Aims The study of the spatial distribution pattern of soil nutrient stoichiometry along the altitude gradient is helpful to clarify the status of nutrient limitation in the mountainous ecosystems, to reveal the potential influencing factors to nutrient limitation, and to provide a scientific basis for regional ecological protection and vegetation restoration.
Methods The sampling plots were established along an altitude gradient from 3 100 m to 3 700 m in Pailugou watershed of the Qilian Mountains. Soil samples from 0-10 and 10-20 cm layers were collected separately to analyze the distribution of soil organic carbon (SOC), total nitrogen (TN), total phosphorus (TP) contents, and stoichiometric characteristics among different altitude and soil layers. Correlations between these nutrient characteristics and climatic factors, aboveground biomass, and soil pH were analyzed.
Important findings Results showed that: (1) Soil nutrients were concentrated on the soil surface and decreased with soil depth. The SOC and TN contents increased initially and then decreased as altitude increased. Soil TP content increased with increasing altitude. (2) Soil N:P increased from 3 100 m to 3 400 m, reached the maximum value at 3 400 m, and then decreased with increasing altitude and soil N:P was less than 14, suggesting an increase in N limitation along the altitude gradient. The C:P in soil was lower at high altitude than that at medium or low altitude, while C:N decreased with increasing altitude. (3) SOC content was positively correlated with TN content and negatively correlated with TP content. TN and TP content had no significant correlation. (4) Mean air temperature and total precipitation in the growing season showed positive correlation with SOC content and C:N:P, negative correlation with TP content, and no correlation with TN content. SOC and TN content had positive correlations with the aboveground biomass of shrublands. Soil pH was negatively correlated with TP content, but did not affect SOC and TN content significantly. Our results indicate that the productivity of alpine shrublands in the watershed was mainly limited by N and an appropriate amount of N application could alleviate the limitation.

Aims Soil extracellular enzymes and enzyme stoichiometry are indicators of soil nutrient availability and microbial substrate limitation. Subalpine treeline ecotones are special areas which are sensitive to global change. However, the patterns in soil enzyme activities and stoichiometry, and their key drivers remain unclear in the subalpine treeline ecotones.

Methods In this study, soils from a subalpine treeline ecotone in Gongga Mountain in Southeast of Qingzang Plateau were collected. The activities of five hydrolases (β-1,4-glucosidase (BG), cellobiohydrolase (CBH), xylosidase (XYL), β-N-acetyl glucosaminidase (NAG), leucine aminopeptidase (LAP)) and two oxidases (polyphenol oxidase (POX), catalase (CAT)) were detected. The stoichiometric ratios of soil extracellular enzyme activities (carbon and nitrogen enzyme activity ratio and carbon quality index) were calculated.

Important findings Our results showed that LAP, POX and CAT activities of the shrub soils were significantly lower than those of the treeline and forest soils, XYL activity was the lowest at the treeline, and the activities of other extracellular enzymes did not differ significantly among locations in the treeline ecotone. The lnBG/lnLAP of the shrub soil was significantly higher than those of the forest and treeline soils, lnBG/ln(NAG + LAP) did not vary significantly at the treeline ecotone, and the carbon quality index was highest at the treeline. Soil extracellular enzyme activity stoichiometric ratios were not significantly related to microbial nutrient status. Non-metric multidimensional scaling analysis showed that total carbon, total nitrogen, nitrate nitrogen content and lignin to nitrogen ratio of plant leaves were the main factors influencing soil extracellular enzyme activities in the treeline ecotone. The main drivers of the stoichiometric ratios of extracellular enzyme activities were soil dissolved nitrogen, carbon to nitrogen ratio, and lignin to nitrogen ratio of plant leaves. In summary, some soil enzyme activities and their stoichiometric ratios varied significantly along the treeline ecotone, which was mainly influenced by the changes in vegetation type, possibly via its influences on plant-associated microbial communities. Treeline migration induced by future climate change may change extracellular enzyme activities and thus affect soil nutrient cycling.

Aims Microbial biomass and their stoichiometric characteristics not only are important parameters of soil nutrient cycling, but also can contribute to prediction of climate changes, improvement of model accuracy, and understanding of terrestrial nutrient cycling. Our objective was to investigate microbial biomass carbon (MBC), microbial biomass nitrogen (MBN), and microbial biomass phosphorus (MBP) concentrations and their stoichiometric characteristics in alpine wetlands in the Three Rivers Sources Region.

Methods Using data from 50 sites, we explored MBC, MBN, MBP, their stoichiometry and their relationships with the controlling factors of alpine wetlands in the Three Rivers Source Region.

Important findings Our results showed that 1) MBC, MBN, MBP concentrations were 105.11, 3.79, 0.78 mmol·kg^-1, respectively, and MBC:MBN, MBC:MBP, MBN:MBP, MBC:MBN:MBP were 50.56, 184.89, 5.42, 275:5:1, respectively. 2) Soil physical and chemical properties could significantly affect MBC, MBN and MBP concentration. Soil moisture had significantly negative effects on both MBC:MBN and MBC:MBP, while soil density had positive effects on both MBC:MBN and MBC:MBP. Soil total nitrogen content had negative relationship with MBC:MBP, while having weak effects on MBC:MBN. Soil physical and chemical properties also had weak effects on MBN:MBP. 3) Generally, soil microbial community composition had significant effects on MBC, MBN and MBP concentration. Soil microbial community composition had similar effects on MBC:MBN and MBC:MBP. Total phospholipid fatty acid (PLFA) content, gram-positive bacteria, gram-negative bacteria, bacteria, actinomycete, arbuscular mycorrhizal fungi concentration, and other PLFA content had negative effects on MBC:MBN and MBC:MBP, while fungi:bacteria had positive effects on both MBC:MBN and MBC:MBP, but fungi had weak relationships with both MBC:MBN and MBC:MBP. Except for arbuscular mycorrhizal fungi, MBN:MBP had weak relationships with soil microbial community composition. Soil physical and chemical properties, and soil microbial community composition had significant effects on soil microbial biomass and their stoichiometric characteristics in Three Rivers Sources Regions in the alpine wetlands, which are greatly helpful for deeply understanding of terrestrial high altitude nutrient cycling.

