Disturbance diminishes the soil conservation potential of biological soil crusts due to patch fragmentation in drylands

Biocrusts, which are a positive factor in soil conservation in drylands, are often distributed as a pattern of patches on the soil surface due to disturbances. The fragmentation of biocrusts patches increases with the intensity and frequency of disturbance, thereby reducing the ecological function of these patches. However, whether and how biocrust fragmentation affects runoff and sediment yield is unclear. Accordingly, we conducted a series of indoor simulated rainfall experiments by constructing 21 erosion pans (1 m x 0.7 m) with bare soil (control) and biocrust with a coverage of 60 % in different fragmentations under a rainfall intensity of 90 mm/h. The fragmentation of biocrust was characterized by patch density (PD) ranging from 1.43 to 31.43. Larger values of PD indicate greater intensity of fragmentation of the biocrust patches. The results showed that biocrust fragmentation significantly influences runoff and soil erosion. Compared to a biocrust PD of 1.43, a PD of 4.29-31.43 shortened the time to runoff by 16.7 %, increased the runoff rate by 0.4 %, and increased the sediment yield rate by 85.8 %. The runoff rate showed a nonlinear relationship with biocrust fragmentation, while the sediment yield rate decreased logarithmically (S = 6.987ln(x) + 18.985, R2 = 0.837). Biocrust fragmentation affected hydrological connectivity (Flowlength, FL) and the overland flow pattern and regime (R2 = 0.525-0.990). The largest FL increased from 95.10 to 110.43 cm as biocrust fragmentation increased, thus indicating increased hydrological connectivity. Generally, biocrust fragmentation indirectly affected sediment yield by reducing Darcy-Weisbach resistance and increasing FL, with path coefficients of -0.71 and 0.80, respectively. The results of our study partly revealed the effect and mechanisms of biocrust fragmentation on their soil conservation function. Such results provide a new perspective for assessing soil erosion on biocrust-covered slopes and provide guidance for revising soil erosion models to include the effectiveness of biocrusts.