Biochar-mediated enhancement of nitrogen retention and antibacterial activity during Chinese medical herb waste composting: production of a pathogen-suppressive soil amendment

Abstract

The disposal of Chinese medical herb waste (CMHW) and continued reliance on chemical agricultural inputs are important environmental concerns. This study evaluated whether biochar amendment could improve nitrogen retention and antibacterial activity during CMHW composting and assessed the resulting compost in soil challenged with a phytopathogen. Compared with the biochar-free control, 9–15% biochar reduced total nitrogen loss to 11.1–17.9% and increased antibacterial activity against plant pathogens by 44.6–51.7%. Microbial co-occurrence analysis showed that potentially beneficial taxa, including Pseudomonas and Monascus, were associated with ammonia monooxygenase activity and antibacterial properties. Structural equation modeling further indicated a positive association between antibacterial activity and NO₃⁻-N (P < 0.05) and identified a strong direct effect of bacterial α-diversity on antibacterial activity (standardized path coefficient = 0.94, P < 0.05). In the subsequent pot experiment, the medium-dose CLD2 compost provided multifunctional benefits relative to the separately tested urea and chemical disease-control reference treatments; Chinese cabbage flavonoid content reached 0.26 μg/cm², and soil nitrogen-fixation-related enzyme activity reached 270.9 U/g. Tax4Fun2 predictions also indicated enrichment of functions related to siderophore production, nitrogen fixation, and indole-3-acetic acid metabolism after the application of biochar-amended CMHW compost. Overall, biochar-amended CMHW compost simultaneously improved nitrogen retention, nutrient cycling, and pathogen suppression. Future studies should include a combined urea-plus-fungicide treatment to determine how this approach compares with conventional integrated chemical management.