Biocrusts enhance soil structural stability during freeze-thaw cycles and improve soil erosion resistance in cold-winter drylands

Seasonally frozen soils are extensively distributed in drylands at middle- and high-latitude regions across the earth and experience intensive freeze-thaw cycles during cold winters. Cyanobacterial and moss biocrusts are critical living skins that cover a large portion of cold-winter drylands and, thus, have potential to regulate soil structure and function within frozen periods. Yet, the biocrust effect on soil structural stability during freezing and thawing and its underlying influencing factors have remained unanswered. Here, we conducted freezing and thawing simulations to elucidate the responses of soil structural stability to cyanobacteria- and moss-biocrust cover under different cycles of freeze-thaw (0–40 cycles) and initial water contents (0.03–0.20 cm3 cm−3). Our findings unveiled that the structural stability of bare soil, cyanobacterial biocrusts, and moss biocrusts decreased by 38.4%, 28.9%, and 24.1%, respectively, as a result of increasing freeze-thaw cycles and initial water content. However, as compared with bare soil, biocrust cover significantly mitigated the adverse impacts of freezing and thawing on soil structural stability. Specifically, biocrusts reduced the relative variation of soil particle-size distribution, porosity, and organic carbon content by 68.9%, 41.1%, and 32.1%, respectively, during increasing freeze-thaw cycles. Meanwhile, the relative variation of these properties caused by increasing initial water content during freezing and thawing was reduced by 39.4% due to the biocrust cover. As the consequences of such changes, biocrusts ultimately reduced the relative degradation of structural distance, aggregate stability, and erodibility by 39.4% in increasing freeze-thaw cycles and by 43.6% in increasing initial water content. This clearly demonstrated the biocrust contributions to enhancing soil structural stability and improving soil erosion resistance during freezing and thawing. Our study highlights the fundamental role of biocrusts in intervening soil structural degradations, caused by freeze-thaw cycles, and in serving as an imperative consideration during frozen periods in cold-winter drylands.