Aims Soil enzymes play an important role in the process of soil nutrient transformation. The main purpose of this study is to explore the chemical composition of soil organic matter and its effect on soil protease and urease activity in alpine grasslands in Northern Xizang, China.
Methods The pyrolysis Gas Chromatograph/Mass Spectrometer (Py-GC/MS) was used to obtain the chemical compositions of soil organic matter, and to analyze the relationships between soil chemical compositions and soil enzyme activity in five alpine grasslands, including alpine meadow, alpine steppe, alpine meadow steppe, alpine desert steppe and alpine desert ecosystems.
Important findings The results showed that the enzyme activities among five alpine grassland soils (0-15 cm) were different. Soil urease activity was significantly higher than soil protease activity in the alpine desert steppe, while the difference between the urease and protease activities was not significant in other types of alpine grasslands. Soil protease activity was significantly different among five alpine grassland types, but soil urease activity was not. The correlation analysis showed that soil protease activity was closely related to the relative abundance of alkanes, alkenes and aromatics in soil organic matter and the ratio of furfural to pyrrole. However, the correlation relationships between urease activity and soil organic matter chemical compositions were not significant. The results indicated that alpine grassland type and soil organic matter chemistry were the key factors affecting soil protease activity, but their effects on soil urease activity was non-significant, which calls for further study on the influencing factors on soil urease activity.

Aims In recent years, under the background of climate change and human activities, the trend of biodiversity loss is increasing. Such accelerated loss in biodiversity could bring serious consequences to ecosystem functions. At present, the research on ecosystem function ignores the important driving role of carbon and nitrogen cycling in soil and microorganism on the above ground ecosystem functions. Any changes of soil carbon, nitrogen and microorganism may affect the ability of belowground community, which can have substantial effects on the aboveground ecosystem functions. Our aim was to explore the driving factors and key mechanism of abovegroud ecosystem functions (AEF) in alpine grassland.
Methods From July to August 2015, we conducted a transect survey in alpine grasslands to measure plant community and soil properties across Qingzang Plateau. There were in total 115 sample sites. The aboveground ecosystem function was calculated based on the aboveground biomass, leaf carbon, leaf nitrogen and leaf phosphorus. The effects of key elements such as soil organic carbon, total nitrogen and biomass on the aboveground ecosystem function were analyzed. Combined with mean annual precipitation and air temperature, we explored important drivers of AEF and related mechanisms.
Important findings Precipitation has a greater impact on aboveground ecosystem functions, while air temperature has a minor impact. Mean annual precipitation, soil microbial nitrogen content and aridity index had relative higher importance to aboveground ecosystem functions. Specificially, mean annual precipitation, soil microbial nitrogen content and aridity index accounted for the variations of 21.1%, 10.9% and 10.1%, respectively. The findings indicated that soil properties might play more important roles than plant community and productivity to aboveground ecosystem functions. Considering the cascading impacts of climate factors on soil nutrients cycling and microorganisms, soil microbial biomass nitrogen content plays an important role in regulating AEF of alpine grassland, Qingzang Plateau.

Aims As a key factor of nutrient cycling in ecosystems, soil enzyme activity is an important indicator of soil quality and ecosystem function. However, there have been very few studies on the differences of soil enzyme activities among different types of alpine grassland ecosystems. Thus, the aims of this study were to compare the differences of soil enzyme activities among five different types of alpine grassland and to reveal their influencing environmental factors on the Qingzang Plateau.
Methods Totally, 21 samples of five alpine grassland types, including alpine meadow, alpine steppe, alpine meadow steppe, alpine desert steppe and alpine desert on northern Qingzang Plateau, were selected for field in-situ investigation and sampling. The activities of 14 enzymes involved in the cycling of carbon (C), nitrogen (N) and phosphorus (P) were determined, and the relationships between enzymatic activities and environmental factors in alpine grassland were established.
Important findings The activities of C-acquisition (invertase, cellulase, β-1,4-glucosidse, polyphenol oxidase and peroxidase), P-acquisition (alkaline phosphatase) enzymes and two N-acquisition (arylamidase and nitrite reductase) were significantly different among different alpine grassland types. Moreover, correlations were found among C-acquisition, N-acquisition and P-acquisition enzymes. A significant positive correlation was found between invertase and alkaline phosphatase, and between cellulase and N-acetyl-α-D-glucosaminidase. A significant negative correlation was found between polyphenol oxidase and nitrite reductase, N-acetyl-β-D- glucosaminidase. Soil organic matter (SOM) content, gram-negative bacteria content, ratio of nitrogen to phosphorus, gram-positive bacteria content, bacteria content, actinomycetes content, total nitrogen content and fungi content were the key factors influencing soil enzyme activity among the 19 environmental indicators, and SOM content had the greatest impact (explained 11.9%). The results demonstrated that the activities of C-acquisition, P-acquisition and two N-acquisition (arylamidase and nitrite reductase) enzymes were significantly different among different types of alpine grassland, and soil enzyme activities were mainly controlled by SOM content, microbes and N elements in alpine grassland ecosystems.

