaf area index and higher soil temperature, with ET regulating the effects of these microclimatic drivers. ANPP showed a monotonic but non-significant decline with decreasing precipitation frequency, consistent with reduced CO2 fluxes. We found an increase in ANPP of xerophyte plants partially compensated for the ANPP decline in the dominant eurytopic xerophyte plants. Our results suggest that extreme temporal repackaging of precipitation into few events with correspondingly long dry intervals may reduce the capacity of steppe ecosystems to assimilate atmospheric CO2, although community diversity may moderate impacts.

Extreme drought can strongly impact belowground communities and biogeochemical processes, including soil microbial community composition and extracellular enzyme activities (EEAs), which are considered key agents in ecosystem carbon (C) and nutrient cycling. However, our understanding of how seasonal timing of drought during the growing season affects soil microbial communities and their activity remains notably poor. In this study, we investigated the responses of soil physicochemical properties, EEAs, and bacterial community composition to extreme-duration drought imposed in the early-, mid-, or late-stages of the growing season in a semiarid grassland ecosystem in Inner Mongolia, China. Compared with the ambient control, the activities of C-, nitrogen (N)-, and phosphorus (P)-acquisition enzymes were significantly decreased in the mid- and/or late-stages of drought. Bacterial community diversity also significantly decreased in the mid- and late-stage drought treatments. Soil water content was the most important factor explaining changes in soil EEAs and bacterial community composition. At the end of the growing season, the activities of C-, N-, and P-acquisition enzymes had mostly recovered, while the bacterial community diversity in the mid- and late-stage drought treatments was still lower than the ambient control. Overall, our study demonstrates that the effects of extreme drought on soil EEAs and bacterial community composition depend on the timing of drought. Our results highlight that understanding the effects of extreme-duration drought at different stages of the growing season may play a vital role in predicting the responses of belowground function to global changes in grassland ecosystems.

In this study, a process-based denitrification-decomposition (DNDC) model was parameterized and validated with CO2 fluxes data measured using eddy covariance (EC) technique in the meadow and desert steppes of Inner Mongolia grasslands, China. The validated model was applied to predict the influence of moderate climate change on carbon (C) budget of the two steppes over the next 30 years. A series of parameters associated with plant growth including maximum biomass production, C/N ratio of plant tissue, and accumulating thermal degree days based on local observations were adjusted to meet the requirements of adapting DNDC for the two targeted steppes. Daily weather data for the studied year together with soil properties for the study sites were employed as inputs to simulate the grass growth and soil C dynamics. A baseline and five alternative climate scenarios were utilized to estimate the effect of climate change on the ecosystem C dynamics for the next 30 years. The coefficient of determination (R 2), root mean square error (RMSE), relative mean deviation (RMD), and modeling efficiency (EF) were used to assess the DNDC model based on the EC measurements. The DNDC model successfully captured the maximum and minimum soil surface temperature and the effects of rainfall pulse on soil water content (SWC). Validation of ecosystem daily CO2 fluxes against the EC observed values resulted in satisfactory results in both of the meadow and the desert steppe. Additionally, the comparison of monthly simulated results is better than that of daily results. Between 2011 and 2040, if local climate becomes wetter, both steppes would become C sinks, leading to a great net primary productivity (NPP) of 821 and 828 kg C ha−1 year−1 sequestered in soil organic C (SOC), respectively. If local climate becomes drier, they would become C-sources, having the lowest NPP with 166 and 121 kg C ha−1 year−1 lost from the soil C pool, respectively. The validated DNDC model can be used to predict C dynamics in two semiarid grassland ecosystems. For the upcoming 30 years, if the local climate became wetter, the meadow and desert steppes would act as C sink, absorbing CO2 from the atmosphere and increase the C sequestration potential. However, both steppes would act as C source, releasing CO2 to the atmosphere, and decrease the C sequestration when the local climate becomes drier.

