Biocrusts regulate surface soil deformations during freezing-thawing and wetting-drying cycles in cold-winter drylands
Description
In cold-winter drylands, seasonally frozen soils undergo recurring freezing-thawing and wetting-drying cycles throughout the cold and warm seasons, resulting in soil deformations such as frost heave, thaw settlement, wet-swelling, and dry-shrinkage. These behaviors are fundamental in determining the functioning of soils and ecosystems. Biocrusts, as ubiquitous living covers in cold-winter drylands, fundamentally reshape the structure and properties of surface soil. However, the role of biocrusts in regulating surface soil deformations remains poorly understood. Here, we conducted a series of laboratory simulations to address this knowledge gap, investigating the deformations of two types of biocrusts (cyanobacterial and moss) and bare soil (0–2 cm depth) during frost heave, thaw settlement, wet-swelling, and dry-shrinkage, as well as their underlying pathways. Our results revealed time-series differences in deformation ratios (i.e., height variation / original height) between biocrusts and bare soil during freezing and thawing. During freezing, the maximum frost heave ratios of cyano and moss crusts were 10.2‰ and 9.4‰, respectively, which were 31% and 37% lower than those of bare soil, and these frost heave ratios were intensified by increasing freezing intensity and initial water content. During subsequent thawing, the maximum thaw settlement ratio of biocrusts (15.0‰) was 48% higher than that of bare soil, with the thaw settlement being intensified by increasing freezing intensity and initial water content. After freezing-thawing, the final deformation ratio was positive (1.6‰) for bare soil but negative (–4.3‰) for biocrusts, indicating that biocrusts settled but bare soil expanded in comparison to their pre-freezing position. Furthermore, throughout wetting, biocrusts manifested a higher swelling ratio than bare soil, with the maximum swelling ratio of biocrusts (26.8‰) exceeding that of bare soil by 13%. Similarly, biocrusts also exhibited higher shrinking ratios than bare soil throughout drying. After wetting-drying, the final deformation ratio was positive (17.7‰) for bare soil but negative (–3.5‰) for biocrusts. Essentially, the clay content, total porosity, and organic matter content were critical factors that were influenced by biocrusts and explained observed deformations. Overall, our findings unveil the critical roles of biocrusts in mitigating soil frost heave and intensifying thaw settlement, wet-swelling, and dry-shrinkage, which thereby generate positive and multifaceted ecological implications on reducing soil erosion, alleviating moisture deficiency, and enhancing ecosystem productivity in cold-winter drylands.