Effects of biological crust coverage on soil hydraulic properties for the Loess Plateau of China

Biological soil crusts (BSCs) are ubiquitous living covers that have been allowed to grow on abandoned farmlands over the Loess Plateau because the Grain for Green project was implemented in 1999 to control serious soil erosion. However, few studies have been conducted to quantify the effects of BSC coverage on soil hydraulic properties. This study was performed to assess the effects of BSC coverage on soil hydraulic properties, which are reflected by the soil sorptivity under an applied pressure of 0 (S-0) and -3 (S-3)cm, saturated hydraulic conductivity (K-s), wetting front depth (WFD), and mean pore radius ((m)), for the Loess Plateau of China. Five classes of BSC coverage (i.e., 1-20%, 20-40%, 40-60%, 60-80%, and 80-100%) and a bare control were selected at both cyanobacteria- and moss-covered sites to measure soil hydraulic properties using a disc infiltrometer under 2 consecutive pressure heads of 0 and -3cm, allowing the direct calculation of S-0, S-3, K-s, and (m). The WFD was measured onsite using a ruler immediately after the experiments of infiltration. The results indicated that both cyanobacteria and moss crusts were effective in changing the soil properties and impeding soil infiltration. The effects of moss were greater than those of cyanobacteria. Compared to those of the control, the S-0, S-3, K-s, WFD, and (m) values of cyanobacteria-covered soils were reduced by 13.7%, 11.0%, 13.3%, 10.6%, and 12.6% on average, and those of moss-covered soils were reduced by 27.6%, 22.1%, 29.5%, 22.2%, and 25.9%, respectively. The relative soil sorptivity under pressures of 0 (RS0) and -3 (RS3)cm, the relative saturated hydraulic conductivity (RKs), the relative wetting front depth (RWFD), and the relative mean pore radius (R-m) decreased exponentially with coverage for both cyanobacteria- and moss-covered soils. The rates of decrease in RS0, RS3, RKs, RWFD, and R-m of cyanobacteria were significantly slower than those of moss, especially for the coverage of 0-40%, with smaller ranges. The variations of soil hydraulic properties with BSC coverage were closely related to the change in soil clay content driven by the BSC coverage on the Loess Plateau. The results are useful for simulating the hydraulic parameters of BSC-covered soils in arid and semiarid areas.