Microbial inoculants modify the functions of soil microbes to optimize plant growth at abandoned mine sites

Mining activities have caused significant land degradation globally, emphasizing the need for effective restoration. Microbial inoculants offer a promising solution for sustainable remediation by enhancing soil nutrients, enzyme activities, and microbial communities to support plant growth. However, the mechanisms by which inoculants influence soil microbes and their relationship with plant growth require further investigation. Metagenomic sequencing was employed for this study, based on a one-year greenhouse experiment, to elucidate the effects of Bacillus thuringiensis NL-11 on the microbial functions of abandoned mine soils. Our findings revealed that the application of microbial inoculants significantly enhanced the soil total carbon (TC), total sulfur (TS), organic carbon (SOC), available phosphorus (AP), ammonium (NH₄⁺), urease, arylsulfatase, phosphatase, β-1,4-glucosidase (BG), β-1,4-N-acetylglucosaminidase (NAG). Moreover, this led to substantial improvements in plant height, as well as aboveground and belowground biomass. Microbial inoculants impacted functional gene structures without altering diversity. The normalized abundance of genes related to the degradation of carbon and nitrogen, methane metabolism, and nitrogen fixation were observed to increase, as well as the functional genes related to phosphorus cycling. Significant correlations were found between nutrient cycling gene abundance and plant biomass. Partial Least Squares Path Model analysis showed that microbial inoculants not only directly influenced plant biomass but also indirectly affected the plant biomass through C cycle modifications. This study highlights the role of microbial inoculants in promoting plant growth and soil restoration by improving soil properties and enhancing normalized abundance of nutrient cycling gene, making them essential for the recovery of abandoned mine sites.