High molecular weight dissolved organic matter drives soil resistome proliferation by enhancing microbial competition...

Soil organic carbon is a key determinant of microbial community structure and function, yet the role of dissolved organic matter (DOM) bioavailability in shaping the soil antibiotic resistome remains poorly understood. Here, we combined previous continental-scale field sampling across 18 provinces in China (n = 141) with additional microcosm experiments to investigate how DOM molecular weight influences soil antibiotic resistance gene (ARG) proliferation. Using Fourier transform ion cyclotron resonance mass spectrometry and metagenomic analyses, we found that soils enriched in high molecular weight (HMW) DOM harbored significantly greater ARG abundance and diversity compared to low molecular weight DOM soils. HMW DOM intensified microbial competition, as evidenced by a higher proportion of negative correlations in the co-occurrence network and lower niche breadth, favoring the enrichment of co-hosts that simultaneously carried ARGs, carbon metabolism genes, and biosynthetic gene clusters for antimicrobial compounds. Microcosm experiments confirmed that HMW DOM (lignin) addition significantly increased ARG transcript abundance (2.4-fold) and co-host relative abundance (2.3-fold), accompanied by a concurrent increase in transcribed viral auxiliary metabolic genes (2.5-fold) involved in complex carbon degradation. Structural equation modeling revealed that HMW DOM abundance and chemodiversity exerted the strongest positive effects on ARG abundance, primarily by shaping microbial community competition and metabolic potential. Collectively, our findings establish DOM bioavailability, particularly its molecular weight, as a critical yet previously overlooked driver of soil resistome development, challenging the conventional focus on total carbon content and highlighting the potential for molecular-level organic matter management to mitigate the spread of ARGs.