Objective The aim was to investigate the effects of biological soil crusts on soil shrinkage properties and to deepen the understanding for the mechanisms of biological soil crusts regulating surface soil structure.

Method This study focused on bare soil and moss-crusted soil in the Loess Plateau region. Using image analysis, the desiccation cracking process of the soil was systematically revealed, the differences in shrinkage characteristics between the two soil types were compared and analyzed, and the impact of environmental conditions on soil crack features was explored.

Result Compared with bare soil, moss-crusted loessal soil exhibited a slower initial evaporation rate but higher cumulative evaporation. The maximum fracture area ratio of bare soil was 3.46%, which was 1.9 times that of moss-crusted loessal soil (1.78%). The relationship between fracture area ratio and soil mass water content for both soils followed a "Z"-shaped curve. Cracking initiated at a water content of approximately 25.0% (98.8% FC_non, 88.3% FC_moss; FC = field capacity). When the water content dropped below 20.0% (79.4% FC_non, 70.7% FC_moss), the cracking rate increased sharply. As the water content decreased to 7.5% (29.8% FC_non, 26.5% FC_moss), the increase in fracture area ratio slowed down. With increasing numbers of wet-dry cycles, both the area and width of soil cracks increased for both soils, while the overall morphology remained stable. The number of secondary cracks in bare soil increased, whereas cracks began to appear on the surface of moss-crusted loessal soil. Compared with the first wet-dry cycle, the maximum fracture area ratio of bare soil increased by 22.3% and 35.0% in the second and third cycles, respectively; for moss-crusted loessal soil, these values increased by 48.9% and 65.7%, respectively.

Conclusion The development of moss crusts significantly inhibited soil shrinkage and reduced the fracture area ratio by altering the structure of the topsoil. The wet-dry cycling process is an important factor affecting soil shrink-swell behavior; it exacerbates crack development, and this effect is more pronounced in moss-crusted loessal soil.