Mechanisms of biochar derived from chicken manure or rice straw that effectively immobilizes cadmium do not apply to sewage-sludge biochar.

In order to explore immobilization characteristic and its possible mechanism of various feedstocks-based biochars, deriving from chicken manure (CMB), rice straw (RSB) and sewage sludge (SSB), the effect of three biochars on speciation transformation of Cd in soil was compared, and soil properties, microbial community diversity were also examined. The treatment of CMB and RSB reduced significantly the content of acid-extractable Cd by 53.62% and 41.18%, respectively, but this available fraction was increased by 23.44% after SSB treatment. Simultaneously, CMB and RSB treatment significantly increased the soil pH, CEC, TOC, AN, AP, TK, microbial biomass carbon, dehydrogenase, phosphatase, catalase and urease enzyme activity, while SSB treatment slightly increased AP and TK content, only decreased urease enzyme activity. Lower bacterial alpha diversity was observed at the initial sampling of day 0, whereas treatment differences in bacterial Shannon diversity were not significant at day 225, it suggested that their effects on microbial alpha diversity were dependent on sampling stage and microbial kingdom. Partial least squares modelling (PLS-PM) speculated that CMB and RSB treatment decreased available Cd through direct effect of biochar and indirect effect of soil, whereas SSB treatment elevated available metal by indirect effect. This increase might be due to the decrease of urease activity indirectly affected by SSB, thus decelerating Cd precipitation and enhancing available Cd. These results deepen our understanding on the degree of immobilization and underlying mechanisms for Cd in soil that is critical for evaluating the application of effective biochar treatments.