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Effects of climate warming and changing precipitation on ecosystem carbon fluxes have been intensively studied. However, how they co‐regulate carbon fluxes is still elusive for some understudied ecosystems. To fill the gap, we examined net ecosystem productivity (NEP), gross ecosystem productivity (GEP,) and ecosystem respiration (ER) responses to multilevel of temperature increments (control, warming 1, warming 2, warming 3, warming 4) in three contrasting hydrological growing seasons in a typical semiarid alpine meadow. We found that carbon fluxes responded to precipitation variations more strongly in low‐level warming treatments than in high‐level ones. The distinct responses were attributable to different soil water conditions and community composition under low‐level and high‐level warming during the three growing seasons. In addition, carbon fluxes were much more sensitive to decreased than to increased precipitation in low‐level warming treatments, but not in high‐level ones. At a regional scale, this negative asymmetry was further corroborated. This study reveals that future precipitation changes, particularly decreased precipitation would induce significant change in carbon fluxes, and the effect magnitude is regulated by climate warming size. [ABSTRACT FROM AUTHOR]

Temperature is conventionally considered as the dominant factor regulating carbon fluxes of the alpine meadow on the Tibetan Plateau, while contribution from water availability is composed of large uncertainty. In this study, eddy covariance (EC) data were used to assess the relative contribution of temperature and water availability to carbon fluxes of the alpine meadow ecosystem. The results showed that soil water content (SWC) was the most important factor controlling carbon fluxes – Net Ecosystem Productivity (NEP), Gross Primary Productivity (GPP) and Ecosystem Respiration (Re). The GPP and Re increased with strengthened SWC under any temperature conditions, indicating the dominant control of water availability on carbon fluxes. In addition, water availability regulated the response size of ecosystem to temperature, and could alleviate the stress caused by low temperature. The photosynthesis capacity of alpine plants at noon was depressed by water stress rather than by high temperature. The structural equation modeling (SEM) analysis further confirmed the dominance of SWC on the carbon fluxes. This study implies that effects of climatic change on this alpine ecosystem might be more induced by changes in water pattern than increased temperature, which provides new insights into the climate controls of carbon fluxes over alpine meadow, and adds to our understanding on climate change impacts on carbon cycling on the Tibetan Plateau. [ABSTRACT FROM AUTHOR]

The effects of warming-shifted plant phenology on ecosystem carbon (C) cycling have received increasing attention in recent years. However, there is a lack of evidence and mechanistic understanding of how warming-shifted plant phenology influences ecosystem C cycling. In this study, we conducted a field experiment to investigate the effects of warming on phenology and ecosystem C exchange in a Tibetan alpine meadow during the 2014 and 2015 growing seasons. Our results indicated that warming led to later green-up in spring by aggravating water limitation but little change in autumn phenology, resulting in shortened growing season length. Interestingly, we found warming caused a seasonal shift of ecosystem C exchange. During the early summer monsoon, ecosystem C uptake was suppressed by warming due to the delay of phenological development. However, warming accelerated ecosystem C uptake and promoted ecosystem C uptake under ample water conditions during the late summer monsoon. As a result, although warming shortened the growing season length, it had no significant effects on gross primary production (GPP) and net ecosystem production (NEP). Our results will improve our understanding of the mechanisms of how warming-shifted plant phenology influences ecosystem C cycling in semiarid alpine ecosystems. [ABSTRACT FROM AUTHOR]

• Cold environment suppressed the accumulation of microbial-derived C in the soil. • Warming increased microbial residues in the soil, but N addition decreased it. • Warming weakened the N-driven negative effect on soil C stability. Microbial-derived soil organic carbon has attracted increased attention as an important source of soil organic matter that may affect soil carbon persistence. Ecosystems at high elevation and latitude contain massive amounts of soil organic carbon that is potentially vulnerable to rapid climate change. However, little is known about the mechanisms that govern the production and accumulation of carbon derived from microbes in the context of global changes for these regions. Here, we investigate soil biomarkers from membrane lipids and microbial residues in an experiment with six-year warming and nitrogen (N) addition treatments in an alpine meadow of the Tibetan Plateau. The results showed low accumulation of amino sugars in soil (167.51 to 297.36 μg·g−1) for this alpine meadow. After the six-year manipulative experiment, warming promoted accumulation of microbial-derived carbon in soil with both increased bacterial (increased by 53.56%) and fungal carbon (increased by 25.60%). Nitrogen addition inhibited microbial-derived carbon accumulation (decreased by 25.37%) and significantly decreased the fungal carbon content relative to bacterial carbon content. Climate warming tended to weaken this N-driven negative effect. As microbial residues relate closely with soil organic matter stability, these results highlight the potential of global change factors (warming and N addition) to alter the structure and persistence of soil organic matter, with important implications in refining carbon cycle-climate models. [ABSTRACT FROM AUTHOR]