Global climate change is projected to increase both the intensity and frequency of extreme precipitation events (EPEs), which are considered to have stronger impacts on ecosystem functions than gradual changes in mean precipitation conditions. In this study, a consecutive 20-day extreme precipitation event (282 mm) was applied during the mid- and late-growing season periods in a semiarid steppe for three years to investigate the effects of extreme large precipitation events on aboveground net primary productivity (ANPP) and ecosystem carbon dioxide (CO 2 ) fluxes, including net ecosystem carbon absorption (NEE), gross primary productivity (GPP) and ecosystem respiration (Re). Although soil moisture was significantly increased by extreme precipitation, and even exceeded field capacity during the treatment periods, ANPP remained stable across all the treatments. There was also little change in mean growing season ecosystem CO 2 fluxes under the two precipitation treatments, despite GPP rates decreased by 34.4 and 26.3%, and NEE rates were suppressed by 77 and 68% during the mid- and late-season treatment periods, respectively. The stable CO 2 fluxes could be attributed to the recovery of GPP and NEE in 7 and 12 days after the end of EPEs. Our study demonstrated that both ANPP and CO 2 fluxes in this semiarid steppe were very stable in the face of extreme large precipitation events, regardless of the timing of events occur. Nevertheless, future, long-term studies need to investigate the potential tipping points or thresholds for ecosystem function shifts, as an increasing occurrence of EPEs has been forecasted in future climate change scenarios.

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

The prevailing trend in pasture areas worldwide is that of mobile pastoralism to settlement, which produces a positive impact on pastoral livelihoods and livestock husbandry. However, the impact of nomad settlement on the grassland ecosystem is not well documented and remains debatable. In response, from 2001 to 2015, the central Chinese government initiated the Nomad Settlement Policy (NSP). In this study, we conducted a case study of the pastoral area of the Qinghai-Tibetan Plateau, to investigate impact of NSP on grassland ecological conditions including plants, soil and microorganisms. Results showed that grassland ecological conditions presented differentiation characteristics, with changes depending on the distance from settlements. The grassland ecological conditions showed heavy degradation near the settlement based on the classification of Qinghai-Tibetan Plateau grassland degradation, and gradual improvement with increasing distance from the settlement. Based on our investigation and previous studies, we found that intervention of NSP decreased the distance in livestock mobility and led to intensive grazing near the settlement, thereby increased grassland degradation. At the same time, the grassland maintained a relatively good ecological condition with the increase in distance from settlement, which may be attributed to short-period grazing and light trampling effects. Our findings provide new insight into the grassland ecological condition in the aftermath of NSP implementation, and also put forward some measures (e.g. multi-household grazing management, pastoral cooperative) to restore the grassland degradation.

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

Huge numbers of microorganisms reside in livestock faeces and constitute one of the most complex microbial ecosystems. Here, faecal microbial communities of three typical livestock in Xilingol steppe grassland, i.e. sheep, cattle, and horse, were investigated by Illumina MiSeq sequencing and quantitative real-time polymerase chain reaction (qPCR). Firmicutes and Bacteroidetes comprised the majority of bacterial communities in three livestock faeces. Sordariomycetes, Leotiomycetes, and Dothideomycetes were dominant in fungal communities, as well as Methanobacteria and Methanomicrobia were dominant in archaeal communities in three livestock faeces. Similar fungal community dominated in these samples, with 95.51% of the sequences falling into the overlap of three livestock faeces. In contrast, bacterial communities were quite variable among three different livestock faeces, but a similar community was observed in sheep and cattle faeces. Nearly all the archaea were identified as methanogens, whilst the most diverse and abundant methanogens were detected in cattle faeces. Potential pathogens including Bacteroides spp., Desulfovibrio spp., and Fusarium spp. were also detected in livestock faeces. Overall, this study provides the first detailed microbial comparison of typical livestock faeces dwelling on single grassland, and may be help guide management strategies for livestock grazing and grassland restoration.

Environmental factors that are important in shaping microbe community structure are less explored along elevation in the alpine grassland ecosystem of Tibet Plateau, which is generally phosphorus limited. Here, we examined soil prokaryote communities at three elevations to explore soil prokaryote community distribution and mediation factors in Noijin Kangsang Peak, Tibetan Plateau. Results showed prokaryote community compositions differed significantly by elevations. Topsoil or subsoil prokaryote richness and Shannon diversity were significantly lower at the middle than other elevations, while significantly higher aboveground biomass (AGB) and available P (AP) were found at the middle elevation. The importance of P for both soil layers was discovered by variation partitioning analysis based on redundancy analysis, finding that soil AP and total phosphorus, interacted with pH, explained 43% the variance in topsoil prokaryote community compositions, while soil AP, as well as AGB, explained 44% in subsoil. Consistently, structural equation model also revealed that AP was a mediating factor for prokaryote community diversity. Other than plant beta diversity, soil prokaryote beta diversity positively correlated with AP difference significantly. Taken together, the distribution patterns of soil prokaryote community were distinct along elevations even in a small scale in Noijin Kangsang Peak and was likely mediated predominantly by soil AP in both topsoil and subsoil.