Aims Nitrogen is the most limiting factor to artificial pastures. It is distributed unevenly in time and space, and has different forms, which are correlated with cultivation approaches and above-ground net primary productivity (ANPP). This study investigated the dynamics of soil nitrogen and productivity in artificial pastures of Bromus inermis, Elymus sibiricus, E. nutans, Festuca ryloviana, F. sinensis, Poa pratensis var. anceps ‘Qinghai’, P. crymophila and Puccinellia tenuiflora in pure species cultivations in the Tongde farm of Qinghai Province. The dynamics of soil soluble nitrogen pools in each artificial pasture type and their relationships with ANPP were examined.
Methods The pastures were planted in 2013 without fertilizer application, and mowed to the level with 5 cm stubble in mid-September every year. The soil ammonium nitrogen (NH₄⁺-N), nitrate nitrogen (NO₃^--N), soluble organic nitrogen (SON) and soluble total nitrogen (STN) content were measured during growing seasons. ANPP was determined in September each year.
Important findings (1) The average ANPP across the eight pasture types ranged between 329.67-794.67 g·m ^-2, with the ANPP of 794.67 g·m ^-2 for the E. nutans significantly higher than other pasture types. (2) From the second to fourth year following planting, the content of soil NO₃ ^--N, SON and STN significantly decreased, but that of the NH₄⁺-N significantly increased. (3) SON accounted for the highest proportion of STN, varying between 45.11%-88.76% in the 0-10 cm soil layer and 47.75%-88.18% in the 10-20 cm soil layer, followed by NO₃^--N in ranges of 5.81%-34.85% (0-10 cm) and 6.08%-40.42% (10-20 cm), respectively; NH₄⁺-N had the least proportion at 3.41%-22.18% (0-10 cm) and 3.09%-19.56% (10-20 cm), respectively. (4) The non-metric multidimensional scale analysis (NMDS) shows that the temporal effect on soil soluble nitrogen content by different pasture types diverged for the 0-10 cm soil layer, but converged for the 10-20 cm soil layer, and that the effect of pasture types on soil soluble nitrogen content was related to soil depth. (5) Soil SON and STN contents were positively correlated with ANPP, and negatively with inorganic nitrogen (IN) content. In summary, nitrogen fertilizer application is one of the key factors for maintaining the productivity of artificial pasture from three to four years. The above results provide a scientific basis for a more in-depth understanding of the dynamics of soil soluble nitrogen and the maintenance of productivity and stability of artificial pastures on the Qingzang Plateau.

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

Aims Biological soil crust is an important type of surface cover in alpine sandy lands. Understanding of the effect of warming on respiration from the biological soil crust-soil system in alpine regions can provide theoretical reference to the assessment of the response and feedback of biological soil crusts to climate changes.
Methods The moss and algae crusts in the artificial vegetation restoration areas were taken as the research objects. The open top chamber (OTC) was used as a passive warming device to simulate warming. The daily and growing season dynamics of respiration rates in two types of biological soil crust-soil systems were measured. The effects of warming on CO₂ emission and its temperature sensitivity were discussed.
Important findings Both the daily and the growing season dynamics of respiration rate of the moss and algae crust-soil system showed “single-peak” curves and were not affected by warming. The daily peaks appeared around 13:00, and the growing season peaks appeared around August. Warming changed the daily peak value of respiration rate of the biological soil crust-soil system. In the relatively dry year (2017), moderate warming increased cumulative CO₂ emission from the two types of biological soil crust-soil system during growing season, but the increase declined under excessive warming. In the relatively wet year (2018), as warming got greater, CO₂ emission from the two types of biological soil crust-soil system increased more. The relationship between respiration rate and temperature of two types of biological soil crust-soil system followed the exponential function. In the relatively dry year, more increase of temperature induced smaller temperature sensitivity of CO₂ emission, and the temperature sensitivity varied from 1.47 to 1.61 and 1.60 to 1.95 in the moss and algae crust soil system respectively. In the relatively wet year, with the increase of temperature, temperature sensitivity of system respiration increased, and the temperature sensitivity varied from 1.44 to 1.68 and 1.44 to 1.76 in the moss and algae crust soil system respectively. This study shows that global warming has greatly increased the respiration of biological soil crust-soil system in alpine ecosystems. Therefore, we should fully consider the impact of climate warming on the wide spread biological soil crusts in this area for better evaluation of carbon cycling processes in alpine ecosystems.

Aims Soil microbe plays key role in mediating terrestrial carbon cycles. It has been suggested that climate warming may affect the microbial community, which may accelerate carbon release and induce a positive feedback to soil climate warming. However, there is still controversy on how microbial community responds to experimental warming, especially in cold and drought environment.

Methods We conducted an open top chambers (OTCs) experiment to explore the effects of warming on soil microbial community in an alpine steppe on Qinghai-Xizang Plateau. During the maximum of the growing seasons (August) of 2015 and 2016, we monitored the biomass and structure of soil microbial community in warming and control plots using phospholipid fatty acids (PLFA) as biomarkers.

