Underground coal mining reduces organic carbon stability and alters microbial metabolic limitation in biocrusts of an arid region in northern China.

Underground coal mining disrupts surface stability, depletes soil organic carbon (SOC) stocks through persistent alterations in topography and hydrological regimes, and adversely affects soil microbial diversity and ecological functioning. Biological soil crusts (biocrusts) play critical roles in soil stabilization, SOC sequestration, and microbial nutrient transformations in coal-mining areas. However, the mechanisms by which different mining disturbance gradients regulate organic carbon composition and stability, microbial metabolic processes, and nutrient limitations within biocrusts remain poorly understood. In this study, we investigated biocrust organic carbon fractions and microbial resource characteristics across coal mining subsidence areas by applying an organic carbon stability index and ecoenzymatic stoichiometry model to evaluate the effects of mining disturbance gradients (unmined, active mining, and 1-2 year mined-out) and vegetation types (herbaceous and shrub) on biocrust properties and microbial resource limitation. Biocrust cover ranged from 26.4 to 40.8%, with the highest moss cover occurring in herbaceous vegetation in unmined areas, whereas cyanobacterial biocrusts were most abundant under shrub vegetation in unmined areas. Compared with unmined sites, active mining and 1-2 year mined-out areas exhibited significantly lower recalcitrant organic carbon (ROC), dissolved organic carbon (DOC), and microbial biomass carbon (MBC) in biocrusts. Active mining markedly reduced biocrust nutrient availability, organic carbon stability, microbial enzyme activities related to carbon (C), nitrogen (N), and phosphorus (P) acquisition, and microbial carbon use efficiency (CUE). Vector analysis of extracellular enzyme activities indicated that moss biocrusts were primarily P-limited. In contrast, cyanobacterial biocrusts were predominantly N-limited, with nutrient limitations most pronounced in active mining and 1-2 year mined-out areas. Moreover, microbial carbon limitation was significantly greater in the moss biocrusts than in the cyanobacterial biocrusts. Partial least squares path modeling further demonstrated that coal mining influences microbial CUE, primarily by altering metabolic limitations and organic carbon stability, with microbial carbon limitation exerting the strongest negative total effect on CUE. Overall, these findings elucidated the key mechanisms governing the stability of biocrust organic carbon and the microbial metabolic constraints under mining disturbance, providing a scientific basis for ecological protection and restoration in arid and semi-arid mining regions.