Climate extremes are expected to increase in frequency and intensity as a consequence of anthropogenic climate change attributed to the rise of atmospheric concentrations of greenhouse gases (GHGs). However, studies on the impacts of climate extremes on terrestrial ecosystems are limited. Here, we experimentally imposed extreme drought and a heat wave (∼60-year recurrence) to investigate their effects on GHGs fluxes of a semiarid grassland in China. We estimated a 16% and 38% percent reduction in net ecosystem CO2 uptake caused by the heat wave and drought respectively, but via different mechanisms. Drought reduced gross ecosystem productively (GEP) and to a lower extent ecosystem respiration (ER). By contrast, the simulated heat wave suppressed only GEP while ER remained stable. The climate extremes also created a legacy effect on GEP and NEE lasting until the end of the growing season, whereas ER recovered immediately. Although CH4 and N2O fluxes were unaffected by the heat wave, drought promoted CH4 uptake and suppressed N2O emission during the treatment period. The effect of drought on GHGs fluxes generally overwhelmed that of the heat wave treatment, and there were no interactive effects of these two types of climate extremes. Our results showed that responses of ecosystem GHGs exchange to climate extremes are strongly regulated by soil moisture status. In conclusion, future amplification of climate extremes could decrease the sink for GHGs, especially CO2, in this semiarid grasslands.

Increasing frequency and magnitude of climate extremes could fundamentally affect terrestrial carbon (C) cycling. However, as the second-largest terrestrial C flux, soil respiration (SR) responses to climate extremes are not well understood. Here, we investigated the effects of drought, heat wave and drought plus heat wave on SR in a semiarid grassland during the growing season over 3 years. The results indicated that drought consistently reduced the growing-season mean SR, especially during the period of drought treatment, while heat wave, alone or when combined with drought, had little effect on SR. The decreased SR under drought at the intra-annual timescale could be primarily attributable to restriction of low soil moisture to microbial biomass, as there were no consistent changes in plant community (aboveground biomass, richness and abundance). In contrast, the inter-annual variation in SR was positively related to plant community abundance, richness and aboveground biomass in additional to soil water availability, but was not significantly affected by microbial biomass. Our study highlighted that incorporating microbial biomass in C cycling models can improve simulation of seasonal dynamics of soil respiration while incorporating plant community characteristics can benefit prediction of variation in SR across multiple years in semiarid grasslands.

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

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

An accurate and synoptic quantification of gross primary production (GPP) in wetland ecosystems is essential for assessing carbon budgets at regional or global scales. In this study, a satellite-based Vegetation Photosynthesis Model (VPM) integrated with observed eddy tower and remote sens- ing data was employed and adapted to evaluate the feasibility and dependability of the model for estimating GPP in an alpine wetland, located in Zoige, Southwestern China. Eddy flux data from 2-year observations showed that temperature explained mostof theseasonal variability incarbonfluxesand that warming increased GPP and ecosystem respiration, and hence affected the carbon balance of alpine wetlands. The comparison between modeled and observed GPP fluxes indi- cated that simulated values were largely in agreement with tower-based values (P