Aims: Vegetation in high-altitude regions is hypothesized to be more responsive to increasing atmospheric CO2 concentrations due to low CO2 partial pressure. However, the underlying mechanisms driving this response at an ecosystem scale are poorly understood. We aimed to explore the plant carbon (C) and nitrogen (N) relationships and biomass allocation in response to elevated CO2 and N addition in a Tibetan meadow. Methods: A 5-year manipulation experiment was conducted in an alpine meadow (4585 m above sea level) to explore the responses of plant carbon (C), nitrogen (N), and biomass dynamics, as well as their allocation schemes, to elevated CO2 (from 380 ppm to 480 ppm) and N fertilization. Results: Elevated CO2 alone significantly enhanced aboveground plant biomass by 98%, exhibiting a stronger CO2 fertilization effect than the global average level (20%) for grasslands. Elevated CO2 favored N accumulation in aboveground parts despite the declined concentration. Nitrogen fertilization alleviated the N constraints on CO2 fertilization effects, which strengthened C sequestration capacity for the aboveground plant tissues. Moreover, our results indicate a decoupling between C and N cycles in alpine ecosystems under elevated CO2, especially in the N-enrichment environments. Conclusions: Overall, this study shows a high sensitivity of aboveground plant biomass and decoupled C-N relationships under elevated CO2 and N fertilization for high-elevation alpine ecosystems, highlighting the need to incorporate altitude effects into Earth System Models in predicting C cycle feedbacks to climate changes. [ABSTRACT FROM AUTHOR]

• The first study to use high-resolution images to survey pika hole size and area. • The first study to explore pika hole distribution pattern at a landscape scale. • Effects of pika hole on plant growth in the surrounding varies with pika hole size. • Vegetation coverage affects pika hole density in alpine meadow. The pika (Ochotona curzoniae) hole is an important landscape feature in the Tibetan Plateau (TP) grasslands, and it indicates grassland degradation levels due to the destruction caused by pika burrowing activities on grasslands. However, no studies have ever explored landscape patterns of pika holes and their effects on adjacent vegetation coverage. Taking meadow grasslands in Northern Tibet as an example, this study gathered unmanned aerial vehicle (UAV) images and explored landscape patterns of pika holes and their effects on grass coverage in the surroundings. The performances of two classification methods, including the decision tree classification based on Fully Constrained Least Squares (FDC) and the object-oriented classification (OBC) were compared in recognizing sizes and shapes of pika holes. The results showed that: (1) The object-oriented classification exhibits higher classification accuracy in identifying pika holes. (2) The average size of pika holes in the study area is 0.01 m2 and they exhibit clustered distribution patterns. The average distance between any two nearest pika hole patches is 0.79 m. (3) It presents a significant quadratic relationship between the number of pika holes and grass coverage. (4) The average effective distance of pika holes on the surrounding grass coverage is 20 cm. The findings of this study can provide guidelines for pika control and improve grassland management on the TP. [ABSTRACT FROM AUTHOR]

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

Highlights • Grazing could preserve the deep-layer soil moisture by reducing transpiration. • Higher productivity was maintained in grazed alpine meadow during droughts. • Younger leaves in grazed meadow could restore productivity quickly after droughts. Abstract Grazing is the primary land use practice in alpine ecosystems on the Tibetan Plateau. However, it remains unclear how grazing regulates levels of carbon and the water cycle in this ecosystem. A paired set of eddy covariance systems were set in adjacent fenced (FM) and grazed meadows (GM) to explore the grazing effects on alpine meadows. Aboveground biomass removed by grazing caused declines in grass transpiration (T), whereas the evaporation (E) was enhanced because of greater exposure to radiation, which in turn led to higher evapotranspiration (ET) in GM. However, the deep-layer soil moisture remained high because of the effects of mattic epipedon, which worked as a water-resistant layer. Therefore, the deep-layer soil moisture in GM was higher than that in FM because of decreased water consumption caused by the grazing-induced reduction in leaf area in GM. As a consequence, the deep-layer soil in GM could provide more water to help plants endure droughts. Additionally, grazing enhanced the sensitivity of productivity to soil moisture during the wet season in drought years, which could restore grassland productivity more quickly after droughts. Therefore, grazing could help this fragile ecosystem to withstand droughts to some extent and maintain higher productivity. This may provide us with an ecological approach for confronting global climate change. [ABSTRACT FROM AUTHOR]

