Microbial Activity Spurred By Silicon Amendment Addition Alters the Biogeochemical Cycling of Arsenic, Iron, and Methane in Flooded Rice Paddies.

Rice is a staple food for approximately half the world. Arsenic (As) accumulation in rice due to arsenite uptake via the rice silicon transporter is an issue in areas where rice is grown in flooded paddies, and especially where irrigation water is arsenic contaminated. One potential arsenic uptake mitigation strategy is use of plant-based silicon (Si) amendments such as rice husk, rice husk ash, and calcium silicate; all increase the amount of plant-available Si in the soil, have been shown to decrease total rice As uptake, and are economically feasible and available for small rice farmers. A side effect of plant-based silicon amendment use is increased soil organic matter, a factor that spurs microbial activity including arsenic methylating microbes, iron oxidizing bacteria, methanogens, and methanotrophs. Arsenic methylating microbes contribute to arsenic cycling by converting inorganic arsenic to organic, methylated forms such as monomethylarsonic acid (MMA) and dimethylarsini acid (DMA), which can account for up to 90% of grain arsenic. Iron oxidizing bacteria at least partially contribute to the formation of an arsenate adsorbing iron plaque (an amalgamation of iron oxy-hydroxides) on the rice root surface. Methanogens and methanotrophs are of interest since methane emissions from rice paddies compose up to 16% of total anthropogenic methane emissions. In this study, levels of microbial abundance and activity (as measured by DNA and RNA levels of specific marker genes) were monitored in response to Si amendment addition in flooded rice paddies over a complete rice growing season. Abundance and activity were also monitored in non-flooded and intermittently flooded paddies. The diversity and abundance of microbes from the bulk soil to the rhizosphere and their influence on As, Fe, and C-cycling will be discussed.