Important findings Short-term warming treatment significantly increased the soil temperature by 1.6 and 1.6 oC and decreased soil moisture by 3.4% and 2.4% (volume fraction) respectively, but did not alter either soil properties or normalized difference vegetation index (NDVI) during the growing season (from May to October) in 2015 and 2016. During the maximum of growing seasons (August) of 2015 and 2016, the magnitude of microbial biomass carbon (MBC) were 749.0 and 844.3 mg·kg^-1, microbial biomass nitrogen (MBN) were 43.1 and 102.1 mg·kg^-1, and the microbial biomass C:N ranged between 17.9 and 8.4. Moreover, all three showed no significant differences between warming and control treatments. The abundance of bacteria was the most in microbial community, while arbuscular mycorrhizal fungi was the least, and warming treatment did not alter the abundance of different microbial group and the microbial community structure. Nonetheless, our result revealed that warming-induced changes in MBC had significant positive correlation with changes in soil temperature and soil moisture. These patterns indicate that, microbial community in this alpine steppe may not respond substantially to future climate warming due to the limitation of soil drought. Therefore, estimation of microbial community response to climate change calls for consideration on the combined effect of warming and drought.

Aims Soil exchangeable base cations (BCs) play important roles in keeping soil nutrient and buffering soil acidification, which may be disturbed by anthropogenic nitrogen (N) input. Considering relatively limited evidence from alkaline soils, this study was designed to explore the effects of N addition on soil exchangeable BCs in a typical alpine steppe on the Qinghai-Xizang Plateau.

Methods From May 2013, eight levels of N addition (0, 1, 2, 4, 8, 16, 24, 32 g·m ^-2·a ^-1) in the form of NH₄NO₃ were added in the alpine steppe, where soil is alkaline. During the following three years (2014-2016), we collected soil samples in mid-August in each year. By measuring the concentrations of exchangeable BCs, we examined their changes along the N addition gradient. We also explored the relationships between BCs and other plant and soil properties.

Important findings Continuous N addition resulted in significant loss of exchangeable BCs, especially Mg ²⁺ in all three years and Na ⁺in two years. The concentrations of BCs were found to be negatively related to above-ground biomass and the concentration of soil inorganic N (p < 0.05). These results indicated that increase in N availability stimulated plant growth, which in turn led to more uptake of BCs by plants. Moreover, enhanced NO₃ ^- leaching resulted in the loss of BCs due to the charge balance in soil solution. In addition, increased NH₄ ⁺ displaced BCs binding to soil surface and made them easy to be leached out of soils. Different from acid soils, soil acidification caused by N deposition in alkaline soils is mainly buffered by calcium carbonate, having less effect on BCs. Our results suggest that N addition results in the loss of exchangeable BCs in alkaline soils, leading to poor buffering capacity and decreased plant productivity over long time period, which needs to be considered during grassland management in the future.

Aims Ligularia virgaurea is an indicator species of alpine meadow degradation. Recently, the vast spreading of L. virgaurea has brought the serious economic loss of grassland ecosystem, but it remains unclear whether soil microbes involve in the spreading of L. virgaurea.

Methods We chose four patches with different density of L. virgaurea to measure the influence of spreading of L. virgaurea on the functional diversity of soil microbial community in the Qinghai-Xizang Plateau.

Important findings The spreading of L. virgaurea increased soil microbial activity, but reduced soil available nitrogen concentration. The Shannon index, utilization number of carbon resource and evenness index of soil microbial community displayed no significant differences among patches, but the utilization structure of carbon resource in high density patch was significantly different from control patch. Our findings indicate that the limitation of soil nitrogen caused by the changing functional diversity of soil microbial community in the distributed sites is one of the mechanisms for the vast spreading of L. virgaurea in alpine meadow ecosystem.

Aims This study was conducted to determine the responses of nutrients in plants and rhizospheric soils to climate in alpine-cold desert on the Qinghai-Xizang Plateau.

Methods Tissue samples for two dominant plant species, Hippophae rhamnoides subsp. sinensis and Artemisia desertorum, and associated rhizospheric soil samples were collected from sites representing semi-arid and sub-humid climates in the alpine-cold desert on the Qinghai-Xizang Plateau. Measurements were made on the contents of carbon, nitrogen and phosphorus in roots and shoots, as well as on organic carbon, total nitrogen, total phosphate, ammonium nitrogen, nitrate nitrogen and available phosphate in rhizospheric soils in the 0-10 cm and 10-20 cm layer. The relationship between nutrients in plant tissues and rhizospheric soils and the influencing factors were analyzed.

Important findings There were significant differences between the semi-arid and the sub-humid sites in tissue nutrients and rhizospheric soil nutrients for the two specie. Specifically, the contents of carbon, nitrogen, phosphorus in plant tissues differed significantly between the semi-arid and the sub-humid sites. Soil organic carbon, total nitrogen, ammonium nitrogen, nitrate nitrogen and available phosphate for the rhizosphere of A. desertorum were significantly higher on site under sub-humid climate than that under semi-arid climate; whereas the trend was reversed for the rhizosphere of H. rhamnoides subsp. sinensis. We found significant relationships between the tissue nutrients and soil nutrients, and significantly different plant nutrient ratios between the two species. There were negative correlations between tissues and rhizosheric soils in N:P ratio for A. desertorum and C:N ratio for H. rhamnoides subsp. sinensis under different climates.

Aims Little information has been available on the soil nitrogen transformation process of alpine scrubland under global warming and changing climate. This study aimed at clarifying seasonal dynamics of the soil nitrate and ammonium contents and their responses to increased temperature under different plant treatments.

