Biochar enhanced phytoremediation efficiency of Salix for soil cadmium: the differentiated responses of bacteria and fungi to biochar and rhizosphere effects

Abstract Biochar is suggested to enhance the phytoremediation of cadmium (Cd) via regulating the rhizosphere environment and plant traits in contaminated soil. However, the effect of phosphorus (P)-modified biochar, the rhizosphere effect, and their interaction in improving phytoremediation efficiency of Salix for Cd remains unclear. Here, the effects of bamboo biochar, phytic acid-modified biochar, and sodium phytate-modified biochar on soil properties, the microbial community, plant traits, and Cd accumulation of Salix J1010 in Cd contaminated soil were comparatively and systematically studied. P-modified biochar significantly increased plant growth, Cd accumulation, and its translocation from roots to the aboveground parts of Salix . Cd concentration, root biomass, net photosynthetic rate, and rhizosphere microbial community variations were identified as critical predictors for phytoremediation efficiency using random forest models. Rhizosphere bacteria were more influenced by biochar amendment, while the fungi were more influenced by the rhizosphere effects. A key bacterial cluster, with a preference for high soil carbon and P, was further found to stimulate root growth and improve the bioavailability of soil Cd. Collectively, the study revealed differentiated responses of bacteria and fungi to biochar and rhizosphere effects of Salix , highlighting the importance of biochar modifications to optimize microbial interactions and enhance the phytoremediation efficiency of Salix in Cd-contaminated soils. Graphical Abstract