Influence of Carbon-Input Level on Cadmium Immobilisation by Wheat Straw-Derived Biochar in Contaminated Soil

This study evaluated how wheat-straw input modifies the effects of 2% (w/w) wheat straw-derived biochar on Cd availability, soil physicochemical properties, enzyme activities, N transformation and microbial communities during a 49-day incubation. Combining 2% biochar with 6% straw reduced soil-available Cd by 7.14–14.28% relative to the biochar-only treatment, whereas increasing straw input from 6% to 9% did not further improve Cd immobilisation. Compared with straw-only treatments, biochar-amended soils maintained higher pH and lower available Cd, indicating that biochar buffered straw-induced acidification and Cd mobilisation. High carbon input increased β-glucosidase activity and AOB-amoA abundance, which was consistent with enhanced nitrification-related processes and may indirectly influence Cd mobility through soil-pH dynamics. These effects were partly buffered by the alkalinity of biochar. Biochar alone supported the highest microbial alpha diversity, whereas increasing carbon input reduced community richness. The BS6 treatment also maintained relatively high microbial biomass during incubation. Soil pH, mineral N and enzyme activities were associated with a variation in microbial community structure. Overall, moderate wheat-straw input combined with biochar improved Cd immobilisation while maintaining microbial activity and soil biochemical stability, suggesting potential for the remediation of Cd-contaminated agricultural soils.