Surface soil moisture (SSM) is a key limiting factor for vegetation growth in alpine meadow on the Qinghai-Tibetan Plateau (QTP). Patches with various sizes and types may cause the redistribution of SSM by changing soil hydrological processes, and then trigger or accelerate alpine grassland degradation. Therefore, it is vital to understand the effects of patchiness on SSM at multi-scales to provide a reference for alpine grassland restoration. However, there is a lack of direct observational evidence concerning the role of the size and type of patches on SSM, and little is known about the effects of patches pattern on SSM at plot scale. Here, we first measured SSM of typical patches with different sizes and types at patch scale and investigated their patterns and SSM spatial distribution through unmanned aerial vehicle (UAV)-mounted multi-type cameras at plot scale. We then analyzed the role of the size and type of patchiness on SSM at both patch and plot scales. Results showed that: (1) in situ measured SSM of typical patches was significantly different (P < 0.01), original vegetation patch (OV) had the highest SSM, followed by isolate vegetation patch (IV), small bare patch (SP), medium bare patch (MP) and large bare patch (LP); (2) the proposed method based on UAV images was able to estimate SSM (0-40 cm) with a satisfactory accuracy (R-2 = 0.89, P < 0.001); (3) all landscape indices of OV, with the exception of patch density, were positively correlated with SSM at plot scale, while most of the landscape indices of LP and IV showed negative correlations (P < 0.05). Our results indicated that patchiness intensified the spatial heterogeneity of SSM and potentially accelerated the alpine meadow degradation. Preventing the development of OV into IV and the expansion of LP is a critical task for alpine meadow management and restoration.

Alpine meadows are vital ecosystems on the Qinghai-Tibet Plateau, significantly contributing to water conservation and climate regulation. This study examines the energy flux patterns and their driving factors in the alpine meadows of the Qilian Mountains, focusing on how the meteorological variables of net radiation (Rn), air temperature, vapor pressure deficit (VPD), wind speed (U), and soil water content (SWC) influence sensible heat flux (H) and latent heat flux (LE). Using the Bowen ratio energy balance method, we monitored energy changes during the growing and non-growing seasons from 2022 to 2023. The annual average daily Rn was 85.29 W m-2, with H, LE, and G accounting for 0.56, 0.71, and -0.32 of Rn, respectively. Results show that Rn is the main driver of both H and LE, highlighting its crucial role in turbulent flux variations. Additionally, a negative correlation was found between air temperature and H, suggesting that high temperatures may suppress H. A significant positive correlation was observed between soil moisture and LE, further indicating that moist soil conditions enhance LE. In conclusion, this study demonstrates the impact of climate change on energy distribution in alpine meadows and calls for further research on the ecosystem's dynamic responses to changing climate conditions.

Freeze-thaw desertification (FTD) as a specific land degeneration form in high elevations is intensifying in alpine meadows due to climate change and human activities. It causes the formation of desertified patches (DPs), and further aggravating alpine meadow patchiness and impairing ecosystem functions such as water conservation, carbon sequestration and biodiversity maintenance. However, the impacts of FTD on the patch pattern, soil properties, and vegetation succession of alpine meadows and the elevation differences of these impacts still lack a comprehensive understanding. Here, we analyzed the patch patterns, soil and vegetation characteristics in typical FTD regions in the Qilian Mountains using aerial photography and field investigations along an elevation gradient. Our results indicated that, as elevation increases, the fragmentation of alpine meadows caused by FTD intensified, which was related to the elevational differentiation of freeze-thaw cycles and soil water holding capacity. DPs not only led to a decrease in soil water holding capacity and an increase in bulk density, but also caused surface soil sandification. Among them, the weakening of soil water holding capacity by DPs was particularly serious in high elevations. Additionally, the degradation of the original vegetation species com-munities in DPs caused the significant loss of vegetation cover, biomass and soil organic carbon, and made DPs exhibit certain alpine desert steppe characteristics, whereas the vegetation diversity of DPs had an increase at low elevations. Our findings highlight the significant impacts of FTD on the water conservation function and vegetation diversity of alpine meadows, and it is necessary to apply ecological protection measures to control DPs expansion such as fenced grazing, biological control and land cover (crop, vegetation, degradable plastic mulch, etc.).

