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

Authors: ABU-ALI, LENA - Cornell University; MAGUFFIN, SCOTT - State University Of New York (SUNY); Rohila, Jai; McClung, Anna; REID, MATTHEW - Cornell University

Submitted to: Environmental Geochemistry and Health
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/18/2023
Publication Date: 8/7/2023
Citation: Abu-Ali, L., Maguffin, S.C., Rohila, J.S., McClung, A.M., Reid, M.C. 2023. 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. Environmental Geochemistry and Health. https://doi.org/10.1007/s10653-023-01702-9.
DOI: https://doi.org/10.1007/s10653-023-01702-9

Interpretive Summary: Conventional irrigation for U.S. rice production uses season long flooded paddies. Recently, alternate wetting-drying (AWD) and furrow irrigation (also called row rice) have come into practice, which have been shown to save water resources, reduce methane emissions, and reduce potential grain arsenic (As) concentrations. We hypothesized that the aerated soil conditions created under AWD may lead to a decrease in soil pH and/or precipitation of iron oxide minerals which could affect the solubility of other trace elements that may be important to plant and human nutrition. To test our hypothesis, we conducted a field study for two consecutive years (2017 and 2018) using flood and AWD irrigation methods to determine: (i) pore water chemistry and (ii) elemental concentrations in rice grain. In general, saturated soil conditions resulted in higher solubility of all the studied trace elements (arsenic, iron, molybdenum, cadmium, manganese, copper), except zinc which was not affected by soil moisture content. The study included five varieties, two of which had been reported to be excluders of arsenic uptake and three that were accumulators of grain arsenic. The varieties that were excluders of grain arsenic were shown to have relatively low levels of total arsenic regardless of irrigation strategy. In general, varieties that were accumulators had higher levels under saturated field conditions, but under any of the AWD strategies, the total arsenic was greatly diminished, to a level found in the excluder varieties. For cadmium, which is considered a toxic hazard for humans, there was a trend for cadmium to increase under AWD conditions, but this was significant only for the varieties that were also arsenic accumulators. However, none of the varieties accumulated cadmium at levels that met the criteria for a human health concern. In contrast, for the other elements, there was no significant difference in grain content in response to flood or AWD management. Although this study was conducted using typical agricultural soils in the southern U.S., results may differ in areas of the world where there is heavy metal contamination of soil or water resources. Our results indicate that AWD management can be safely used without negatively impacting elements that are important for human nutrition. For arsenic and cadmium, which are concerns for human health, both can be effectively controlled under the AWD system. Thus, rice can be grown under a management system that does not require season long flooded fields that will save water resources, have a reduced environmental effect, and will not compromise the nutrients needed for human consumption.

Technical Abstract: Rice is a global dietary staple and its traditional cultivation under flooded 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 grain As concentration and concentrations of micronutrient (e.g., zinc, manganese, molybdenum) trace elements. We analyzed pore water chemistry and rice grain composition data from a field experiment conducted in Arkansas, 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. 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 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 growth cycle of rice plants, with dry-downs during heading stage of rice leading to larger impacts on grain composition of multiple elements. We also observed significant interannual variability in grain elemental composition from continuously flooded fields and postulate the warmer night temperature in 2018 may have played a role in these differences.