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 (<i>P</i> < 0.05) with the transcribed <i>arsM</i> 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 <i>arsM</i> and <i>arsC2</i> genes, which were mostly assigned to <i>Acidobacteriota</i>, <i>Pseudomonadota</i>, <i>Planctomycetota,</i> and <i>Bacteroidota</i>. Notably, the transcriptional activity of <i>arsM</i>-harboring <i>Methanosarcinales</i> and <i>Moorellales</i> 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 <i>arsM</i>, which has been confirmed by the incubation experiment with pure culture of <i>Paraclostridium benzoelyticum</i> 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.