Diurnal variation of land surface temperature (LST) is essential for land surface energy and water balance at regional or global scale. Diurnal temperature cycle (DTC) model with least parameters and high accuracy is the key issue in estimating the spatial-temporal variation of DTC. The alpine meadow is the main land cover in the Tibetan Plateau (TP). However, few studies have been reported on the performance of different DTC models over alpine meadows in the TP. Four semi-empirical types of DTC models were used to generate nine 4-parameter (4-para) models by fixing some of free parameters. The performance of the nine 4-para DTC models were evaluated with four in situ and MODIS observations. All models except GOT09-dT-t(s) (dT means the temperature residual between T-0 and T (t ->infinity); t(s) means the time when free attenuation begins) had higher correlation with in situ data (R-2 > 0.9), while the INA08-t(s) model performed best with NSE of 0.99 and RMSE of 2.04 K at all sites. The GOT09-t(s)-tau (tau is the total optical thickness), VAN06-t(s)-omega(1) (omega(1) means the half-width of the cosine term in the morning), and GOT01-t(s) models had better performance, followed by GOT09-dT-tau, GOT01-dT, and VAN06-t(s)-omega(2) (omega(2) means the half-width of the cosine term in the afternoon) models. All models had higher accuracy in summer than in other seasons, while poorer performance was produced in winter. The INA08-t(s) model showed best performance among all seasons. Models with fixing t(s) could produce higher accuracy results than that with fixing dT. The comparison of INA08-ts model driven by in situ and Moderate Resolution Imaging Spectroradiometer (MODIS) data indicated that the simulation accuracy mainly depended on the accuracy of MODIS LST. The daily maximum temperature generated by the nine models had high accuracy when compared with in situ data. The sensitivity analysis indicated that the INA08-dT and GOT09-dT-t(s) models were more sensitive to parameter dT, while all models were insensitive to parameter t(s), and all models had weak relationship with parameters omega and tau. This study provides a reference for exploring suitable DTC model in the TP.

To explore the effects of mattic epipedon (ME) on soil moisture and hydraulic properties in the alpine meadow of three-river source region, the soil moisture, water infiltration, evapotranspiration, soil bulk density and soil water holding capacity of original vegetation (OV), light degradation (LD), moderate degradation (MD) and severe degradation (SD) was conducted in this study, respectively. The results showed that: (1) the alpine meadow degradation reduced the soil moisture in the shallow layer (0-10 cm) and had no significant effects on the soil moisture in the deep layer (20-30 cm). (2) The effects of alpine meadow degradation on infiltration was depend on the presence of ME or not, when the ME existed on the land surface (from OV treatment to MD treatment), the alpine meadow degradation had no significant effects on infiltration. Once the ME disappeared on the land surface (from MD treatment to SD treatment), the alpine meadow degradation mainly increased the infiltration. (3) With the aggravation of alpine meadow degradation, the daily evapotranspiration first decreased and then significantly increased when the gravimetric soil water content at 0-5 cm in SD treatment (GWC5) was exceeded 19.5%, the daily evapotranspiration gradually decreased when GWC5 ranged from 9.3% to 19.5%, and had no significant changes on the evapotranspiration when GWC5 was less than 9.3%. Considering the characteristics of precipitation in alpine meadow, it was concluded that the alpine meadow degradation accelerated the evapotranspiration during the plant-growing season. (4) The effect of alpine meadow degradation on soil bulk density and saturated water capacity was concentrated at 0-10 cm. With the aggravation of alpine meadow degradation, the bulk density at 0-10 cm was first stable and then significantly increased and the saturated water capacity at 0-10 cm was first gradually increased and then significantly decreased. Our results suggested that the ME is vital for water conservation of alpine meadow and the protection of ME should be emphasized to promote the sustainable development of the ecosystem and the water supply of water towers in China.

Soil moisture dynamics play an active role in ecological and hydrological processes. Although the variation of the soil water moisture of multiple ecosystems have been well-documented, few studies have focused on soil hydrological properties by using a drying and weighing method in a long time series basis in the Qinghai-Tibet Plateau (QTP). In this study, 13 year (2008-2020) time-series observational soil moisture data and environmental factors were analyzed in a humid alpine Kobresia meadow on the Northern Qinghai-Tibetan Plateau. The results showed no significant upward trend in soil water content during the 2008-2020 period. In the growth season (May-October), the soil water content showed a trend of decreasing firstly, then increasing, and finally, decreasing. Correlation analysis revealed that five meteorology factors (temperature, humidity, net radiation, dew point temperature, and vapor pressure) and a biomass element (above-ground biomass) had a significant effect on the soil moisture, and air temperature impacted the soil water variation negatively in 0-50 cm, indicating that global warming would reduce soil moisture. Humidity and net radiation made a difference on shallow soil (0-10 cm), while dew point temperature and vapor pressure played a role on the deep soil (30-50 cm). Above-ground biomass only effected 30-50 cm soil moisture variation, and underground biomass had little effect on the soil moisture variation. This indirectly indicated that below-ground biomass is not limited by soil moisture. These results provide new insights for the rational allocation of water resources and management of vegetation in alpine meadows, in the context of climate change.

The hydrological properties of the active soil layer are the key parameters that regulate soil water-heat-solute migration and alter hydrologic cycles in a permafrost region. To date, much remains unknown about the interaction mechanism between permafrost degradation and eco-hydrological processes in the permafrost regions of the Qinghai-Tibet Plateau (QTP). In this study, the soil texture, soil hydrological properties, the soil moisture status, and the hydrothermal processes were measured and analyzed in different degradation degrees of alpine meadow soils on the QTP. The results showed a close relationship between soil hydrological properties and soil physicochemical properties. Freeze-thaw cycles changed the physicochemical and hydrological properties, that is, frequent freeze-thaw cycles promote to permafrost degradation in terms of soil basis properties of active layer. In addition, vegetation on the ground delayed the degradation of frozen soil. The actual available soil water content (SWC) in the root layer was a key factor in the ecohydrological process. The actual effective SWC in the root layers of different alpine meadows was ranked as follows: non-degraded meadow (NDM) > moderately-degraded meadow (MDM) > seriously degraded meadow (SDM) (1.8-5.0% at NDM and 0.0-4.2% at SDM). In addition, the weak soilpermeability in an SDM intensified the deficiency of the available SWC, thereby increasingthe difficulty of ecological restoration. This study provides a basis for ecological environmental protection in permafrost regions and provides a hydrological process model for cold regions under future climate change scenarios.

