title: Selenium inhibits sulfate-mediated methylmercury production in rice paddy soil.
abstract: There is increasing interest in understanding factors controlling methylmercury (MeHg) production in mercury-contaminated rice paddy soil. Sulfate has been reported to affect MeHg biogeochemistry under anoxic conditions, and recent studies revealed that selenium (Se) could evidently reduce MeHg production in paddy soil. However, the controls of sulfate and Se on net MeHg production in paddy soil under fluctuating redox conditions remain largely unknown. Microcosm experiments were conducted to explore the effects of sulfate and Se on net MeHg production in rice paddy soil. Soil was added with 0-960 mg/kg sulfate, in the presence or absence of 3.0 mg/kg selenium (selenite or selenate), and incubated under anoxic (40 days) or suboxic conditions (5 days), simulating fluctuating redox conditions in rice paddy field. Sulfate addition moderately affected soil MeHg concentrations under anoxic conditions, while reoxidation resulted in evidently higher (18-40%) MeHg levels in sulfate amended soils than the control. The observed changes in net MeHg production were related to dynamics of sulfate and iron. However, Se could inhibit sulfate-mediated MeHg production in the soils: Se addition largely reduced net MeHg production in the soils (23-86%, compared to the control), despite of sulfate addition. Similarly, results of the pot experiments (i.e., rice cultivation in amended soils) indicated that soil MeHg levels were rather comparable in Se-amended soils during rice growth period, irrespective of added sulfate doses. The more important role of Se than sulfate in controlling MeHg production was explained by the formation of HgSe nanoparticles irrespective of the presence of sulfate, confirmed by TEM-EDX and XANES analysis. Our findings regarding the effects of sulfate and Se on net MeHg production in rice paddy soil together with the mechanistic explanation of the processes advance our understanding of MeHg dynamics and risk in soil-rice systems.
authors.affiliation: State Key Laboratory of Pollution Control and Resources Reuse, School of the Environment, Nanjing University, Nanjing, 210023, PR China.
authors.forename: Yong-Jie
authors.initials: YJ
authors.lastname: Wang
authors.affiliation: Key Laboratory of Soil Environment and Pollution Remediation, Institute of Soil Science, Chinese Academy of Sciences, Nanjing, 210008, PR China.
authors.forename: Fei
authors.initials: F
authors.lastname: Dang
authors.affiliation: Key Lab for Biomedical Effects of Nanomaterial and Nanosafety, Laboratory of Metallomics and Metalloproteomics, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing, 100049, PR China.
authors.forename: Jia-Ting
authors.initials: JT
authors.lastname: Zhao
authors.affiliation: State Key Laboratory of Pollution Control and Resources Reuse, School of the Environment, Nanjing University, Nanjing, 210023, PR China; Environmental and Life Sciences Program (EnLS), Trent University, Peterborough, Ontario, Canada. Electronic address: zhonghuan@nju.edu.cn.
authors.forename: Huan
authors.initials: H
authors.lastname: Zhong
mesh_terms: D004787:Environmental Pollution / Q000032:analysis; D007501:Iron / Q000737:chemistry; D008628:Mercury / Q000032:analysis; D008767:Methylmercury Compounds / Q000737:chemistry*; D012275:Oryza; D010084:Oxidation-Reduction; D010100:Oxygen; D012643:Selenium / Q000737:chemistry*; D012987:Soil / Q000737:chemistry*; D012989:Soil Pollutants / Q000737:chemistry* / Q000378:metabolism; D013431:Sulfates / Q000032:analysis
chemical_list: D008767:Methylmercury Compounds; D012987:Soil; D012989:Soil Pollutants; D013431:Sulfates; D007501:Iron; D008628:Mercury; D012643:Selenium; D010100:Oxygen
keywords: Methylmercury; Rice paddy soil; Selenium; Sulfate