Synergistic Utilization of KOH-Modified Biochar and Nitrogen-Fixing Bacteria for Recovering Tetracycline-Contaminated Agricultural Soil and Promoting Crop Growth

Tetracycline (TC) contamination in agricultural soils poses a serious threat to crop production and ecosystem health. However, sustainable remediation strategies capable of both efficiently degrading antibiotics and simultaneously restoring impaired nitrogen cycling functions remain lacking. This work developed a synergistic system combining KOH-modified biochar (KOB) with two TC-resistant nitrogen-fixing bacteria (TCNFB): Chryseobacterium lathyri Cbl and C. elymi Cbe. Cbl showed superior TC degradation (47.3%) and nitrogenase activity (0.08 mmol of C2H4 g–1 min–1). In pot experiments, the KOB-Cbl composite increased soybean biomass by 45.21% through rhizosphere microbiome restructuring, enhanced community stability, upregulation of nifD/H/K genes, suppressed denitrification, and elevated phytohormone levels. KOB’s high surface area and pyridinic nitrogen enriched Chryseobacterium, enabling colocalized TC degradation and nitrogen fixation. This work reveals an agricultural soil remediation strategy based on synergistic interactions between functional materials and microorganisms, providing an effective pathway for concurrently addressing antibiotic pollution and enhancing crop growth.