Methods We conducted a field experiment including two plant treatments (removal- or unremoval-plant) subjected to two temperature conditions (increased temperature or control) in Sibiraea angustata scrub ecosystem on the eastern Qinghai-Xizang Plateau. The contents of soil nitrate and ammonium were measured at the early, middle and late growing seasons.

Important findings The results showed that soil nitrate and ammonium contents exhibited obvious seasonal dynamics. Throughout the entire growing season, the soil nitrate contents increased firstly and then decreased, while the soil ammonium contents increased continually. Particularly, in the early and middle growing season, the soil nitrate contents were significantly higher than those of ammonium, regardless of increased temperature and plant treatments; however, in the late growing season, the soil nitrate contents were significantly lower than those of ammonium. These results implied that soil nitrification was the major process of soil nitrogen transformation in the early and middle growing season; soil ammonification contributed mostly to soil nitrogen transformation in the late growing season. Furthermore, different responses of soil nitrate and ammonium contents to increased temperature and plant removal treatments were observed at the different stages in the growing season. The effects of increased temperature on soil nitrate contents mainly occurred in the middle and late growing season, but the effects varied with plant treatments. Increased temperature only significantly increased soil ammonium contents in the unremoval-plant plots during the middle growing season. The effects of plant treatments on soil nitrate contents only occurred in the control plots (controlled temperature). Plant removal only increased soil nitrate contents in the early and middle growing season, but significantly decreased soil nitrate contents in the late growing season. Plant removal significantly decreased soil ammonium contents in the increased temperature plots during the middle growing season. Probably, in the early and middle growing season, scrub vegetation mainly absorbed soil nitrate and the absorption process was not affected by increased temperature. These results would increase our understanding of the soil nitrogen cycling process in these alpine scrub ecosystems under global warming and changing climate.

Aims Nitrous oxide (N₂O) is one of the most important greenhouse gases, which contributes a lot to global warming. However, considerable variations are observed in the responses of soil N₂O emissions to experimental warming, and the underlying microbial processes remain unknown.

Methods A warming experiment based on open-top chambers (OTCs) was set up in a typical alpine steppe on the Qinghai-Xizang Plateau. The static chamber combined gas chromatography method was applied to investigate soil N₂O flux under control and warming treatments during the growing seasons in 2014 and 2015. Gene abundances of ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB) were quantified using quantitative real-time PCR.

Important findings Our results showed that the warming treatments increased soil temperature by 1.7 and 1.6 °C and decreased volumetric water content by 2.5% and 3.3% respectively during the growing season (May to October) in 2014 and 2015. However, there were no significant differences in other soil properties. Our results also revealed that, the magnitude of soil N₂O emissions exhibited substantial variations between the two experimental years, which were 3.23 and 1.47 μg·m ^-2·h ^-1in 2014 and 2015, respectively, but no significant difference in N₂O fluxes was observed between control and warming treatments. AOA and AOB abundances are 15.2 × 10 ⁷and 10.0 × 10 ⁵copies·g ^-1 in 2014, and 5.0 × 10 ⁷and 4.7 × 10 ⁵copies·g ^-1in 2015, with no significant differences between control and warming treatments during the experimental period. Furthermore, warming-induced changes in N₂O emissions had no significant relationship with the changes in soil temperature, but showed a significant positive correlation with the changes in soil moisture at seasonal scale. Overall, these results demonstrate that soil moisture regulates the responses of N₂O emissions to experimental warming, highlighting the necessity to consider the warming-induced drying effect when estimating the magnitude of N₂O emissions under future climate warming.

Aims Although acquisition of soil organic nitrogen (N)(mainly amino acids) by plants is a widespread ecological phenomenon in many terrestrial ecosystems, the rate of organic N uptake and their contributions to plant nutrient supply are poorly understood. Our objective was to determine the relative contributions of inorganic N (NO₃^–-N and NH₄⁺-N) and organic N (amino acids) to plant N uptake in a high-frigid forest ecosystem.Methods The differences in the uptake rate of three different forms of N (NO₃^–-N, NH₄⁺-N and glycine) were quantified by exposing seedlings of two dominant tree species (Picea asperata and Betula albo-sinensis) in subalpine coniferous forests of western Sichuan, China, to trace quantities of K¹⁵NO₃,¹⁵NH₄Cl and (U-¹³C₂/¹⁵N) glycine.Important findings Both ¹³C and ¹⁵N were significantly enriched in fine roots 2 h after tracer application, indicating the occurrence of glycine uptake in P. asperata and B. albo-sinensis seedlings. The seedlings of two tree species had a significant preference for NO₃^–-N compared with glycine and NH₄⁺-N, and the uptake rate of NO₃^–-N was 5 to 10 times greater than that of glycine and NH₄⁺-N. The roots of seedlings in the two species took up glycine more rapidly than NH₄⁺-N, implying that soil organic N (i.e., amino acids) could be an important N source for the two species in subalpine coniferous forests. The results of this study are of great theoretical significance for understanding N utilization strategies and nutrient regulation processes in plants of the high-frigid forest ecosystems.

