Towards the influences of three types of biocrusts on soil water in drylands: Insights from horizontal infiltration and soil water retention

As a “living skin” of soil in drylands, biocrusts possibly change the retention and movement of soil water, and thus critically influence hydrological and erosion processes as well as soil moisture regime. Contrary to vertical infiltration, little attention was paid to horizontal infiltration, which may provide important information on soil water diffusivity and sorptivity. In this study, the one-dimensional horizontal infiltration experiments for bare soil and three types of biocrusts (cyanobacterial, cyanobacterial-moss mixed, and moss crusts) were conducted with disturbed soil samples in a semiarid dryland. Undisturbed soil samples were also taken for each treatment, and their soil water retention capacity were determined in the laboratory. The substantial differences that were found were analyzed, and the differences were further validated through monitoring and comparing their in situ soil moisture regimes at depth of 0–20 cm. Our results showed that the three types of biocrusts had significantly lower constant infiltration rate (0.11 vs. 0.18 cm min−1) and infiltration water amount (6.75 vs. 10.01 cm) than bare soil, and their wetting front moved much slower (0.60 vs. 1.14 cm min−1) in comparison to bare soil, with bare soil > cyanobacterial crusts > cyanobacterial-moss mixed crusts > moss crusts. Total sorptivity of cyanobacterial, mixed, and moss crusts were 21%, 32%, and 53% lower than that of bare soil, respectively. More importantly, at the same level of water content, all crust types had lower water diffusivity, sorptivity, and conductivity than the bare soil, and exhibited a higher water content than bare soil at a same level of matric potential (across the range of −15,000–0 hPa), indicating an increase in water-holding capacity and water availability of the biocrusts in contrast to bare soil. Furthermore, the in-situ soil water content of all biocrusts increased on average by 30% at depth of 0–10 cm but decreased by 23% at 10–20 cm. Confirming the lower infiltrability and higher water-holding capacity of the biocrusts in comparison to bare soil, biocrusts increase surface soil water retention capacity and water availability at the surface, thus playing a vital role in reshaping the surface soil water balance, which subsequently may affect surface hydropedological and biochemical processes in drylands.