The zero-curtain period (ZCP), occuring during seasonal freeze-thaw cycles, plays a crucial role in energy and water exchanges, biogeochemical and hydrological cycles, and ecosystem dynamics. However, its temporal variations and controlling factors remain poorly understood, particularly in high-altitude regions of the Qinghai-Xizang Plateau (QXP). This study investigates the ZCP within seasonal freeze-thaw cycles in the central Headwater Area of the Yellow River (HAYR), using high-precision soil temperature observations from two seasonally frozen ground sites (TCM-2 and ZLH-WS) in 2011-2024. Results demonstrate that at TCM-2, the thawed duration generally decreases with depth, ranging from 163.2 d at 200 cm to 183.6 d at 20 cm, with no significant temporal trends. At ZLH-WS, the thawed duration was 13.2-33.8 d longer than that at TCM-2 but exhibits a distinct decreasing trend in 2016-2023. ZCP duration exhibits strong, albeit contrasting, correlations with soil water storage (SWS) at both sites, negative at TCM-2 (R = -0.86, p < 0.01) and positive at ZLH-WS (R = 0.83, p < 0.01). Notably, the highest recorded soil unfrozen water content in 2019 corresponded to an extended ZCP duration. ZCPs were observed during both thawing and freezing periods. The duration of the freezing period ZCP showed a marked temporal decline, particularly near the maximum frost penetration (MFP) depth and becoming more pronounced at greater depths. A substantial reduction in ZCP duration was detected in the middle and lower portions of the MFP at both sites, with decreasing rates of -1.86 d per year (p < 0.001) at TCM-2 and -2.7 d per year (p < 0.001) at ZLH-WS. These findings suggest that the shortening ZCP altered subsurface water phase dynamics, potentially leading to reduced water retention capacity within the shallow frozen ground profile. This highlights the significant implications of ongoing frozen ground degradation for subsurface hydrology in seasonally frozen ground.

The monitoring of permafrost is important for assessing the effects of global environmental changes and maintaining and managing social infrastructure, and remote sensing is increasingly being used for this wide-area monitoring. However, the accuracy of the conventional method in terms of temperature factor and soil factor needs to be improved. To address these two issues, in this study, we propose a new model to evaluate permafrost with a higher accuracy than the conventional methods. In this model, the land surface temperature (LST) is used as the upper temperature of the active layer of permafrost, and the temperature at the top of permafrost (TTOP) is used as the lower temperature. The TTOP value is then calculated by a modified equation using precipitation-evapotranspiration (PE) factors to account for the effect of soil moisture. This model, referred to as the TTOP-LST zero-curtain (TLZ) model, allows us to analyze subsurface temperatures for each layer of the active layer, and to evaluate the presence or absence of the zero-curtain effect through a time series analysis of stratified subsurface temperatures. The model was applied to the Qinghai-Tibetan Plateau and permafrost was classified into seven classes based on aspects such as stability and seasonality. As a result, it was possible to map the recent deterioration of permafrost in this region, which is thought to be caused by global warming. A comparison with the mean annual ground temperature (MAGT) model using local subsurface temperature data showed that the average root mean square error (RMSE) value of subsurface temperatures at different depths was 0.19 degrees C, indicating the validity of the TLZ model. A similar analysis based on the TLZ model is expected to enable detailed permafrost analysis in other areas.

During winter when the active layer of Arctic and alpine soils is below 0 degrees C, soil microbes are alive but metabolizing slowly, presumably in contact with unfrozen water. This unfrozen water is at the same negative chemical potential as the ice. While both the hydrostatic and the osmotic components of the chemical potential will contribute to this negative value, we argue that the osmotic component (osmotic potential) is the significant contributor. Hence, the soil microorganisms need to be at least halotolerant and psychrotolerant to survive in seasonally frozen soils. The low osmotic potential of unfrozen soil water will lead to the withdrawal of cell water, unless balanced by accumulation of compatible solutes. Many microbes appear to survive this dehydration, since microbial biomass in some situations is high, and rising, in winter. In late winter however, before the soil temperature rises above zero, there can be a considerable decline in soil microbial biomass due to the loss of compatible solutes from viable cells or to cell rupture. This decline may be caused by changes in the physical state of the system, specifically by sudden fluxes of melt water down channels in frozen soil, rapidly raising the chemical potential. The dehydrated cells may be unable to accommodate a rapid rise in osmotic potential so that cell membranes rupture and cells lyse. The exhaustion of soluble substrates released from senescing plant and microbial tissues in autumn and winter may also limit microbial growth, while in addition the rising temperatures may terminate a winter bloom of psychrophiles. Climate change is predicted to cause a decline in plant production in these northern soils, due to summer drought and to an increase in freeze-thaw cycles. Both of these may be expected to reduce soil microbial biomass in late winter. After lysis of microbial cells this biomass provides nutrients for plant growth in early spring. These feedbacks in turn, could affect herbivory and production at higher trophic levels. (C) 2009 Elsevier Ltd. All rights reserved.