Seasonal freeze–thaw front dynamics and effects on hydrothermal processes in diverse alpine grasslands on the northeastern Qinghai–Tibet Plateau

The dynamics of the freeze–thaw front strongly affect the transfer and exchange of water and energy in seasonally frozen zones. This study aimed to characterize the variations in the seasonal freeze–thaw fronts of diverse alpine grasslands and their effects on hydrothermal processes in the Qinghai Lake Basin (QLB). Field experiments were carried out on alpine meadow and steppe transition (AMS), alpine steppe (ASP) and Achnatherum splendens steppe (AST) ecosystems in the QLB, with 2 years of continuous high-frequency year-round observations. The results revealed that the seasonal freeze–thaw front was characterized by three distinct stages, in which the durations of slow freezing period (SF) and thawing period (T) were much shorter than that of rapid freezing period (RF). The maximum seasonal freezing depth in 2018–2020 were ranged from AMS (322.65 cm), AST (271.98 cm) and ASP (200.00 cm). The liquid soil volumetric water content (SVWC) decreased in all three ecosystems during the RF period, but increased during the SF and T periods. During the RF period, the SVWC of the AMS, ASP and AST decreased the most by −13.28 %, −9.52 % and −15.29 %, respectively. In the SF period, the SVWC of the AMS, ASP and AST increased the most, by 3.97 %, 9.52 % and 6.33 %, respectively, and by 13.37 %, 12.73 % and 12.44 % in the T period. During the freeze–thaw process, the SVWC and soil temperature mainly exhibit logarithmic and quadratic functions. The changes in the freeze–thaw front and SVWC in AMS were largely consistent, while those in ASP and AST experienced time lags. The freeze–thaw depth and net radiation of the RF, SF and T periods were fitted via the quadratic polynomial method. The freeze–thaw depth and soil heat flux were linearly fitted in the RF and SF periods, and the quadratic polynomial method was used in the T period. The effect of net radiation and soil heat flux accumulation on thawing front were more obvious in AMS than in ASP and AST. These findings are important for improving our understanding of water and heat transfer processes during the freeze–thaw period.