Rice husk biochar reduces Cd availability by affecting microbial community activity and structure in Cd-contaminated soils

PurposeBiochar was used widely to deal with cadmium (Cd) pollution in soils. An indoor incubation experiment to investigate the effects of biochar on the structure and Cd-related functions of soil microbial communities under different concentration of Cd stress.Materials and methodsA 180-day soil incubation experiment was performed by applying rice husk biochar (BC). At different time periods, Cd forms, chemical properties, and enzyme activities in soils were determined, and the effects of BC on the structure and function of microbial communities in soils with various Cd concentrations were explored by high-throughput sequencing.Results and discussionCompared to CK, BC significantly facilitated the conversion of bioavailable Cd to residual Cd, especially in medium Cd-contaminated soil where the residual Cd content increased by 76.35%. Biochar also significantly enhanced pH, soil organic matter, cation exchange, available phosphorus, rapidly available potassium, and catalase activities, except for ammonium nitrogen content and sucrase activities. PCoA and PERMANOVA revealed that incubation time and pollution levels significantly affected bacterial community structure, whereas pollution level was the only variable to significantly influence fungal community structure. According to co-occurrence network analysis, BC addition increased the microbial association in light and heavy Cd-contaminated soils but inhibited in medium Cd-contaminated soil. Moreover, the Cd-related functions of the microbial community predicted by PICRUSt2 suggested that BC significantly affected Cd transport-related functions in microbial communities, with higher Cd-contaminated soils showing more prominent expression of functions related to Cd transport. Pseudomonadota may have played an important role in the efflux of Cd, while Chloroflexota, Bacillota, and Actinomycetota participated in microbial detoxification and Acidobacteriaota, Armatimonadota, and Planctomycetota for Cd tolerance.ConclusionsBC not only effectively immobilized Cd in light and medium Cd-contaminated soil but also improved three soil properties. BC had an insignificant effect on the microbial community structure in three soils but significantly affected the expression of Cd transport-related functions in the microbial communities of three soils.