Contrasting effects of biocrusts on winter unfrozen water dynamics within aeolian sand and loess soil and their ecohydrological implications

In cold-winter drylands, a substantial portion of soil water remains unfrozen throughout the frozen period despite subzero temperatures. This unfrozen water plays a fundamental role in governing hydrological, thermal, and ecological processes during winter. Biocrusts, as widespread and multifunctional communities in global drylands, are well-known to fundamentally influence soil moisture regimes and hydrological processes during warm and wet seasons. However, the effects of biocrust cover on unfrozen soil water dynamics during cold periods remain poorly understood. Here, we conducted a two-year in-situ and continuous experiment to monitor unfrozen soil moisture and temperature dynamics across 0–100 cm profiles in two contrasting dryland soils (aeolian sand and loess soil) with and without biocrust cover. We also performed soil pore analyses using X-ray computed tomography (CT) scanning to investigate the underlying mechanisms of biocrusts-induced soil water dynamics. Our findings revealed that biocrust cover regulated the temporal and depth dynamics of unfrozen soil water during winter, and these effects depended on soil type and were partially related to the lingering effects from the later warm season. The biocrusts-covered aeolian sand exhibited a 2.9%–23.7% reduction in daily maximum, mean, and minimum unfrozen water content across 0–70 cm depths compared to bare aeolian sand. In contrast, on loess soil, biocrust cover enhanced unfrozen water content by 26.6% in upper layers (0–30 cm) but decreased it by 36.5% in deeper layers (50–90 cm). Furthermore, biocrusts changed unfrozen soil water storage (0–30 cm) during winter, as evidenced by a 13.5% reduction in aeolian sand but a 30.9% increase in loess soil. By comparing unfrozen water storage between the pre- and post-frozen periods, biocrusts were found to promote freezing-induced water accumulations within loess soil, thereby increasing soil water availability for microbiomes, grasses, and shrubs in the subsequent spring. Essentially, these contrasting effects were derived from the differential pore structures between biocrusts-covered and bare soils. Compared to bare soil, biocrust cover decreased the total and plane porosity within most surface layers (1–4 cm) of aeolian sand, whereas it increased them within surface loess soil (0–4 cm). Importantly, biocrusts-induced alterations in unfrozen soil water further influenced soil temperature regimes and freezing-thawing processes, resulting in increased temperature, shortened frozen period, and reduced freezing depth in aeolian sand, whereas temperature decreased, frozen period increased, and freezing depth increased in loess soil. Overall, our findings highlight the critical role of biocrusts in regulating soil water dynamics during frozen periods, which could have implications for erosion processes, carbon emissions, plant phenology, and ecosystem productivity throughout the year in cold-winter drylands.