Silicon-rich soil amendments impact microbial community composition and the composition of arsM bearing microbes

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. 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. We found that Si amendments influenced the 16S rRNA and arsM community composition. Increased C storage from calcium silicate amendment drove differences in the 16S rRNA community, whereas low soil redox potential drove differences in the arsM community. Differences in grain As were observed independent of Si-rich amendments, and did not correspond to differences in either the 16S rRNA or arsM community. Instead, methane flux and soil redox potential correlated with differences in grain DMA. Si-rich amendments drove changes in the microbial community composition and the subset of arsM-bearing organisms, but higher grain DMA levels were not directly caused by Si-rich amendments. Our findings imply that microbes active at lower soil redox potentials where As is mobilized are likely involved in DMA production, and future work should focus on linking the active community with DMA production.

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