Aims Seasonal snow cover is one of the most important factors that control winter soil respiration in the cold biomes. The warming-induced decreases in snowpack could affect winter soil respiration of subalpine forests. The aim of this study was to explore the effects of snow removal on winter soil respiration in a Picea asperata forest.Methods A snow removal experiment was conducted in a P. asperata forest stand in western Sichuan during the winter of 2015/2016. The snow removal treatment was implemented using wooden roof method. Soil temperatures, snow depth and soil respiration rate were simultaneously measured in plots of snow removal and controls during the experimental period.Important findings Compared to the control, snow removal increased the fluctuations of soil temperatures. The average daily temperature of the soil surface and that at 5 cm depth were 1.12 °C and 0.34 °C lower, respectively, and the numbers of freeze-thaw cycles of the soil surface and that at 5 cm depth were increased by 39 and 12, respectively, in plots of snow removal than in the controls. The average rate of winter soil respiration and CO₂ efflux were 0.52 μmol·m^-2·s^-1 and 88.44 g·m^-2, respectively. On average, snow removal reduced soil respiration rate by 21.02% and CO₂ efflux by 25.99%, respectively. More importantly, the snow effect mainly occurred in the early winter. The winter soil respiration rate had a significant exponential relationship with soil temperature. However, snow removal significantly reduced temperature sensitivity of the winter soil respiration. Our results suggest that seasonal snow reduction associated with climate change could inhibit winter soil respiration in the subalpine forests of western Sichuan, with significant implications for the carbon dynamics of the subalpine forests.

Aims Understanding the responses of root exudative carbon (C) to increasing nitrogen deposition is important for predicting carbon cycling in terrestrial ecosystems. However, fewer studies have investigated the dynamics of root exudation in shrubbery ecosystems compared to forests and grassland ecosystems. This objective of this study was to determine the effects of nitrogen fertilization on the rate and C flux of root exudates.Methods Three levels of nitrogen addition treatments were applied to a Sibiraea angustata shrubbery ecosystem situated at the eastern fringe of Qinghai-Xizang Plateau, including N₀ (without nitrogen application), N₅ (nitrogen addition rate of 5 g·m^-2·a^-1), and N₁₀ (nitrogen addition rate of 10 g·m^-2·a^-1), respectively, in 5 m ´ 5 m plots. Root exudates were collected in June, August and October of 2015, using a modified culture-based cuvette system. Root biomass in each plot was measured with root core method.Important findings The rates of root exudates on biomass, length, and surface area basis all displayed apparent seasonal variations during the experimental period, with the magnitude ranked in the order of: August > June > October, consistent with changes in soil temperature at 5 cm depth. With increases in the nitrogen addition rate, the rate of root exudates on biomass, length, and area basis all trended lower. Compared with the control (N₀), the N₅ and N₁₀ treatments significantly reduced fine root biomass in the Sibiraea angustata shrubbery, by 23.36% and 33.84%, respectively. The decreasing root exudation and fine root biomass in response to nitrogen addition significantly decreased C flux of root exudates. Our results provide additional evidences toward a robust theoretical foundation for better understanding soil C-nutrient cycling process mediated by root exudation inputs in Alpine shrubbery ecosystems under various environmental changes.

Aims To estimate the size and spatial patterns of 3-m-deep soil inorganic carbon (SIC) stock across alpine grasslands on the Qinghai-Xizang Plateau.Methods We conducted a comprehensive investigation and collected soil samples from 342 3-m-deep cores and 177 50-cm-deep pits across the study area. Using Kriging interpolation, we interpolated site-level observations to the regional level. The distribution of SIC density was then overlaid with the regional vegetation map at a scale of 1:1000000 to calculate SIC stock of the alpine steppe and alpine meadow. Kruskal-Wallis tests were further conducted to examine the differences of SIC density between the two grassland types and among soil depths with 50 cm-depth intervals.Important findings The total SIC stock at depths of 50 cm, 1 m, 2 m and 3 m were estimated at 8.26, 17.82, 36.33 and 54.29 Pg C, with SIC density being 7.22, 15.58, 31.76 and 47.46 kg C·m^-2, respectively. SIC density exhibited large spatial variability, with an increasing trend from the southeastern to the northwestern plateau. Much larger SIC stock was observed in the alpine steppe than alpine meadow, with the former accounting for 63%-66% of the total stock at depths of 50 cm, 1 m, 2 m and 3 m. A large amount of SIC stock was found in deep soils (1-3 m), amounting to approximately 2 times as much carbon stored in the top 1-m-deep soil layer. The vertical distributions of SIC density differed between the two grassland types. The highest proportions of SIC occurred in the upper 50 cm layer for the alpine steppe while the highest proportions occurred in 100-150 cm layer for the alpine meadow. These results highlight that a large amount of SIC is stored in deep soil layers, which should be considered in evaluating terrestrial carbon balance under global change scenario.

Aims Over the past 20 years, alpine wetlands have been subjected to a rapid change in climate, resulting in water table drawdown and increased nitrogen deposition. In wetland ecosystems, the water table drawdown can improve soil aeration, hence leading to a higher soil respiration rate; whereas an increased nitrogen deposition could reduce the microbial biomass and pH value, suppressing soil respiration. Understanding the responses of soil respiration to reduced water table and increased nitrogen deposition in alpine wetlands is thus critical to predicting the carbon cycle of wetland ecosystems and its feedbacks to ongoing climate changes. This study tests the effects of water table reduction and nitrogen addition on soil respiration in the Luanhaizi wetland on the Qinghai-Xizang Plateau.
Methods We imposed four treatments, including control (WT⁰N⁰), reduced water table (WT^-N⁰), nitrogen addition (WT⁰N⁺), and a combination of reduced water table and nitrogen addition (WT^-N⁺), on 20 peat monoliths collected from the Luanhaizi wetland at the Haibei station. Soil respiration was measured from late July through mid-September under all treatments.
Important findings A reduction in water table significantly increased the rate of soil respiration. In contrast, nitrogen addition suppressed soil respiration only when water table was not reduced. A positive correlation was found between the aboveground biomass and soil respiration, while no correlation was detected between root biomass and soil respiration. The temperature sensitivity of soil respiration was increased by reduced water table, but was not affected by nitrogen addition. Our results suggest that nitrogen deposition is likely to reduce soil CO₂ emission in alpine wetlands where water level remains high. However, future warmer and drier conditions could result in reduced water table, and consequently alpine wetlands would be predicted to release substantially more CO₂ than previously estimated.

