Abstract: Soil erosion is severe in the red soil hilly region of southern China, where biocrusts widely develop on the surface of degraded and exposed soil, playing a critical role in regional eco-hydrological processes. From July to October 2024, we conducted an experiment with uncrusted degraded red soil and typical moss-crusted degraded red soil at the Ecological Experiment Station of Red Soil, Chinese Academy of Sciences, in Yujiang District, Yingtan City, Jiangxi Province. To elucidate the differences of soil water movement characteristics between biocrusts and uncrusted soil in degraded red soil regions, we investigated the infiltration processes and water flow types of uncrusted soil and biocrusts (moss crusts) through double-ring infiltration and in-situ dye-tracing experiments. The results showed that moss crusts significantly altered the physicochemical properties of the 0-10 cm depth of soil. Specifically, soil bulk density and saturated hydraulic conductivity of moss crusts was significantly reduced by 9.1% and 176.5%, while organic matter content, clay content, silt content, and surface roughness were 58.2%, 35.3%, 45.0%, and 201.7% higher than that of uncrusted soil, respectively. Compared to uncrusted soil, moss crusts greatly diminished soil infiltration, with initial infiltration rate, steady-state infiltration rate, average infiltration rate, and cumulative infiltration being reduced by 11.8%, 53.3%, 50.0%, and 50.9%, respectively. Moss crusts suppressed infiltration, resulting in a 29.7% reduction in the 0-10 cm stained area ratio and a 57.9% decrease in the stained path width <100 mm in comparison to uncrusted soil. Furthermore, the stained path number of moss crusts was 1.5 times of uncrusted soil. Moreover, the horizontal stained images also revealed that moss-crusted soil exhibited a higher number of water flow paths and a larger equivalent width compared to the uncrusted soil at 0-10 cm depth. The water flow pattern of moss crusts was predominantly exhibited heterogeneous finger flow (concentra-ted at 0-1 cm depth of soil) and highly interacted macropore flow, and without any homogeneous flow. Uncrusted soil exhibited predominantly homogeneous flow in the 0-3 cm depth, heterogeneous finger flow in the 3-8 cm, and highly interacted macropore flow below 8 cm. Results of Mantel test and structural equation modeling revealed that infiltration rate exhibited significant correlations with stained area ratio, surface type, and organic matter content. Especially, surface type (uncrusted soil and moss-crusted soil) exerted a direct and significant effect on soil infiltration and also indirectly influenced stained area ratio and soil infiltration by modifying other soil properties. In conclusion, moss crusts that developed on degraded and exposed red soil surfaces in subtropical regions could significantly alter the basic physicochemical properties and hydraulic parameters of surface soils. It markedly reduced the infiltration rate, suppress infiltration, and ultimately change soil water flow type. Thus, moss crusts play a cardinal role in surface ecological restoration in degraded red soil regions.

Key words: biological soil crust, red soil, infiltration rate, dye tracer, stained area ratio