Effects of alternate wetting and drying on oxyanion-forming and cationic trace elements in rice paddy soils: impacts on arsenic, cadmium, and micronutrients in rice.

Rice is a global dietary staple and its traditional cultivation under flooded soil conditions leads to accumulation of arsenic (As) in rice grains. Alternate wetting and drying (AWD) is a widely advocated water management practice to achieve lower As concentrations in rice, water savings, and decreased methane emissions. It is not yet clear whether AWD leads to tradeoffs between concentrations of As and micronutrient elements (e.g., zinc, manganese, molybdenum) in rice grain. We analyzed pore water chemistry and rice grain composition data from a field experiment conducted in Arkansas, USA, in 2017 and 2018 to test the hypothesis that AWD will have diverging effects on oxyanion-forming (arsenic, molybdenum) vs. cationic (cadmium, zinc, manganese, copper) trace elements. This was hypothesized to occur via decreases in soil pH and/or precipitation of iron oxide minerals during oxidizing conditions under AWD. Solubility of all trace elements, except zinc, increased in more reducing conditions. Consistent with our hypothesis, AWD tended to increase grain concentrations of cationic elements while decreasing grain concentrations of oxyanionic elements. Decreases in total As in rice grains under AWD were mainly driven by changes in dimethylarsinic concentrations, with negligible changes in inorganic As. Linear mixed-effects modeling showed that effects of AWD on grain composition were more significant in 2017 compared to 2018. These differences may be related to the timing of dry-downs in the developmental stage of rice plants, with dry-downs during the heading stage of rice development leading to larger impacts on grain composition of certain elements. We also observed significant interannual variability in grain elemental composition from continuously-flooded fields and postulate the warmer temperatures in 2018 may have played a role in these differences.