Effects of artificial biological soil crust establishment on spoil substrate structure, carbon sequestration, and bacterial community assembly on a high-altitude slope.

Engineering spoil generated by construction projects constitutes a fragile ecosystem that is highly susceptible to degradation and the development of secondary geohazards. Artificial biological soil crusts (BSCs) offer a nature-based strategy to enhance degraded substrates; however, the underlying mechanisms of their effects remain poorly understood. In this study, an in situ artificial BSC establishment experiment was conducted on a high-altitude spoil slope. Different moss-algal and moss-cyanobacterial inoculum combinations were applied to evaluate their effects on spoil substrate. The results showed that artificial BSCs increased water content and nutrient availability, reduced bulk density and pH, and promoted SOC accumulation, including ROC, POC, TOC, and DOC. FTIR and SEM analyses indicated that BSC establishment enhanced EPS accumulation, increased organic cementing substances, strengthened particle bonding, and promoted more stable crust structures. BSC establishment also enhanced the activities of enzymes involved in carbon and nitrogen transformation, particularly catalase and nitrate reductase. In addition, BSC treatments increased bacterial richness, expanded the detectable ASV pool, altered bacterial community composition, and changed the relative contributions of community assembly processes, although stochastic processes remained dominant overall. Stochastic processes dominated community assembly in both BSC-treated and untreated soils, but BSC establishment modified the relative contributions of stochastic processes, with undominated processes and dispersal limitation becoming more evident. PLS-PM indicated that BSC establishment indirectly promoted carbon accumulation through improved crust properties, nutrients, bacterial communities, and enzyme activities, with enzymes showing the strongest direct effect. These findings provide a basis for optimizing BSC-based restoration strategies for spoil slopes in engineering disturbance areas.