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).

Aims Currently, the temperature sensitivity of soil carbon (C) mineralization and the factors that control it are the focus of studies on soil C cycle and global climate change. The main objectives of this study were to: (1) investigate the effects of temperature and land-use (fenced grassland vs. grazing grassland) on soil C mineralization and its temperature sensitivity (Q₁₀) in the grasslands of Qinghai-Xizang Plateau and (2) determine the relationships of the rate of soil C mineralization with soil properties (e.g. soil organic carbon content (SOC), soil total nitrogen content (STN)).
Methods Eleven pairs of plots (fenced sites vs. grazing sites) were selected along an east-west transect in northwest of Qinghai-Xizang Plateau. Soil samples were collected at a depth of 0-20 cm, to measure soil C mineralization rates under a temperature gradient (i.e. 5, 10, 15, 20, and 25 °C) in laboratory. Data for soil C mineralization rate on the 7th day and 56th day, respectively, were used to assess the short- and long-term effects.
Important findings Soil C mineralization rates declined from east to west on fenced sites, but varied slightly on the grazing sites. Soil C mineralization rates increased significantly with increasing incubation temperature, and were strongly related to SOC and STN; higher the SOC and STN, greater the accumulative soil C mineralization. Q₁₀ showed no apparent spatial pattern along the east-west transect, and was not susceptible to land-use with average Q₁₀ values of 1.83 and 1.86 on the fenced sites and the grazing sites, respectively. Moreover, Q₁₀ was not correlated with either SOC or STN. Findings in this study provide new insights on the responses of soil C mineralization and its temperature sensitivity to land-use change on the Qinghai-Xizang Plateau, contributing important information for evaluating soil C sequestration and its response to warming scenarios in this region.

Aims The forest-alpine tundra ecotone is one of the most conspicuous climate-driven ecological boundaries. However, dynamics of soil microbial biomass and quantity during different stages of the growing season in the ecotone remain unclear. Our objective was to understand the temporal and spatial variations of microbial biomass and quantity to explore the main drivers in the ecotone.
Methods We collected soil samples in a forest-alpine tundra ecotone (dark-conifer forest, timberline, treeline, dense shrub, sparse shrub and alpine meadow) during early, mid and late growing season (EGS, MGS and LGS). The number and species composition of soil microorganisms were determined by means of the plate count method. Soil microbial biomass carbon (MBC) and nitrogen (MBN) were measured by the chloroform fumigation leaching method.
Important findings Vegetation and seasonality significantly influence MBC, MBN and microbial community structure. Microbial biomass distribution among vegetation types was different in the three stages of the growing season. MBC above treeline was higher than below during EGS and MGS. The MBC of dark-conifer forest, timberline and treeline during LGS was significantly increased, and MBC differences among different vegetation types decreased. There were significant differences in measured soil microbial quantity between above- and below-treeline vegetation types; bacteria of dense shrub were highest among vegetation types. The amount of cultivated microorganisms was LGS>EGS>MGS. The ratio of MBC to MBN was the highest and the quantity of fungi increased largely late in the growing season. Statistical analysis showed that there were significant correlations between MBN and bacteria, fungi and actinomyces quantity, while only MBC and fungi quantity were significantly correlated (p < 0.05). Litter input and snow cover late in the growing season were external factors of microbial seasonal variation. Soil microbes and alpine plants competing for nitrogen may be internal factors. Plant nitrogen absorption early in the growing season and microorganisms’ nitrogen fixation late in the growing season enhanced the alpine ecosystem’s nitrogen fixation and utilization. Climate warming may extend the growing season of alpine plants, increasing the alpine soil microbial biomass, and accelerate the decomposition of soil organic matter, which may change soil carbon sequestration rates in the alpine ecosystem.

Grasslands in China cover vast, continuous areas and account for about 40% of Chinese land area. Most are located in the eco-geographical fragile region, are sensitive to climate change, and play important roles in regulating the carbon dioxide concentration in the atmosphere. Our objective was to review recent studies on soil respiration of grassland in China. Most studies were conducted in Northeast Plain, Inner Mongolia and Tibetan Plateau. Diurnal dynamics of soil respiration are controlled by temperature, seasonal patterns are controlled by temperature and/or water depending on the limiting environmental factors, and inter-annual variability is mainly determined by water. In addition, there is great spatial heterogeneity driven by mean annual precipitation and soil total nitrogen content. Responses of soil respiration to global changes were complicated and depended on the interaction of each factor. Most recent soil respiration models failed to incorporate the modulation of soil and biotic factors and their interaction. Key issues and suggested future research topics are 1) soil respiration in temperate desert grassland, 2) soil respiration during non-growing season, 3) comparison study of grassland soil respiration on different spatial and temporal scales, 4) simulation study of grassland soil respiration and 5) remote sensing of grassland soil respiration.