To explore grazing effects on carbon fluxes in alpine meadow ecosystems, we used a paired eddy-covariance (EC) system to measure carbon fluxes in adjacent fenced (FM) and grazed (GM) meadows on the Tibetan plateau. Gross primary productivity (GPP) and ecosystem respiration (Re) were greater at GM than FM for the first two years of fencing. In the third year, the productivity at FM increased to a level similar to the GM site. The higher productivity at GM was mainly caused by its higher photosynthetic capacity. Grazing exclusion did not increase carbon sequestration capacity for this alpine grassland system. The higher optimal photosynthetic temperature and the weakened ecosystem response to climatic factors at GM may help to facilitate the adaption of alpine meadow ecosystems to changing climate. [ABSTRACT FROM AUTHOR]

Climate warming is predicted to considerably affect variations in soil organic carbon (SOC), especially in alpine ecosystems. Microbial necromass carbon (MNC) is an important contributor to stable soil organic carbon pools. However, accumulation and persistence of soil MNC across a gradient of warming are still poorly understood. An 8‐year field experiment with four levels of warming was conducted in a Tibetan meadow. We found that low‐level (+0–1.5°C) warming mostly enhanced bacterial necromass carbon (BNC), fungal necromass carbon (FNC), and total MNC compared with control treatment across soil layers, while no significant effect was caused between high‐level (+1.5–2.5°C) treatments and control treatments. The contributions of both MNC and BNC to soil organic carbon were not significantly affected by warming treatments across depths. Structural equation modeling analysis demonstrated that the effect of plant root traits on MNC persistence strengthened with warming intensity, while the influence of microbial community characteristics waned along strengthened warming. Overall, our study provides novel evidence that the major determinants of MNC production and stabilization may vary with warming magnitude in alpine meadows. This finding is critical for updating our knowledge on soil carbon storage in response to climate warming. [ABSTRACT FROM AUTHOR]

• Atmospheric and soil droughts together greatly depressed GPP. • Atmospheric water deficits had stronger instant inhibited effects on GPP. • Drought-induced H enhancements provided positive feedback and aggravated GPP loss. Under global climate change, warmer temperatures and changing precipitation patterns will increase the relative importance of soil and atmospheric droughts in limiting productivities across different ecosystems, especially in the fragile and sensitive ecosystem on the Tibetan Plateau. Therefore, the present study examined the effects of atmospheric drought, soil drought and compound drought on alpine meadow productivity based on seven years of continuous observational data. The results demonstrated that water conditions dominated the variations in alpine meadow productivity in northern Tibet. Gross prime productivity (GPP) in the growing season was significantly inhibited during soil drought (defined as soil water content (SWC) < 0.11 m3 m−3) or atmospheric drought (defined as vapor pressure deficit (VPD) > 0.61 kPa) conditions. Pooling of the moisture data according to these thresholds revealed that soil drought occurred more frequently and accounted for 20.4% of all observed days. Atmospheric drought occurred in 2.4% of all days, and the frequency of compound drought (when SWC < 0.11 m3 m−3 and VPD > 0.61 kPa) was 8.7%. Compound drought reduced GPP by the largest extent to as much as 1.47 g C m−2 d−1. Atmospheric drought reduced GPP by 1.10 g C m−2 d−1. However, the more frequent soil drought reduced GPP by the smallest extent, with a value of 0.69 g C m−2 d−1. In spite of this, soil drought played an important role in restricting the accumulated GPP of this alpine meadow via its higher occurrence frequency and longer duration. All three types of drought significantly increased sensible heat flux (H), which provided positive feedbacks to droughts and led to further decreases in GPP. The results of the present study suggest that drought greatly threatens the alpine meadow ecosystem in northern Tibet. Therefore, more attention should be paid to water conditions under the global climate change context. [ABSTRACT FROM AUTHOR]

