The promotion mechanism of how biochar affects the reclaimed soil microbial carbon sequestration capacity in coal mining areas

Biochar application is a key measure for enhancing soil quality. However, the impact of biochar applications on the reclaimed soil for improvement on soil physicochemical properties, enzyme activity and microbial diversity is still unclear, especially for the promotion mechanism of microbial carbon sequestration capacity. This study applied three kinds of biochar originated from straw containing rice straw, wheat straw, and corn straw to mine reclaimed soil, measured the effects of biochar addition on the physicochemical properties, enzyme activity, and carbon management index of reclaimed soil, and analyzed the variation of soil microbial community structure and carbon sequestration functional genes. From the experimental results, the main conclusions are shown as follows: (1) The soil pH, electrical conductivity, ammonium nitrogen, nitrate nitrogen, available phosphorus, and available potassium content in the biochar-added groups significantly increased (P < 0.05), and the activities of β-glucosidase (BG), cellobiohydrolase (CBH), and leucine aminopeptidase (LAP) were enhanced, whereas the activity of β–N-acetylglucosaminidase (NAG) decreased by 15.0% to 25.0%. (2) Biochar addition increased the α diversity index of soil microbial community, while the effect on bacterial α diversity index was significantly higher than that of fungi. Biochar addition increased the relative abundance of Proteobacteria and Chloroflexi (P < 0.05), while decreased the relative abundance of Actinobacteriota. In addition, it reduced the relative abundance of Ascomycota in fungi and significantly increased the relative abundance of Basidiomycota (P < 0.05). The three biochar treatments enhanced bacterial network complexity, but biochar addition did not significantly affect the fungal network complexity. (3) The soil carbon management index of rice straw biochar, wheat straw biochar, and corn straw biochar treatments increased by 4.7%, 4.8%, and 24.0%, respectively. Compared to the control group, the absolute abundance of carbon sequestration functional gene CBBL (the encoding gene of ribulose bisphosphate carboxylase large subunit) in the straw biochar treatment group significantly increased (P < 0.05). The absolute abundance of carbon sequestration functional gene PMOA (the encoding gene of particulate methane monooxygenase ß subunit) in the corn straw biochar treatment group significantly increased (P < 0.05). Biochar addition significantly improved the correlations among environmental factors, carbon sequestration functional genes, and carbon management index, with the microbial community being the main controlling factor to regulate soil carbon sequestration potential, which could provide important basis for the future ecological restoration of mines, carbon sequestration and sink enhancement.