AbstractAlthough methylation is regarded as one of the main detoxification pathways for arsenic (As), current knowledge about this process during manure composting remains limited. In this study, two pilot-scale compost piles were established to treat manure contaminated with As. An overall accumulation of methylated As occurred during 60 day-composting time. The concentration of monomethylarsonic acid (MMA) increased from 6 to 190 μg kg⁻¹ within 15 days and decreased to 35 μg kg⁻¹ at the end of the maturing phase; while the concentration of dimethylarsinic acid (DMA) continuously increased from 33 to 595 μg kg⁻¹ over the composting time. The arsM gene copies increased gradually from 0.08 × 10⁹ to 6.82 × 10⁹ copies g⁻¹ dry mass over time and correlated positively to the concentrations of methylated As. 16S rRNA gene sequencing and arsM clone library analysis confirmed the high abundance and diversity of arsM genes. Many of these genes were related to those from known As-methylating microbes, including Streptomyces sp., Amycolatopsis mediterranei and Sphaerobacter thermophiles. These results demonstrated that As methylation during manure composting is significant and, for the first time, established a linkage between As biomethylation and the abundance and diversity of the arsM functional genes in composting manure.