• Droughts enhanced the control effects of biotic factors on carbon and water fluxes. • Droughts induced changes in the driving paths of biotic and abiotic factors on RUE. • RUE responded differently to droughts in different periods in the growing season. • Droughts in the mid-growing season reduced the RUE. Drought is a threat universally faced by global terrestrial ecosystems under global climate change. The habitat of alpine ecosystems is harsh, and they necessitate to use the limited resources efficiently to survive. However, drought occurred frequently during the short growing season, which greatly affected their resource use efficiency (RUE). In this study, based on long-term flux and microclimate observations in a typical alpine meadow ecosystem in northern Tibet, the effects of drought on RUE, including water use efficiency (WUE), light use efficiency (LUE), and carbon use efficiency (CUE), were studied. The results indicate that droughts could change the driving paths of abiotic and biotic factors on carbon and water fluxes and hence affect RUE. When vegetation exhibits robust physiological activity, droughts could enhance the control effects of biotic factors. The droughts that occurred at different periods of the growing season showed divergent effects on RUE. In the early-growing season, the strong photosynthetic capacity of the nascent leaves facilitated enhancements in WUE and LUE in response to drought conditions. However, the CUE did not increase significantly due to the heightened consumption of organic matter by respiration. The mid-growing season droughts led to a sharp decrease in gross primary productivity (GPP), consequently leading to a decline in WUE, LUE and CUE. During the late-growing season, droughts together with the decomposition of chlorophyll reduced the photosynthetic capacity and led to a significant decrease in LUE. The diminished physiological activity of the senescent leaves led to insensitive responses of WUE and CUE to droughts. These findings indicate that RUE might primarily be regulated by factors affecting GPP rather than others. This study would be beneficial for assessing the responses and adaptation characteristics of alpine meadow ecosystems to future climate change and could help accurately predict their possible evolution trends. [ABSTRACT FROM AUTHOR]

• The soil water content and temperature determined the maximum photosynthesis. • Sufficient soil water elevated the optimal temperature and maximum photosynthesis. • The optimal environmental configuration for the alpine meadow was detected. • If soil water is high, higher temperature and VPD are beneficial for carbon uptake. The changes in the carbon sink function of sensitive and fragile alpine ecosystems under climate change are widely concerned. Radiation, temperature, and water resources play important roles in regulating carbon assimilation. Therefore, it is important to detect the optimal configuration of environmental factors for the photosynthesis of alpine meadow ecosystems. In this study, based on 10-year flux and corresponding environmental observations in a typical alpine meadow ecosystem, the optimal environmental configuration that could realistically occur in the natural world for photosynthesis was detected. The results indicated that the temperature and water conditions were the primary limiting factors, as radiation resources are abundant on the Tibetan Plateau. The optimal temperature for photosynthesis (TA opt) was determined to be 11.92 ℃ based on the entire observation dataset, with a maximum photosynthetic capacity (GPP max) of 0.20 mg CO 2 m−2 s−1. However, the TA opt and the corresponding GPP max could be elevated when the soil water content (SWC) was abundant. When SWC exceeded 0.26 m3 m−3, TA opt increased to 13.41 ℃, and the corresponding GPP max reached as high as 0.30 mg CO 2 m−2 s−1, which is the maximum the ecosystem could reach. Thus, the optimal environmental configuration for photosynthesis that could realistically occur in the natural world in this alpine meadow was air temperature (TA)=13.41 ℃, SWC=0.28 m3 m−3, vapor pressure deficit (VPD)=0.61 kPa, net radiation (RN)=411.53 J m−2 s−1. According to this study, it could be inferred that within a reasonable range, higher temperatures and VPD would benefit photosynthesis in this alpine meadow as long as the soil water supply is sufficient. The climate on the Tibetan Plateau is arid with high VPD and is experiencing warming due to climate change. Consequently, alpine ecosystems situated in relatively humid regions with sufficient soil water resources may exhibit greater potential for carbon sequestration than previously estimated. [ABSTRACT FROM AUTHOR]

