Biocrusts’ impact on hydrological processes and erosion dynamics: A review

• Biocrusts are recognized as “ecosystem engineers” in fragile or degraded restoration. • Biocrust formation and succession are continuous, dynamic processes without distinct boundaries. • Biocrusts exhibit both positive and negative impacts on soil hydrological processes. • Biocrusts’ “armoring effect” minimizes soil particle detachment and transport. • Broader global research on biocrusts in diverse climates is recommended to address existing debates. Biological soil crusts (BSCs), functioning as the “living skin” and “ecosystem engineers” of arid and semi-arid desert regions, are composite structures formed by cyanobacteria, lichens, mosses, and their symbiotic microorganisms. Through mechanisms including extracellular polymeric substances (EPSS) secretion, microtopography construction, and photosynthetic carbon–nitrogen fixation, they significantly alter soil physicochemical properties, thereby regulating hydrological characteristics and enhancing erosion resistance. The widespread development of biocrusts not only stabilizes soil structure via “biological cementation-physical interception-chemical modification” but also provides moisture and nutrients for soil microbiota and vascular plants, driving positive ecological succession in desert ecosystems. Despite consensus on their soil improvement benefits, substantial controversies persist regarding biocrusts’ hydrological and erosional regulatory effects—particularly their dual hydrological roles, threshold-dependent erosion responses, and spatial heterogeneity—stemming from interactive complexities among crust types, developmental stages, environmental factors, and methodological approaches. Although scholars advocate integrating biocrusts into soil erosion prediction models (e.g., RUSLE or WEPP), limited mechanistic understanding of their hydrological and erosional processes constrains model accuracy. This review systematically synthesizes multi-scale impacts of biocrusts on soil physicochemical properties, hydrological regulation, and erosion dynamics, aiming to clarify their ecological functions, research advancements, and limitations while proposing future directions to deepen understanding of ecohydrological-erosion processes and mechanisms in arid and semi-arid regions, thereby providing critical insights for ecosystem restoration and erosion prediction.