Climate change is causing measurable change in rainfall patterns with uncertain implications for key processes in ecosystem function and carbon cycling. We conducted a modeling analysis to identify how variation in amount and frequency of precipitation affected on CO2 fluxes and net primary productivity. The denitrification-decomposition model was used to quantify the effects of altered precipitation quantity and frequency under 12 climate scenarios over a period of 30-years on both a fenced and a moderately grazed temperate steppe in Inner Mongolia, China. The modeling results show the 12 climate scenarios had an obvious effect on gross primary productivity (GPP), ecosystem respiration (Re) and net primary productivity (NPP) in both a fenced and a grazed site. GPP, Re and NPP increased in both sites under increased precipitation scenarios called A3 (+20% precipitation) and A4 (+40%) when compared with baseline conditions, while GPP, Re and NPP declined under decreased precipitation scenarios A1 (−40% precipitation) and A2 (−20%) scenarios. The changed rainfall frequency resulted in a decline in GPP, Re and NPP compared with those parameters under the base conditions at both sites. The ecologically effective rainfall (ER), not total rainfall, controls the ecosystem CO2 sink/source function. When ER exceeded 318 mm yr−1 in the fenced site and 224 mm yr−1 in the grazed site, the steppe switched from CO2 emission to CO2 absorption. CO2 fluxes in the typical steppe which was fenced and moderately grazed are relatively responsive to changes in the amount of rainfall. However, in terms of the long-term modeling analysis, the modeled results suggest the effects of altered rainfall quantity, frequency and the interaction of rainfall quantity and frequency on CO2 fluxes and plant productivity had no significant difference because of the fluctuating interannual biotic and abiotic factors.

Summary The Zoige Plateau is characterized by its high altitude, low latitude and low annual mean temperature of approximately 1°C and is a major source of atmos- pheric methane in the Qinghai-Tibetan Plateau. Meth- anotrophs play an important role in the global cycling of CH 4 , but the diversity, identity and activity of meth- anotrophs in this region are poorly characterized. Soils were collected from hummocks and hollows in the Riganqiao peatland and the methanotroph com- munity was analysed by qPCR and sequencing methane monooxygenase (pmoAandmmoX) genes. ThepmoAgenes ranged between 10 7 and 10 8 cop- ies g -1 fresh soil, with a somewhat greater abundance in hummocks than hollows. ThepmoAgenes were analysed by amplicon pyrosequencing and themmoX genes by cloning and sequencing.Methylocystis species were found to be the most abundant meth- anotrophs, but numerous clades were present includ- ing three novelpmoAand three novelmmoXclusters. There were differences between the methanotroph communities in the hummocks and hollows, with the most significant being an increased abundance of uncultivated type Ib methanotrophs in the hollows. The results indicate that aerobic methanotrophs are abundant in Riganqiao peatland and include previ- ously undetected clades in this geographically iso- lated and distinctive environment.

Purpose Carbon (C) dynamics in grassland ecosystem contributes to regional and global fluxes in carbon dioxide (CO2) concentrations. Grazing is one of the main structuring factors in grassland, but the impact of grazing on the C budget is still under debate. In this study, in situ net ecosystem CO2 exchange (NEE) observations by the eddy covariance technique were integrated with a modified process-oriented biogeochemistry model (denitrification–decomposition) to investigate the impacts of grazing on the long-term C budget of semiarid grasslands.

We used a minirhizotron system to investigate the influence of three major factors—root morphology, root depth, and season of root emergence—on root survivorship and longevity in a Kobresia humilis meadow on the Tibetan Plateau during the growing season of 2009. Root longevity was assessed by survival analysis, Kaplan–Meier analysis, and Cox proportional hazards regression. Root longevity was correlated positively with root diameter. A 17.5 % decrease in the risk of mortality was associated with a 0.1-mm increase in diameter. Roots distributed in the top 10 cm of the soil had significantly shorter longevities than roots at greater depths, with a 48 % decrease of mortality risk for each 10-cm increase in soil depth from the surface to 40 cm. Of all the factors examined, the season of root emergence had the strongest effect on root lifespan. Roots that emerged in May and June had shorter longevity than roots that emerged later in the year, and roots that emerged in September and October were more likely to survive over winter. Our findings indicated that life-history traits of roots in K. humilis meadows are highly heterogeneous, and this heterogeneity should be considered when modeling the contribution of roots to carbon and nitrogen fluxes in this type of meadow ecosystem. Moreover, temporal, spatial, and compositional variations in root longevity must be considered.

Sedge-dominated wetlands on the Qinghai–Tibetan Plateau are methane emission centers. Methanotrophs at these sites play a role in reducing methane emissions, but relatively little is known about the composition of active methanotrophs in these wetlands. Here, we used DNA stable isotope probing to identify the key active aerobic methanotrophs in three sedge-dominated wetlands on the plateau. We found that Methylocystis species were active in two peatlands, Hongyuan and Dangxiong. Methylobacter species were found to be active only in Dangxiong peat. Hongyuan peat had the highest methane oxidation rate, and cross-feeding of carbon from methanotrophs to methylotrophic Hyphomicrobium species was observed. Owing to a low methane oxidation rate during the incubation, the labeling of methanotrophs in Maduo wetland samples was not detected. Our results indicate that there are large differences in the activity of methanotrophs in the wetlands of this region.

