Alternate Wetting and Drying Decreases Methylmercury in Flooded Rice (Oryza sativa) Systems

• Published in: Soil Science Society of America journal Vol. 82; no. 1; pp. 115 - 125
• Main Authors: Tanner, K. Christy, Windham-Myers, Lisamarie, Marvin-DiPasquale, Mark, Fleck, Jacob A., Linquist, Bruce A.
• Format: Journal Article
• Language: English
• Published: The Soil Science Society of America, Inc 01.01.2018
• ISSN: 0361-5995; 1435-0661
• DOI: 10.2136/sssaj2017.05.0158

Core Ideas We studied how alternate wetting and drying (AWD) water management effects methylmercury (MeHg) dynamics in rice fields. Alternate wetting and drying reduced MeHg concentrations in soil, water, and rice grain. Iron speciation indicated that AWD oxidized the soil and regenerated electron acceptors. Rice yield did not differ between AWD and the control over 4 yr. In flooded soils, including those found in rice (Oryza sativa L.) fields, microbes convert inorganic Hg to more toxic methylmercury (MeHg). Methylmercury is accumulated in rice grain, potentially affecting health. Methylmercury in rice field surface water can bioaccumulate in wildlife. We evaluated how introducing aerobic periods into an otherwise continuously flooded rice growing season affects MeHg dynamics. Conventional continuously flooded (CF) rice field water management was compared with alternate wetting and drying, where irrigation was stopped twice during the growing season, allowing soil to dry to 35% volumetric moisture content, at which point plots were reflooded (AWD‐35). Methylmercury studies began at harvest in Year 3 and throughout Year 4 of a 4‐yr replicated field experiment. Bulk soil, water, and plant samples were analyzed for MeHg and total Hg (THg), and iron (Fe) speciation was measured in soil samples. Rice grain yield over 4 yr did not differ between treatments. Soil chemistry responded quickly to AWD‐35 dry‐downs, showing significant oxidation of Fe(II) accompanied by a significant reduction of MeHg concentration (76% reduction at harvest) compared with CF. Surface water MeHg decreased by 68 and 39% in the growing and fallow seasons, respectively, suggesting that the effects of AWD‐35 management can last through to the fallow season. The AWD‐35 treatment reduced rice grain MeHg and THg by 60 and 32%, respectively. These results suggest that the more aerobic conditions caused by AWD‐35 limited the activity of Hg(II)‐methylating microbes and may be an effective way to reduce MeHg concentrations in rice ecosystems.