As global temperatures continue to rise, the impact of water availability on vegetation productivity remains unclear. This study aims to assess the long-term trends of vegetation productivity response to wet and dry changes and the time-scale thresholds of vegetation response in China from 1982 to 2018, which will be important for reducing the management costs of terrestrial ecosystems and achieving the goals of carbon peaking and carbon neutrality. Results show that: (1) Water stress on vegetation productivity in China has increased throughout the study period. (2) In multiple time scale dry and wet variations, 61.18% of the vegetated cover areas had water deficit significantly inhibiting vegetative photosynthesis. In contrast, 28.29% of the vegetated cover areas had water surplus significantly inhibiting vegetative photosynthesis. (3) The minimum response time for vegetation productivity significantly stressed by aridification has been shortened, while the maximum response time for vegetation productivity significantly constrained by water surplus has been lengthened. These observations indicate that it became easier for aridification to suppress vegetation productivity effects and more difficult for the water surplus to produce suppressive effects on vegetation productivity. Consequently, the water constraint on vegetation productivity in China has been intensifying over the past 37 years. These findings shed light on the evolving trend of water availability in the face of ongoing climate warming, providing a scientific basis for understanding the coupling relationship between water and carbon, as well as the water-carbon cycle.

Transpiration (Tc) is a critical component of the global water cycle. Soil moisture (SM) and vapor pressure deficit (VPD) are key regulators of Tc, and exploring their contributions to changes in Tc can deepen our understanding of the mechanisms of water cycling in terrestrial ecosystems. However, the driving roles of VPD and SM in Tc changes remain debated because of the coupling of SM and VPD through land-atmosphere interactions which restrict the quantification of the independent effects of SM and VPD on Tc. By decoupling the correlations between SM and VPD using a novel binning approach, this study analyzed the dominant drivers of vegetation transpiration in subtropical China from 2003 to 2018 based on multi-source data, including meteorological reanalysis, remotely sensed soil moisture, transpiration, and land cover data. The results show that Tc first increased and then remained stable with an increase in SM across the study area but changed slightly with increasing VPD. Overall, the relative contribution of SM to the change in Tc was approximately five times that of VPD. The sensitivities of Tc to SM and VPD differed among vegetation types. Although the sensitivity of Tc to SM was greater than that of VPD for all four vegetation types, the thresholds of Tc in response to SM were different, with the lowest threshold (approximately 35%) for the other forests and the highest threshold (approximately 55%) for short wood vegetation. We infer that this is associated with the differences in ecological strategies. To verify the reliability of our conclusions, we used solar-induced chlorophyll fluorescence (SIF) data as a proxy for Tc based on the tight coupling between photosynthesis and transpiration. Consistent results were obtained by repeating the analyses. The results of this study, in which the impacts of SM and VPD on Tc were decoupled, are beneficial for further understanding the critical processes involved in water cycling in terrestrial ecosystems in response to climate change.

The response of the ecological environment on the Qinghai-Tibet Plateau to global climate change is a crucial topic in global change research. Given the close relationship between surface weathering and climate change, conducting quantitative research on silicate weathering and pyrite oxidation in glacial catchments can provide valuable insights into understanding climate change on the plateau. In this study, we focused on a typical glacial catchment, the Laohugou glacier catchment, located in the Qilian Mountains, north of the Qinghai-Tibet Plateau. Through the analysis of hydrochemistry, water isotopes of the river and precipitation, we explored the sources of glacier runoff, the origin of river solutes, the influence of hydrological conditions on solutes, the rate of chemical weathering in the catchment, and the impact of pyrite oxidation on the carbon sink effect of chemical weathering. Using isotopic hydrograph separation, we found that groundwater, glacial meltwater, and atmospheric precipitation contributed 18.2%, 58.3%, and 23.5%, respectively to glacial runoff. By employing a solute production modeling framework, we further examined the influence of discharge changes on the solutes. Additionally, utilizing an inverse model, we estimated the average mass contributions of carbonate, silicate, evaporite, and precipitation to the cations as 59.1%, 20.1%, 8.3%, and 12.5%, respectively. For $SO_{4}^{2-}$, the contributions of evaporite, precipitation and meltwater, and pyrite were 14.4%, 8.0%, and 77.6%, respectively. Finally, we estimated that the weathering fluxes of carbonate rock and silicate rock were 50.8 g/s and 7.8 g/s, respectively, with relative CO₂ consumption rates of 24.4 t/(km² a) and 5.9 t/(km² a), respectively. As for the impact of pyrite oxidation, the total CO₂ consumption rate of the Laohugou basin was estimated at 8.3 t/(km² a) in the short term (<10⁶ a), while the total CO₂ release rate was 16.2 t/(km² a) in the long term (>10⁶ a). Overall, this study provides an evaluation of silicate weathering and pyrite oxidation in the Laohugou glacier catchment under the influence of current climate change. It also sheds light on the impact of pyrite oxidation on the carbon cycle, thereby enhancing our understanding of the weathering and carbon cycle processes on the Qinghai-Tibet Plateau.

