Maintaining biocrusts in grasslands above a threshold coverage is vital for soil erosion control in drylands

Biocrusts (biological soil crusts) are ubiquitous living surface covers in dryland grasslands that play critical roles in erosion control. However, the performance of biocrusts in soil conservation in grasslands remains uncertain due to the limited knowledge of the linkage between biocrust coverage and soil loss. We established eight treatments, including bare soil, 30 % grass alone, and 30 % grass with biocrusts, with coverage ranging between 10 % and 60 %. The runoff rate, hydrodynamic parameters, and sediment yield rate were investigated via simulated rainfall experiments at an intensity of 90 mm h⁻¹. We found that biocrusts significantly increased the soil surface roughness and Darcy–Weisbach resistance coefficient, leading to a decrease in flow velocity, Reynolds number, and Froude number in grasslands. Thus, biocrusts were found to significantly reduce runoff rate and sediment yield rate from grasslands compared to bare soil. Both the decrement of runoff rate (R) and sediment yield rate (S) were exponentially correlated with increasing biocrust coverage (R=0.981exp(−0.012x), R² = 0.639; S=13.515exp(−0.045x), R² = 0.889). There was a threshold coverage (35 %) for the impact of biocrusts on the sediment yield rate. Further analysis by the structural equation model (SEM) revealed that biocrusts influenced sediment yield through direct cover (path coefficient of −0.49) and indirectly by reducing runoff and velocity with path coefficients of −0.55 and −0.60, respectively. Results of this study break our preconceived notion that grasses are often thought to be the key to erosion control, when in fact biocrusts play an unignorable role in erosion control in grasslands. The study highlighted the importance of maintaining biocrusts to a threshold coverage in grasslands for soil erosion control.