Tibetan Plateau is one of the largest and most unique habitats for organisms including arbuscular mycorrhizal fungi (AMF). However, it remains unclear how AMF communities respond to key environmental changes in this harsh environment. To test if precipitation could be a driving force in shaping AMF community structures at regional scale, we examined AMF communities associated with dominant plant species along a precipitation gradient in Tibetan alpine steppe. Rhizosphere soils were collected from five sites with annual precipitation decreasing from 400 to 50 mm. A total of 31 AMF operational taxonomic units (OTUs) were identified. AMF community composition varied significantly among sites, whereas AMF community composition did not vary among plant species. Path analysis revealed that precipitation directly affected AMF hyphal length density, and indirectly influenced AMF species richness likely through the mediation of plant coverage. Our results suggested that water availability could drive the changes of AMF communities at regional scale. Given the important roles AMF could play in the dynamics of plant communities, exploring the changes of AMF communities along key environmental gradients would help us better predict the ecosystem level responses of the Tibetan vegetation to future climate change.

The alpine meadow on the Tibetan Plateau is the highest and largest pasture in the world, and its formation and distribution are mainly controlled by Indian summer monsoon effects. However, little is known about how monsoon-related cues may trigger spring phenology of the vast alpine vegetation. Based on the 7-year observations with fenced and transplanted experiments across lower to upper limits of Kobresia meadows in the central plateau (4400–5200 m), we found that leaf unfolding dates of dominant sedge and grass species synchronized with monsoon onset, regardless of air temperature. We also found similar patterns in a 22-year data set from the northeast plateau. In the monsoon-related cues for leaf unfolding, the arrival of monsoon rainfall is crucial, while seasonal air temperatures are already continuously above 0 °C. In contrast, the early-emerging cushion species generally leafed out earlier in warmer years regardless of precipitation. Our data provide evidence that leaf unfolding of dominant species in the alpine meadows senses the arrival of monsoon-season rainfall. These findings also provide a basis for interpreting the spatially variable greening responses to warming detected in the world’s highest pasture, and suggest a phenological strategy for avoiding damages of pre-monsoon drought and frost to alpine plants.

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

In water-limited ecosystems, an ecologically significant rainfall pulse was defined as a rainfall event that altered both soil water status and plant physiological activity. We developed a new threshold concept of an ecologically effective precipitation pulse (EEPP) applicable to both plant individual and ecosystem scales. The concept was tested in a typical steppe on Inner Mongolia plateau. Two EEPPs, single 3-mm rainfall and 5-mm rainfall, were applied to investigate their effects on soil and plant water status, CO 2 assimilation of five species (four C 3 plants and one C 4 plant), whole-plot soil respiration ( R s ), and net ecosystem CO 2 exchange (NEE) on 1 June and 28 July 2009, respectively. Both EEPPs increased leaf water potential ( Ψ l ) of all the species, which peaked 1–3 days after rainfall pulses. Soil water content ( SWC ) in two depths (5 cm and 20 cm) significantly increased after the two EEPPs for 1–3 days. Soil water potential ( Ψ s ) within 20‐cm soil layer in EEPP treatments significantly differed ( p A net ) of all C 3 plants had a slight increase at the next day after two EEPPs, in contrast to the C 4 species. R s elevated and peaked 1–3 days later after water supply. Ecosystem net CO 2 absorption rate rose to maximum value 3 days after the 5-mm pulse on 28 July, higher than the response to 3-mm pulse on June 1. The grassland turned to net emission of CO 2 after 3-mm pulse on 28 July. The results supported that there was an ecosystem level threshold for EEPP, and the threshold was temporally variable. It also highlighted the necessity of considering the response threshold of EEPP in rainfall manipulative experiment. In addition, effective rainfall amount was more approriate than total rainfall amount in modeling ecosystem carbon balance.

