Community dynamics of As(V)-reducing and As(III)-oxidizing genes during a wet-dry cycle in paddy soil amended with organic matter, gypsum, or iron oxide

Microbe-mediated redox transformations regulate arsenic mobility in paddy soil. However, the community dynamics of the related genes, which might be affected by soil ameliorants, have not been systematically investigated during a wet-dry cycle. This study incubated arsenic-contaminated paddy soil amended with organic matter (OM), gypsum, or hematite in microcosms under alternate watering conditions. Added gypsum and hematite reduced arsenic mobility in the soil by 8-60% during the wet and dry periods. However, added OM increased arsenic mobility by 70-130% during the first 4 weeks (not the last 4 weeks) of submergence and the dry period. The results of quantitative real-time polymerase chain reaction (qPCR) depended heavily on the primers used, so the contribution of relevant genes to arsenic transformation cannot be compared using only the gene abundance assessed by qPCR. However, correlation analyses showed that the abundance and community members of the arrA gene, which mediates dissimilatory As(V) reduction [i.e., As(V) respiration], were related to soil arsenic concentrations. This was not the case for the arsC gene, which mediates cytoplasmic As(V) reduction, or the aioA gene, which mediates As(III) oxidation. These suggest that the dissimilatory pathway was mainly responsible for arsenic reduction and release in the soil studied.