To understand the water, energy, and carbon cycles in the Tibetan Plateau (TP), it is essential to estimate seasonal and inter-annual variations in energy fluxes and evapotranspiration (ET) for alpine meadow ecosystems. The multiyear (2014-2019) energy fluxes and ET, for a typical alpine meadow at Arou station (northeastern TP), and their environmental and biophysical controls were evaluated using the eddy covariance method in this study. Latent heat flux (LE) was the dominant component of energy consumption during the growing season, whereas sensible heat flux (H) dominated energy partitioning during the non-growing season. H showed the opposite trend to LE, while the seasonal variation of soil heat flux (G) was small. The daily ET was primarily controlled by the available energy on the seasonal scale. Soil water content (SWC) and normalized difference vegetation index (NDVI) displayed secondary effects on ET during the non-growing and growing seasons, respectively. The inter-annual ET was relatively stable, ranging from 562.6 to 661.9 mm (coefficient of variation; CV = 7.4 %); this was slightly higher than the annual precipitation despite large variations in inter-annual precipitation (CV = 19.9 %) and was most likely due to snow and frozen ground melting. The cumulative ET in the growing season was about 77 % of the annual ET. There was a nonlinear increase in the daily Priestley-Taylor coefficient (alpha = ET/ETeq, where ETeq is the equilibrium evaporation) with an increase in bulk surface conductance (g(c)), which was insensitive to increases in g(c) that exceeded 15 mm s(-1). There was a good relationship between gc and NDVI. This study provides insights into the driving mechanisms of long-term variations in the energy partitioning and biophysical controls on ET in alpine meadow ecosystems.

By altering the physical properties of soil through root activity, plants can act as important agents in affecting soil hydrothermal properties. However, we still know little about how plant roots regulate these properties in certain ecosystems, such as alpine meadows. Thus, we studied the influence of roots on soil hydrothermal properties in the Qinghai-Tibet Plateau (QTP). Root biomass as well as soil physicochemical and hydrothermal properties were examined at a depth of 0-30 cm at three study sites in the QTP. The relationship between root biomass and saturated soil hydraulic conductivity (K-s) was examined, as was the applicability of common soil hydrothermal properties models to the alpine meadow system. Results revealed that approximately 91.10%, 72.52%, and 76.84% of root biomass was located in the top 0-10 cm of soil at Maqu, Arou, and Naqu, respectively. Compared with the bulk soil, the water-holding capacity of rhizosphere soil was enhanced by 20%-50%, while K-s was decreased by at least 2- to 3-fold. The thermal conductivity (lambda) of rhizosphere soils was lower than that of the bulk soil by 0.23-0.82 W m(-1).K-1 on average. Lastly, soil hydrothermal properties models that do not explicitly consider root effects overestimated the Ks and lambda in the rhizosphere soil of these systems. Overall, our results revealed distinctive differences in soil hydrothermal properties between the rhizosphere soil and the bulk soil in the QTP. This research has important implications for future modeling of soil hydrothermal processes of alpine meadow soils.

Wet alpine meadows generally act as a significant carbon sink, since their low rate of soil decomposition determines a much smaller ecosystem respiration (Re) than photosynthesis. However, it remains unclear whether the low soil decomposition rate is determined by low temperatures or by nearly-saturated soil moisture. We explored this issue by using five years of measurements from two eddy-covariance sites with low temperature and significantly different soil water conditions. The results showed that both sites were carbon sinks. However, despite a smaller annual gross primary productivity, the wet site with a shallow groundwater showed a much higher carbon use efficiency and larger carbon sink than the dry site (which had a deeper water table) due to its much lower Re. Our analyses showed that Re of the wet site was significantly decreased under the nearly-saturated soil condition during the unfrozen seasons. This effect of nearly-saturated soil water on Re increased with soil depths. In contrast, at the dry site the high soil water content favored Re. The corresponding soil temperature at both sites expectedly showed large and positive effects on Re. These results demonstrated that the high carbon sink of the wet alpine meadow was mainly caused by the inhibiting effects of the nearly-saturated soil condition on soil respiration rather than by the low temperatures. Therefore, we argue that a warming-induced shrinking cryosphere may affect the carbon dynamics of wet and cold ecosystems through changes in soil hydrology and its impact on soil respiration. In addition, our study highlights the different responses of soil respiration to warming across soil depths. The thawing of frozen soil may cause larger CO2 emission in the top soil, while it may also partially contribute to slowing down soil carbon decomposition in the deep soil through decreasing metabolic activity of aerobic organisms.