Microbial necromass carbon enhances arsenic methylation in paddy soils

Microbial necromass carbon (MNC) constitutes a critical component of soil organic carbon. Yet, how MNC regulates microbial arsenic (As) methylation processes in soil remains unclear. Across major Chinese rice-growing regions, bacterial and fungal necromass carbon showed significant positive correlations ( P < 0.05) with the transcribed arsM gene. Soil incubation experiments with seven soils explored how straw and three types of MNC—gram-positive bacterial necromass carbon (G + -NC), gram-negative bacterial necromass carbon (G − -NC), and fungal necromass carbon (F-NC)—affect As methylation. Our results demonstrated that all types of MNC enhanced As methylation, and G − -NC exhibiting the most pronounced effect on methylated As accumulation. The addition of 10 to 60 mg G − -NC maximally increased As(III) by 43.0 to 75.9% and enhanced methylated As by 4.4- to 18.0-fold in soil porewater vs. the control. Further, metagenomic and metatranscriptomic analyses demonstrated that G − -NC addition upregulated the relative abundance of transcribed arsM and arsC2 genes, which were mostly assigned to Acidobacteriota , Pseudomonadota , Planctomycetota, and Bacteroidota . Notably, the transcriptional activity of arsM -harboring Methanosarcinales and Moorellales was markedly enhanced at the order level. By promoting As reduction process, G − -NC provides more substrates for As methylation process in soil. Furthermore, G − -NC could be used as a carbon source for As-methylating microorganisms, stimulating the transcriptional activity of arsM , which has been confirmed by the incubation experiment with pure culture of Paraclostridium benzoelyticum TC8. This study highlights the critical role of MNC in regulating As biogeochemistry, establishing a basis for predicting the extent of As methylation and risk of rice straighthead disease in paddy ecosystems.