Mercury cycling in a flooded rice paddy

Abstract

In 2008 and 2009, mercury (Hg) cycling was investigated in a flooded rice paddy in the Wanshan Hg mining region of eastern Guizhou, China, in the rice‐planted (2008 and 2009) and fallow (2009) sections of the same paddy. In the rice‐planted section, pore water was more acidic and pore water methylmercury (MeHg) concentrations were higher compared to the fallow section. However, iron (Fe) and sulfur (S) cycling differed in 2008 and 2009, with higher sediment Fe concentrations in 2009, when pore water MeHg and sulfate concentrations were more strongly correlated in the rice‐planted section. We explored whether elevated sediment Fe contributed to S cycling and hence, Hg(II)‐methylation. Critical pH values for formation of FeS(s) were estimated. Based on pore water pH collected in both sections of the paddy, the fallow section was more often a sink for FeS(s), while FeS(s) did not form in the rice‐planted section, although sulfide concentrations were low in both sections in both years (i.e.,<10 μM). We hypothesized Fe(III) oxidized sulfide, and intermediate S species (e.g., polysulfides) were further oxidized to sulfate instead of forming FeS(s), thus prolonging sulfate reduction and promoting Hg(II)‐methylation in the rice‐planted section in 2009. Results suggested Fe(III) reduction increased electron acceptors for sulfate‐reducing bacteria, which indirectly enhanced Hg(II)‐methylation. Additionally, highest sediment MeHg concentrations were observed in the fallow section after the paddy was dried and re‐wetted, indicating water‐saving rice cultivation practices (e.g., alternating wetting and drying), may cause MeHg concentrations in paddy soil to spike, which should be further investigated.