Soil is an important component of the terrestrial ecosystem and plays a critical role in global carbon cycle. Better understanding the distribution pattern of soil carbon storage along environmental gradients will facilitate the projection of global change on terrestrial C cycling. This study was conducted to examine soil organic carbon and nitrogen contents in major grassland types along elevation gradients in the source region of Yangtze, Yellow and Lantsang Rivers. Soil organic carbon and nitrogen contents were greater at the highest (5 120 m a.s.l.) and lowest (4 176 m a.s.l.) sites and lower at middle site. Soil organic carbon and nitrogen contents increased with soil moisture along the altitudinal gradient. Partial correlation analysis showed that spatial variability of soil organic carbon and nitrogen contents at 0-30 cm soil layers could be primarily explain by soil moisture with partial correlation coefficients of 0.946 5、0.905 9 (p<0.01), respectively. In addition, soil organic carbon and nitrogen contents showed positive linear correlations with plant cover and productivity and negative correlation trend with soil pH and total salt content.

Systematic analysis of the relation between wheat residual decomposition and soil biochemistry factors in the heavy frigid region of North East China found the following: changes through time in the rate of wheat residual decomposition showed a distinct seasonal trend with a single peak, in July (12.14×10-3g·g-1·d-1). 14 soil biochemistry factors were observed and these also changed in a distinct seasonal pattern. A step effect was an obvious feature of the residual decomposition and soil organic matter synthesis. The grey correlative effects of the 14 soil biology chemistry factors on the wheat residual decomposition were arranged as follows: x1(0.914)>x4(0.880)>x3(0.855)>x12(0.852)>x14(0.802)>x2(0.799)=x11(0.799)>x8(0.788)>x10(0.775)>x9(0.760)>x13(0.709)>x5(0.700)>x7(0.694)>x6(0.657) This allowed the delimitation of the GM(0,6) predictive model: y(k)＝13.5x1(k)+23.75x4(k)-15.0x3(k)-16.5x12(k)-0.5x14(k)+5.0x2(k)-1.6

Aims Our objectives were to measure the soil microbial biomass carbon (SMB C), soil microbial biomass nitrogen (SMB N) content and soil microbial activity, and determine the relationship between these parameters and other soil properties (including organic carbon, total nitrogen content and water content) in montane forest (dominated by Picea crassifolia), steppe and alpine meadow ecosystems in Qi Lian Mountains, China.
Methods We measured SMB C and SMB N content using fumigation-incubation method and soil microbial activity using substrate-induced respiration.
Important findings The SMB C content under forest was 60% and 120% higher than under steppe and alpine meadow, respectively, and it was 40% higher under steppe than alpine meadow. The SMB N content was 64% and 111% higher in 0-5 cm soil depth under forest than alpine meadow and steppe, respectively, and it was 29% higher under alpine meadow than steppe. Also, it was 7% and 191% higher in 5-15 cm soil depth under forest than steppe and alpine meadow, respectively, and it was 171% higher under steppe than alpine meadow (p<0.05). The ratio SMB C (SMB C-to-SOC (Soil organic carbon), 0.4%-2.8%) was 32% higher under forest and steppe than alpine meadow, and the ratio of SMB N (SMB N-to-total soil N, 0.5%-2.8%) in 0-5 and 5-15 cm soil depths was 150% higher under forest and steppe than alpine meadow (p<0.05). Soil microbial activity in 0-5 or 5-15 cm soil depth was 26% higher under forest or alpine meadow than steppe, and in 15-35 cm soil depth it was 28% higher under forest than steppe and alpine meadow (p<0.05). The SMB C and SMB N content was positively correlated with SOC content, and the SMB N content or its ratio was also positively correlated with the SMB C content and its ratio (r²>0.30, p<0.000 1). The SMB N content, SMB C ratio, SMB N ratio and microbial activity were significantly negatively correlated with soil pH. The SMB C content, SMB N content and their ratio and microbial activity were positively correlated with soil water content.

Aims Our objectives were to compare vegetation biomass and total carbon and nitrogen content of severely degraded grassland and undisturbed Kobresia meadow, and to measure the relative influence of various rehabilitation practices on vegetation biomass and carbon and nitrogen content in early secondary succession.

Methods The research was conducted on alpine meadows in Dari County, Qinghai Province, China, using five treatments: undisturbed native meadow, severely degraded grassland, and three grasslands rehabilitated by different practices (mixed seeded, single seeded and natural recovery). In each treatment, vegetation C and N contents were calculated on an area and depth basis from biomass samples and plant concentration analyses.

Important findings In the undisturbed native meadow treatment, total aboveground biomass was 265.1 g·m^-2 and root biomass in the uppermost 30 cm averaged 6 982 g·m^-2. In the severely degraded grassland treatment, above ground biomass was only 139.9 g·m^-2 and root biomass was only 916 g·m^-2. Total aboveground biomass in the mixed seeded, single seeded and natural recovery treatments was 307.1, 179.9 and 200.4 g·m^-2, and root biomass was 1 323, 1 169 and 1 412 g·m^-2, respectively, after seven growing seasons. Total C content of vegetation in the undisturbed native meadow was 3 067.42 g·m^-2, while that of the severely degraded grassland treatment was only 414.07 g·m^-2. Therefore, land degradation leads to loss of 86.5% of the original plant tissue C. In addition, land degradation leads to loss of 68.3% of the original plant tissue N. Compared with the severely degraded land, mixed seeded and natural recovery treatments partly recover C and N content, indicating that they may be alternative approaches to sequestering C in former degraded alpine meadow.