Understanding the responses of soil C mineralization to climate change is critical for evaluating soil C cycling in future climatic scenarios. Here, we took advantage of a multifactor experiment to investigate the individual and combined effects of experimental warming and increased precipitation on soil C mineralization and 13C and 15N natural abundances at two soil depths (0–10 and 10–20 cm) in a semiarid Inner Mongolian grassland since April 2005. For each soil sample, we calculated potentially mineralizable organic C (C 0) from cumulative CO2-C evolved as indicators for labile organic C. The experimental warming significantly decreased soil C mineralization and C 0 at the 10–20-cm depth (P

The CO(2) flux was measured by the eddy covariance method on a temperate Leymus chinensis steppe over a period of 17 months spanning two consecutive growing seasons. The amount of precipitation was nearly normal, but it was low in the early and high in the late growing period in 2006. In the 2007 growing season, the amount of precipitation was about 45% less than the multi-year average and more evenly distributed. Comparisons were made between a moderately grazed site and a 28-year-old fenced site. The maximum instantaneous CO(2) release and uptake rates were 0.12 (May) and -0.11mg CO(2)m(-2)s(-1) (July) at the fenced site, and 0.11 and -0.16mg CO(2)m(-2)s(-1) (both in July) at the grazed site. In both growing seasons, the grazed site always had a higher daily uptake rate or lower release rate than the fenced site. The grazed site was a CO(2) sink during the growing season of 2007 and a CO(2) source in the growing season of 2006, whereas the fenced site was a CO(2) source in both seasons. Lower precipitation decreased CO(2) loss during the growing season more in the grazed site than in the fenced site, mainly because of depression of total ecosystem respiration (R(e)) in the former and stimulation in the latter. During the dormant season (from October to April), the fenced and grazed sites released 60.0 and 32.4g of C per m(2), respectively. Path analysis showed that temperature had the greatest effect on daily variation of ecosystem CO(2) exchange during the growing seasons at the two study sites. The results suggest that decrease of precipitation and/or increase of temperature will likely promote C loss from L. chinensis steppes, whether fenced or grazed, and that a grazed site is more sensitive.

In this study, a process-oriented biogeochemistry model, denitrification–decomposition (DNDC), was employed and adapted to interpret and integrate the field observations that the tested ecosystem was a weak sink of atmospheric carbon dioxide (CO2) in 2004 but a strong source in 2005 during the growing seasons. Then we applied the model to predict long-term impacts of climate change on carbon (C) dynamics in the semiarid grassland. To adapt DNDC for the targeted grassland, we modified the default values of several grass parameters such as maximum biomass production, biomass partitions, plant tissue C/N ratio, and accumulative thermal degree days based on local observations. Daily weather data for 2004 and 2005 in conjunction with soil properties and management practices for the location were utilized as inputs to simulate the grass growth and soil C dynamics. The modeled C fluxes were compared with the eddy tower data. Sensitivity tests were conducted with a baseline and twelve alternative climate scenarios of 100 years for the target grassland. The observed and modeled CO2 fluxes data were well in agreement (P

Net ecosystem exchange (NEE) of carbon dioxide (CO2) was measured at Zoige wetland using the eddy covariance technique. Analysis of CO2 fluxes in two years showed Zoige wetland was a net CO2 sink of −47.1 and −79.7 g C m−2 a−1 in 2008 and 2009, respectively. The peak NEE value was −0.54 mg CO2 m−2 s−1 (the negative value signifies net ecosystem carbon gain from air). The maximal daily integrated NEE was −4.1 g C m−2 d−1 during the peak growth season (from July to August). Gross ecosystem photosynthesis was likely more variable than ecosystem respiration at both seasonal and interannual timescales in this wetland. Our data strongly suggested that the combination of precipitation and temperature, as well as phenological stage of vegetation, controlled the dynamics of ecosystem carbon gain, even in drought years. Therefore, an accurate representation of these parameters in climate models is critical to the success of forecasting carbon budgets of alpine wetlands.