The grain size of sediments in sand seas is influenced by both sand provenance and various surface processes. This study collected 224 sedimentary samples from the Hunshandake Sandy Land including aeolian sand, fluvial and lacustrine sediments. Grain size measurement and end member analysis found that the mean grain size and sorting of aeolian sand generally decreased across the Hunshandake Sandy Land downwind in the SE direction. The spatial variations of skewness and kurtosis are more complex, indicating that the grain size parameters of aeolian sand are affected by the local provenance, depositional environment, and vegetation coverage. The end member analysis shows that the aeolian sand contains six end members (EM). EM1 is related to lacustrine outcrops in the western sandy land, while EM2 is likely to originate from loose fluvial and lacustrine sediments, controlled by both wind and topography. Fluvial deposits contribute to the widely distributed EM3, which is the dominant aeolian sand fraction found across the region. EM4 is predominantly found in the outer regions of the sand sea and closely associated with alluvial/fluvial deposits. EM5 and EM6 have the coarsest grain size and may originate from lacustrine/fluvial sediments that accumulated during past interglacials. Our findings suggest that the dune sand in Hunshandake mainly comes from the lacustrine/fluvial deposits around the sand sea, while the sand belt is unlikely to be a major source of dune sand. Aeolian sand with different sources is mixed in the western Hunshandake with fine fractions moved by the prevailing wind to the central and eastern parts of the sand sea. Rivers originating from the southern and eastern mountains also contribute aeolian sand to the adjacent areas. This study highlights the importance of extensive sampling combined with end member modeling for discriminating aeolian sand sources in desert areas.

Climbing dunes are important barrier dunes that are widely distributed in highland mountain regions, and their formation is mainly controlled by topography, sand sources and wind regime. Existing researches mainly focus on simulation experiments, distribution patterns, field observations, and morphological characteristics. From a regional scale, there is a lack of research to analyze the variations in sediment characteristics and environmental significance of climbing dunes in different climatic regions. In this study, the wind regime, near-surface airflow, sediment characteristics and material sources of climbing dunes on the Qinghai-Tibet Plateau were comprehensively analyzed from the aspects of aeolian geomorphology, grain-size sedimentology, and geochemistry. The results show that: (1) Due to differences in material sources, the grain-size composition of climbing dunes sediments in different climatic areas varies, primarily fine and medium sands, with good sorting in the semi-humid areas and poor sorting in arid areas. The grain-size difference of sediments in different geomorphic parts is small, and it responds well to near-surface airflow, and in-situ sand accumulation is the basic formation form, with fine sand having the strongest climbing ability. (2) The chemical element content of sediments is influenced by the parent rock characteristics and the climatic environment, and the particle size is also important to its spatial variation, and trace elements are primarily concentrated in river floodplains, with little variation between geomorphic sites. (3) Except for SiO₂ enrichment, all macronutrients in the sediment show varying degrees of leaching or enrichment, and the majority of trace elements are deficient, as leaching and enrichment are closely related to particle size. (4) Climbing dunes sediments are in the early stage of continental weathering, with higher weathering levels in the semi-humid zone. The weathering of sediments in different geomorphic sites varies substantially, which is related to the parent rock, climatic environment, and grain size.