Selective application of biochars to realize biochar–microbe synergistic immobilization of soil cadmium

Biochar–microbe interactions play a pivotal role in governing heavy metal (HM) behavior in soil, yet how to realize this synergy remains unclear. Herein, we aimed to elucidate how temperature-dependent biochars for selective application regulate the biochar–microbe synergy in cadmium (Cd) immobilization. A pot experiment was conducted using kitchen-waste biochar prepared at 300, 500, and 700 °C, applied either alone or in combination with Escherichia coli (E. coli). Low-temperature biochar (300 °C) exhibited limited Cd immobilization, reducing the acid-extractable Cd fraction by only 4.22% ± 0.20% compared with the control, but markedly enhanced soil fertility. In contrast, high-temperature biochar (700 °C) reduced the acid-extractable Cd fraction by 28.34% ± 0.50% and the total bioavailable Cd (F1 + F2) by 22.25% ± 0.04% relative to the water-treated control. The enhanced Cd stabilization of the 700 °C biochar was attributed to its well-developed pore structure and high alkalinity, which created favorable habitats for beneficial taxa such as Bacillus, Rhodococcus, and Mucor while suppressing competitors. Although E. coli alone had negligible effects on Cd mobility, its co-application with low-temperature biochars even increased Cd bioavailability and plant uptake, whereas combination with the 700 °C biochar strongly immobilized Cd and minimized translocation risk. Collectively, these findings suggest that the temperature-dependent physicochemical properties of biochar influence microbial communities, contributing to enhanced Cd immobilization in soil and providing a new theoretical basis for the selective application of biochar and efficient Cd immobilization in HM-contaminated soils.