Effects of the co-application of biochar and trace elements on cadmium passivation and nitrogen biogeochemical processes in contaminated soil cultivated with soybean

Cadmium (Cd) contamination of soil poses food safety concerns through its bioaccumulation and by disrupting biological nitrogen (N) fixation; therefore, reducing productivity and altering microbial community structure. This study evaluated the co-application of biochar (BC) with molybdenum (Mo), iron (Fe), or sulfur (S) to regulate Cd accumulation, N fixation, nitrification, rhizosphere microbial communities, and plant physiological responses in Cd-contaminated soybean cultivation. The effects of BC co-application differed depending on the trace element supplied. Indeed, BC–Mo treatment increased rhizosphere porewater (hereafter, porewater) inorganic N concentrations, suggesting enhanced N mineralization and nitrification-related transformation, but was associated with increased plant Cd burden and stress responses. Furthermore, BC–Fe₂O₃ treatment maintained a nitrate (NO₃⁻)-dominant porewater N composition but reduced total inorganic N accumulation and did not recover nitrogenase activity. By contrast, BC–FeS₂ co-application was the only Cd-contaminated treatment that maintained nitrogenase activity at control-comparable levels. This treatment shifted porewater inorganic N composition toward the control, with a NO₃⁻/NO₂⁻ ratio of 2.39, closer to the control value of 1.82 than other BC-based treatments (3.65–7.11), and was associated with the lowest genus-level loss and retention of N-fixation- and nitrification-associated taxa. Phytohormone responses further suggest alleviated stress under BC–FeS₂ conditions. Overall, this treatment was the most effective treatment for simultaneously maintaining nitrogenase activity and porewater inorganic N balance under Cd-contaminated conditions. To conclude, BC–FeS₂ co-application may represent a potentially useful amendment approach for mitigating Cd risk while supporting N fixation and nitrification, although further validation under controlled-temperature or field conditions, including appropriate single-amendment controls, is needed.