Biocrusts regulate surface soil deformations during freezing-thawing and wetting-drying cycles in cold-winter drylands

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.

在寒冷干旱的冬季干草原地区，季节性冻结的土壤在冷热季节中反复经历冻融和湿润干燥周期，导致土壤变形，如冻胀、融沉、湿涨和干缩。这些行为是决定土壤和生态系统功能的基础。生物结皮作为冷冬季干旱地区普遍的活覆盖层，从根本上改变了表层土壤的结构和性质。然而，生物结皮在调节表层土壤变形方面的作用尚不明确。本研究通过一系列实验室模拟实验，旨在填补这一知识空白，探究了两种类型生物结皮（蓝细菌和苔藓）以及裸土（0-2厘米深度）在冻胀、融沉、湿涨和干缩过程中的变形情况及其潜在机制。我们的研究结果揭示了生物结皮与裸土在冻融过程中变形率（即高度变化/原始高度）的时间序列差异。在冻结过程中，蓝细菌和苔藓结皮的冻胀最大比率为10.2‰和9.4‰，分别比裸土低31%和37%，并且这些冻胀比率随着冻结强度的增加和初始含水量的提高而加剧。在随后的融沉过程中，生物结皮的最大融沉比率为15.0‰，比裸土高出48%，融沉过程同样随着冻结强度和初始含水量的增加而加剧。冻融后，裸土的最终变形率为正值（1.6‰），而生物结皮的最终变形率为负值（-4.3‰），这表明与冻结前位置相比，生物结皮发生沉降，而裸土则发生膨胀。此外，在整个湿润过程中，生物结皮的膨胀率高于裸土，生物结皮的最大膨胀率（26.8‰）比裸土高出13%。同样，在干燥过程中，生物结皮的收缩率也高于裸土。湿润干燥后，裸土的最终变形率为正值（17.7‰），而生物结皮的最终变形率为负值（-3.5‰）。本质上，粘土含量、总孔隙率和有机质含量是受生物结皮影响的关键因素，并解释了观察到的变形。总体而言，我们的发现揭示了生物结皮在缓解土壤冻胀、加剧融沉、湿涨和干缩中的关键作用，从而在减少土壤侵蚀、缓解水分短缺和提升生态系统生产力方面在冷冬季干旱地区产生积极且多方面的生态影响。