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

In water-limited grassland ecosystems, discrete and occasional precipitation events trigger brief but important episodes of biological activity. Differential responses of above- and below-ground biota to precipitation pulses may constrain biogeochemical transformations at the ecosystem scale. We examined the short-term dynamics of the whole ecosystem response to small precipitation events during 2003 and 2004 in a steppe on the Inner Mongolia Plateau. The results indicate that changes in soil moisture occur, with a 1–2 day time lag, only when the amount of precipitation exceeds 3 mm (from day of year [DOY] 120 to DOY 180) or 5 mm (after DOY 180). The interception of the developing plant canopy is a primary reason for the different temporal precipitation threshold. The lower threshold of effective rain is different between Net Ecosystem Exchange (NEE, 3 mm), Ecosystem Respiration (R e, 3 mm) and Gross Ecosystem Production (GEP, 5 mm). The NEE reached a maximum 4–5 days after the end of effective rain events and dropped to 60–70% of the original fluxes after 10 days. However, the drop in GEP was greater than that of NEE and reached 30–50% of the original fluxes after 10–15 days without “effective rainfall.” The characteristics of the response time can be attributed to the variation in soil water content and the time of readjusting for the ecological processes after the effective rainfall. In addition, the independent responses of photosynthesis, respiration and evapotranspiration to precipitation probably contributed to this time lag. The results support the hypothesis that the concept of an ecologically significant rainfall event can be developed for an ecosystem.

We monitored UVA, UVB, and solar radiation from August 2001 to 2003 on the northern Qinghai-Tibetan Plateau to characterize the diurnal and seasonal variations of UV radiation on the world's highest plateau. Daily UVB radiation and the ratio of UVB to total solar radiation increased significantly when the atmospheric ozone concentration decreased as estimated by the total ozone mapping spectrometer (TOMS), as well as when cloud coverage decreased. The UVB/UVA ratio also showed a significant increase when the TOMS ozone concentration decreased in the morning. The seasonal variation pattern of UVB, however, was closely correlated with solar elevation but was little affected by the seasonal pattern of the atmospheric ozone amount. Compared to observations from the central plateau, the magnitude of the UVB increase attributed to ozone depletion was smaller at the northern edge. The study suggests that the temporal variation of ground UV radiation is determined by both solar elevation and the ozone amount, but the spatial difference on the plateau is likely to be ascribed mainly to the spatial variation of the ozone amount. (c) 2007 Published by Elsevier B.V.

Eddy covariance technique was used to measure carbon flux during two growing seasons in 2003 and 2004 over typical steppe in the Inner Mongolia Plateau, China. The results showed that there were two different CO2 flux diurnal patterns at the grassland ecosystem. One had a dual peak in diurnal course of CO2 fluxes with a depression of CO2 flux after noon, and the other had a single peak. In 2003, the maximum diurnal uptake and emitting value of CO2 were −7.4 and 5.4 g·m−2·d−1 respectively and both occurred in July. While in 2004, the maximum diurnal uptake and release of CO2 were −12.8 and 5.8 g·m−2·d−1 and occurred both in August. The grassland fixed 294.66 and 467.46 g CO2·m−2 in 2003 and 2004, and released 333.14 and 437.17 g CO2·m−2 in 2003 and 2004, respectively from May to September. Water availability and photosynthetic active radiation (PAR) are two important factors of controlling CO2 flux. Consecutive precipitation can cause reduction in the ability of ecosystem carbon exchange. Under favorable soil water conditions, daytime CO2 flux is dependent on PAR. CO2 flux, under soil water stress conditions, is obviously less than those under favorable soil water conditions, and there is a light saturation phenomena at PAR=1200 μmol·m−2·s−1. Soil respiration was temperature dependent when there was no soil water stress; otherwise, this response became accumulatively decoupled from soil temperature.

It was the objective of this study to compare the suitability of different extractants for predicting the availability of sulfur (S) in natural grassland in a sulfur response trial on three different soil types in the Inner Mongolia steppe of China. For soil analysis, seven different extractants have been employed. The inorganic SO4–S concentration was determined by ion chromatography. Additionally, in the Ca(H2PO4)2 extract the total soluble S was determined employing turbidimetry. Weak salt solutions (0.15% CaCl2, Ca(H2PO4)2, and KH2PO4) extracted similar amounts of SO4–S. Extraction with 0.025 M KCl provided the lowest SO4–S values. Deionized water dissolved significantly more SO4–S in the control plots than most weak salt extractants. The concentration of soluble organic S decreased in the control plots after 100 days of plant growth, indicating that the organic S pool contributed significantly to the S nutrition of the forage crops. Significant relationships among the SO4–S in the soil deter...