Soil erosion is the most concerned environmental problem in the world, especially in islands where land resources are incredibly scarce. Biological soil crusts (biocrusts) play a vital ecological role in soil conservation, especially by enhancing the horizontal stability of biocrusts through the secretion of microbial extracellular substances, protecting the soil from shear stress such as water erosion. Indeed, the protective mechanisms of biocrusts against soil erosion are intricate and multifaceted. The interconnections between soil erosion, extracellular polymeric substances (EPS), and microbial community structure during the formation and development of biocrusts remain insufficiently understood. Here, we collected 30 dark biocrust, light biocrust, and bare soil samples to evaluate the potential anti-erosion role of a tropical coral island biocrust, South China Sea. We applied a cohesive strength meter to measure the soil shear stress among different stages of biocrust development. We observed a significantly increased soil erosion resistance during biocrust formation. Compared to bare soil, soil shear stress increased from 0.33 N m^-2 to 0.61 and 1.31 N m^-2 in the light-biocrusts and dark-biocrusts, respectively. Mechanistically, we measured the microbial EPS contents and revealed a positive correlation between EPS and soil anti-erodibility. To explore the source of EPS, we performed high-throughput amplicon sequencing to identify microbial contributing to biocrust formation. Correlation analysis detected Cyanobacteria, Chloroflexi, Deinococcota, and Patescibacteria as potential microbials promoting EPS and reducing soil erosion. Together, our study presents the first evidence that biocrust from tropical coral reef island in the South China Sea promotes resistance to soil erosion, pinpointing key EPS-producing microbials against soil erosion. The findings would provide insights for island soil restoration.

Biocrust; Microbial community; Extracellular polymeric substances; Soil anti-erodibility; Soil nutrients