Objective This study aims to elucidate the influence of the natural restoration process on changes in soil microbial biomass and organic carbon fractions in coal mining subsidence areas. This effort will provide a crucial scientific foundation for understanding the natural ecological restoration process and carbon accumulation dynamics in arid coal mining subsidence areas.

Methods Arid coal mining subsidence areas in Xinjiang, China were investigated in this study. Using the space-for-time substitution approach, sample plots were selected from an unmined area and areas having undergone natural restoration for one, three, eight, and 12 years. Subsequently, systematic analyses were conducted on the dynamic changes in the general physicochemical properties, microbial biomass carbon (MBC), microbial biomass nitrogen (MBN), microbial biomass phosphorus (MBP), and organic carbon fractions of soils at depths of 0–10 cm and 10–20 cm in the sample plots. The purpose is to provide a scientific basis for accelerating ecological restoration in coal mining areas.

Results and Conclusion The natural restoration across different years exhibits distinct phased changes. After one year of natural restoration, the soil organic carbon (SOC), total nitrogen (TN), total phosphorus (TP), and available phosphorus (AP) rapidly declined to their lowest levels. Then, these metrics increased slowly, recovering to only 60%–90% of their pre-mining levels after 12 years of natural restoration. Soil microbial biomass in the coal mining subsidence areas exhibited temporal accumulation during natural restoration. This characteristic, as well as the stoichiometric characteristics of soil microbial biomass, implies the carbon source limitation in the early-stage natural restoration. During the later stage of natural restoration, the MBN proved more deficient compared to the MBP, while the limitation of MBC and MBN gradually decreased with increasing years of natural restoration. Additionally, after 12 years of natural restoration, particulate organic carbon (POC) and mineral-associated organic carbon (MAOC) accounted for 21%–33% and 67%–79% of the SOC, respectively, suggesting a higher restoration rate of MAOC compared to POC. The surface soil POC was affected by soil TN and TP. In contrast, the microbial utilization of dissolved organic carbon (DOC) and AP led to elevated MBC, ultimately driving the MAOC accumulation in the surface soil. The subsurface soil POC and MAOC accumulated synchronously under the influence of TN, dissolved organic nitrogen (DON), TP, and AP, reflecting the impact of nutrient-microbial coupling on carbon dynamics. Overall, soil microbial biomass and organic carbon fractions were subjected to severe damage during the early-stage natural restoration. In the late-stage natural restoration, they were improved but failed to completely recover to the pre-mining levels. It is recommended to accelerate ecological restoration and promote carbon sink functions by combining artificial vegetation.