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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]

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]

Cold biome ecosystems, extensively distributed on our planet, are highly sensitive to global changes. Fluctuations caused by climate change would inevitably affect the ecosystems' structure and functions. However, the linkage between cold biome ecosystems and global changes demonstrates high spatial heterogeneity, especially between high-latitude ecosystems (HL) and high-altitude alpine ecosystems (HA). A comparative analysis of their response patterns would provide deeper insight into the underlying mechanisms at play. We used meta-analysis to synthesize ecosystems' response to warming and altered precipitation performed in HL and HA. Warming and enhanced precipitation increases ecosystem biomass and carbon fluxes in HL and HA. Warming significantly stimulates aboveground biomass (AGB), root biomass (RB), total biomass (TB), aboveground net primary productivity, gross ecosystem productivity (GEP), soil respiration (SR), and net ecosystem productivity (NEP) in HL and HA. Similarly, AGB, GEP, and NEP increase significantly with enhanced precipitation. Respondent of ecosystem carbon storage and fluxes in HL and HA showed diverse results to warming treatment. Warming increases AGB and RB in HA while RB remains unaltered in HL. GEP and ER exhibit a positive response to warming in HL but an insignificant response in HA. In general, HL is sensitive to warming, and HA is sensitive to precipitation. The differential responses of HL and HA to climate change imply specific ecosystem traits and particular environmental constraining factors. Future cold biome ecosystem studies should further consider specific conditions like microtopography, soil moisture, and local climate unique to high-latitude and high-altitude ecosystems. [ABSTRACT FROM AUTHOR]

Investigating elevational gradient of climate driving effects on cropland ecosystem net primary productivity (NPP) plays an important role in food security in alpine region. We simulated cropland NPP by coupling a remote sensing model with an ecosystem process model and explored elevational gradient of climate driving effects on it in an alpine region of the southwest China during 1981–2014. The results showed that cropland NPP increased significantly with a rate of 3.85 gC m−2 year−1 year−1 under significant increasing solar radiation and climate warming and drying, among which the increasing solar radiation was the main driving factor of the increasing NPP. The driving effect of climate warming on cropland NPP shifted from negative at low elevations to positive at high elevations, which was caused by the fragile ecosystem characteristics and frequent drought at low elevations and a higher temperature sensitivity of cropland ecosystem at high elevations. Different effects of climate warming on NPP change at different elevations caused different results when we analyzed the climate-driving effects on cropland NPP at different spatial scales. These results reminded us that we should take the elevational gradient of climate driving effects into account when we manage food security in the alpine region. [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]

The alpine ecosystems on the Qinghai-Tibetan Plateau are quite sensitive to climate change. The increasing temperature and changing precipitation patterns greatly affect the gross primary productivity (GPP) and disturb the carbon balances of these alpine ecosystems. To clarify the impacts of future climate change across the Qinghai-Tibetan Plateau, it is important to address the scientific issue "Which factor would dominate interannual variability (IAV) in GPP and through which path does it work?" To clarify this issue, two key processes, growing season length (GSL) and maximum photosynthetic capacity (GPP max), were introduced to reveal the underlying mechanisms, and which of the environmental factors dominated their variations were studied specifically based on the flux and corresponding environmental observation data obtained in different types of alpine ecosystems across the Qinghai-Tibetan Plateau in this study. The results indicated that across the temperature- and water-limited alpine ecosystems, the temperature controlled the GSL, but the water conditions dominated the variations in GPP max. The soil water content (SWC) dominated GPP max , which could explain 89% of the variation in GPP max. The GSL alone was incapable of explaining IAV in GPP. Conversely, GPP max is robust in explaining IAV in GPP, which could explain 94% of the annual GPP. Therefore, climate change would probably drive IAV in GPP through the path of "SWC → GPP max → annual GPP" on the Qinghai-Tibetan Plateau. In addition, GPP max together with GSL (GSL × GPP max) could explain 99% of IAV in GPP, as they indicated the length of the carbon uptake time and the capacity of carbon sequestration, respectively. This study provides a new perspective on the predominant causes of IAV in GPP in alpine ecosystems, indicating that the changing precipitation patterns under future climate change will play a dominant role in affecting the carbon sink function of the Qinghai-Tibetan Plateau. [Display omitted] • Temperature dominated the growing season length (GSL) in alpine ecosystems. • Soil water content (SWC) dominated the maximum photosynthetic capacity (GPP max). • GPP max rather than GSL dominated the annual GPP. • GSL and GPP max coexplained 99% of the interannual variability in annual GPP. • Climate change drove GPP through two key underlying processes: GSL and GPP max. [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]