Alternate Wetting and Drying Decreases Methylmercury in Rice Ecosystems.

In flooded soils, such as those found in rice fields, microbes convert inorganic mercury to its more toxic form, methylmercury (MeHg). MeHg can then be transported in rice drainage water to downstream ecosystems where it can negatively impact wildlife. Additionally, MeHg is accumulated in rice grain, potentially affecting human health. We conducted a replicated (n = 3) plot experiment comparing conventional continuous flood (CF) rice field water management to alternate wetting and drying (AWD). For the AWD treatment, irrigation was stopped twice during the growing season, allowing soil to dry to 35% moisture content, at which point plots were re-flooded. For both treatments, straw as incorporated into the soil and the plots were flooded during the fallow season. Soil, water and plant samples were collected from the plots throughout the year and analyzed for MeHg and total mercury (THg).  Soil THg remained constant throughout the year and did not differ significantly between treatments. Following periods when the treatments received similar management, such as the early growing season and the end of the fallow season, soil MeHg concentration did not differ significantly between treatments. However, after drying events in the AWD plots, soil MeHg concentrations in AWD plots were 76% lower than CF plots. In surface water, MeHg concentrations in AWD plots were 68% and 39% lower than CF in the growing and fallow seasons, respectively. Rice grain from AWD plots had 60% and 32% lower MeHg and THg concentration than CF, respectively. These results suggest that AWD may be an effective way to reduce MeHg levels in rice ecosystems.