Si-induced DMA desorption is not the driver for enhanced DMA availability after Si addition to flooded soils

Silicon (Si) addition to flooded rice paddy soil tends to decrease grain inorganic arsenic (iAs) and increase grain dimethylarsinic acid (DMA) concentrations, but the mechanism for the increase in plant-available DMA is unresolved. It has been suggested that Si displaces DMA from soil solids, rendering it plant-available; however, we hypothesize that Si desorbs primarily iAs from soil solids, which stimulates methylation to DMA. We added silicic acid to a contaminated paddy soil and a flooded upland soil that had been historically contaminated with lead arsenate in a batch incubation experiment, and measured changes in solid-phase As speciation, porewater As speciation, and As-methylating microbial (AsMM) abundance over time. We found that DMA was not detectable in soils prior to the start of the experiment nor throughout the experiment, so it comprised a trace amount of total soil As. Upon Si addition to paddy soil, total As increased in porewater following Si spike and this increase was mainly due to iAs desorption, and an order of magnitude less MMA and DMA was desorbed. The upland soil transitioned to reducing conditions throughout the experiment, but when they were achieved, iAs was desorbed first and this was followed by an increase of MMA and then DMA compared to control soils. Total microbial community abundance increased over the course of the experiments and arsM gene abundance increased from initial conditions, but did not differ between treatments. In the paddy soil, the ratio of arsM:16S gene abundance decreased from the initial conditions, but it increased in the upland soil with historic As contamination. These results suggest that Si-induced desorption of DMA is small and likely does not explain the increases of plant-available DMA upon Si fertilization in prior work. Likely, Si-induced iAs desorption drives microorganisms to methylate iAs, but degree of methylation will differ between soils.