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Abstract Purpose: Arsenic (As) cycling in flooded rice paddies is driven by soil microbes which among other transformations can cause conversion between inorganic and organic As species. Silicon (Si)-rich soil amendments cause increased methylated As species, particularly DMA, in grain likely because they influence the microbial community responsible for As methylation, but the mechanism remains unclear. Methods: To investigate how Si-rich amendments influenced the microbial community, we sequenced the 16S rRNA and arsM genes from rhizosphere soil collected at grain ripening from unamended rice paddy mesocosms or those amended with Si-rich rice husk, charred husk, or calcium silicate, and paired these data with geochemistry and As speciation in grain. Results: We found that Si-amendments influenced the 16S and arsM community composition. Increased C storage from calcium silicate amendment drove differences in the 16S community, whereas low redox from husk amendments drove differences in the arsM community. High grain DMA was not associated with treatment or microbial community, but with low redox. Conclusions: Differences in grain As were observed independent of amendments, and did not correspond to differences in either the 16S or arsM community. Instead, methane flux and redox correlated with differences in grain DMA, implying that methanogen activity and redox are more important factors than community composition in determining grain As speciation. Silicon amendments did not impact grain As, but impacted the microbial community composition, and the subset of arsM -bearing organisms. These findings imply that redox, porewater As, and methanogen activity are likely more important factors than arsM or overall microbial community composition in determining grain DMA levels.