• Water conditions dominated plant phenology and photosynthetic capacity. • Plant phenology and photosynthetic capacity co-determined to annual GPP. • Plant photosynthetic capacity dominated annual GPP. Under global climate change, climate warming and changing precipitation patterns would greatly affect plant phenology and photosynthetic capacity and hence affect the gross primary productivity (GPP) of the sensitive alpine meadow. To deeply understand the variations in GPP, it is important to clarify the joint effects of plant phenology and photosynthetic capacity. In this study, 10-year continuous flux and microclimatic data and in situ phenology period observation data were used to explore the roles of growing season length (GSL) and the seasonal maximal capacity of carbon uptake (GPP max) on annual cumulative gross primary productivity (GPP ann). The results indicated that the temperature and water conditions, especially water conditions, dominated GSL and GPP max and hence determined GPP ann. Both GSL and GPP max were dominated by annual precipitation, and the soil water content in August also showed a strong influence over them. GSL and GPP max together could explain 87% of the variation in GPP ann. Compared with GSL, GPP max had stronger effects on the interannual variability (IAV) in GPP ann. This study provides a new perspective on clarifying the proximate causes of IAV in GPP ann. The IAV in GPP ann of alpine ecosystems could be determined by these two key underlying processes, the length of the carbon uptake period and amplitude. These results are of critical importance for improving our ability to predict future changes in alpine meadow ecosystems. [ABSTRACT FROM AUTHOR]

Summary Cosmic-ray neutron sensing (CRNS) is a new method for continuously monitoring mean soil water content (SWC) on a hectometer scale. To evaluate the application and accuracy of the method for SWC observation in an alpine meadow ecosystem (AME), we installed the CRNS in a flat meadow near the Naqu prefecture on the northern Tibetan Plateau. We collecting soil samples and applying the system by the oven-drying method. A weather station was also installed near the CRNS for monitoring basic meteorological variables and the soil temperature and water content at various depths. Three Em-50 instruments for monitoring SWC and soil temperature were buried in three sub-quadrats northwest, northeast and southeast of the CRNS at distances of 460, 370 and 373 m, respectively, to observe the variation of SWC at the various depths. The footprint of the CRNS for SWC observation in the meadow was about 580 m, and the mean measuring depth was about 31 cm according to the general calculation equations. The reference neutron flux for dry soil ( N 0 ) had a mean and coefficient of variation of 8686 and 3%, respectively, and remained substantially invariant throughout the measuring period. The five SWCs from the independent field samples almost passed through the SWC trend of the CRNS, the root mean square error (RMSE) was 0.011 m 3 m −3 for the CRNS and oven-drying method. The time series of SWC measured by the CRNS agreed well with the mean SWC series to a depth of 20 cm measured by the weather station. The trend of SWC measured by the Em-50s generally agreed with the trend of SWC measured by the CRNS, but some values and variations of SWC differed between the Em-50s and CRNS data. Because of the good agreement between the CRNS and independent field samples, we suspect that this disagreement is due to an insufficient representativeness of point observations and the distances of the points from the CRNS. The diurnal variation of hourly SWC from the CRNS was sinusoidal during a dry period, peaking at 11:00 and was minimum at 18:00 (Beijing time), with a range of 1%. Overall, the CRNS measured SWC in the AME with an acceptable accuracy, providing a scientific basis for the promotion and application of the CRNS in high, cold ecosystems. [ABSTRACT FROM AUTHOR]

Abstract: Although grazing is a common land use type, few studies are available about the response of soil microbial biomass to grazing especially above 4300 m on the Tibetan Plateau. Therefore, three fenced enclosures were made at three alpine meadow sites along an elevation gradient (4313 m, winter pasture; 4513 m and 4693 m, summer pasture) in July 2008. Soil samples inside and outside the fenced enclosures were gathered in July, August and September 2011. Microbial biomass C (MBC) and N (MBN) were determined using the chloroform fumigation–extraction method. Grazing marginally declined MBC by 21.60%, 4.83% and 5.36% across sampling dates at elevation 4313 m, 4513 m and 4693 m, respectively. Grazing significantly declined MBN by 39.58% and 18.88% across sampling dates at elevation 4313 m and 4693 m, respectively, whereas it slightly declined MBN by 1.50% at elevation 4513 m. Microbial biomass at elevation 4693 m was significantly higher in comparison with elevation 4513 m and 4313 m, whereas soil temperature at elevation 4693 m was 2.3 °C and 2.8 °C lower than that at elevation 4513 m and 4313 m, respectively. Our findings suggest that MBN may respond more rapidly to grazing than MBC and that climate warming and grazing may decline microbial biomass for the alpine meadow. [Copyright &y& Elsevier]