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.