Differential regulation of soil antibiotic resistance genes by biochar types and their derived dissolved organic matter

Agricultural soil represent a critical hotspot for the emergence and dissemination of antibiotic resistance genes (ARG), posing significant threats to food safety, public health, and agricultural sustainability. Biochar, a carbon-rich material derived from biomass pyrolysis, has emerged as a promising soil amendment capable of modulating the fate and transport of ARG in terrestrial ecosystems. Nevertheless, the underlying mechanisms by which biochar and its biochar-derived dissolved organic matter (BDOM) influence ARG dynamics remain poorly understood. It is hypothesized that biochar and BDOM regulate ARG dissemination through their effects on soil microbial community composition, functional potential, and metabolic activity. To test this hypothesis, we conducted a controlled microcosm experiment in which corn stover biochar (CBC), reed straw biochar (RBC), corn stover BDOM (CBDOM), and reed straw BDOM (RBDOM) were applied to agricultural soil amended with organic fertilizer. Our results revealed contrasting effects of the two biochars: CBC increased the relative abundance of ARGs by up to 2.48-fold compared to organic fertilizer control, whereas RBC consistently suppressed ARG levels by up to 91 ​%. In contrast, BDOM exhibited a comparatively weaker influence on ARG abundance than its solid-phase biochar counterpart. Partial least-squares path modeling identified mobile genetic elements as the primary drivers of ARG dissemination across all treatments. Notably, the CBC application was associated with a marked enrichment of IntI1, a clinical class 1 integron-integrase gene. Conversely, both RBC and BDOM suppressed polyunsaturated fatty acid metabolism and ATP synthesis, potentially reducing microbial antibiotic resistance. Furthermore, CBC promoted the potential ARG hosts involved in xenobiotic degradation, while RBC enhanced the potential hosts associated with the nitrogen cycle. Collectively, these findings elucidate the complex and feedstock-dependent roles of biochar in shaping ARG dynamics in agricultural soils, offering a strategic, cost-effective, and environmentally sustainable approach to mitigate ARG pollution in agroecosystems.