Special Microbial Communities Enhanced the Role of Aged Biochar in Reducing Cd Accumulation in Rice

Biochar exhibits a good adsorption ability for heavy metals in soil and has been widely used as a remediation material in Cd-contaminated soil. However, the status of Cd uptake by rice driven by soil physicochemical properties and rhizosphere microbial communities after years of biochar application is not well understood. In this study, the relationship between the rhizosphere microbial community and soil physicochemical properties and rice Cd accumulation were investigated during the main rice growth stages. The results showed that in comparison to the non-biochar treatment (control), a noticeable reduction in Cd content in rice stem sheaths, leaves, rice husks and milled rice with different growth stages were observed in the biochar treatment after four years, which decreased by 38.76–66.18%, 40.93–70.27%, 43.64–47.92% and 31.91–34.38%, respectively. Compared to non-biochar treatment (control), the properties of the soil in different growth stages by biochar treatment of the soil pH, soil organic matter (SOM), total nitrogen (TN) and available phosphorus (AP) were significantly increased, which increased by 10.5–16.13%, 8–25%, 75–130.13% and 132.95–191.43%, respectively. The content of available Cd (ACd) concentration in different stages by biochar treatment was significantly decreased, which decreased by 26.57–44.24%. Biochar application after four years changed the rhizosphere bacterial community structure composition (phyla level) in all stages. The relative abundance of Proteobacteria, Bacteroidetes and Nitrospirae was increased, while the relative abundance of Chloroflexi, Acidobacteria and Actinobacteria was decreased. Meanwhile, the biochar application enriched Rhodocyclaceae, Burkholderiaceae, Nitrosomonadaceae, Anaerolineaceae, Ignavibacteriales and Bacteroidales, which may contribute to the reduction of Cd uptake and accumulation in rice. These results suggest that biochar treatment after four years changed the rhizosphere microbial community structure and soil physicochemical properties and promoted the colonization of specific microbial populations in the rice rhizosphere to form a special protective system in the rice rhizosphere, which reduced Cd uptake by rice.