State Key Laboratory of Pollution Control and Resources Reuse, School of the Environment, Nanjing University, Nanjing 210023, PR China; Key Laboratory of Geographic Information Science, Ministry of Education, East China Normal University, Shanghai 200241, PR China.
State Key Laboratory of Pollution Control and Resources Reuse, School of the Environment, Nanjing University, Nanjing 210023, PR China; College of Resources and Environmental Engineering, Guizhou University, Guiyang 550002, PR China.
Sci Total Environ. 2019 Dec 20;697:134143. doi: 10.1016/j.scitotenv.2019.134143. Epub 2019 Aug 27.
Paddy soil is a key area of methylmercury (MeHg) production and is dominated by fluctuating redox conditions following rice plant growth or rice-wheat rotation planting in eastern and southern Asia. The role of organic matter in the formation of MeHg under these biogeochemical redox cycles remains poorly understood, especially in certain mercury (Hg)-contaminated paddy soils. Here, we provide a detailed understanding of the formation of MeHg and its accumulation in crops (i.e., wheat and rice) in rice-wheat rotation systems under straw return. Two series of experiments, pot and microcosm experiments, were performed using Hg-contaminated paddy soil with 1% (w/w) wheat or rice straw addition under aerobic or anoxic conditions. The results showed that straw amendments increased MeHg levels in wheat (by 225%) and rice (by 20%) grains, most likely due to the elevated soil MeHg following straw amendment. Microcosm experiments further confirmed that fresh straw-derived organic matter enhances MeHg net production in soil through an overall increase in the activity of sulfate-reducing bacteria (SRB), particularly under anoxic conditions. Our study clearly demonstrated that straw amendment enhanced MeHg accumulation in wheat and rice grains and highlighted that straw return in Hg-contaminated soils may increase the health risk of MeHg exposure to local residents via crop consumption. Thus, some guidance should be provided for crop residue return in rice-wheat rotation system farming practices.
稻田是甲基汞（MeHg）生成的重要区域，在东亚和南亚地区，水稻生长或稻麦轮作种植会导致其氧化还原条件不断波动。在这些生物地球化学氧化还原循环中，有机质在 MeHg 形成过程中的作用仍知之甚少，特别是在某些汞（Hg）污染的稻田土壤中。本研究详细探讨了在稻麦轮作系统中，秸秆还田条件下，氧化还原条件波动对 MeHg 形成及其在作物（小麦和水稻）中积累的影响。通过向受 Hg 污染的稻田土壤中添加 1%（w/w）的小麦或水稻秸秆，采用盆栽和微宇宙实验，在有氧或缺氧条件下进行了两个系列的实验。结果表明，秸秆添加使小麦（增加 225%）和水稻（增加 20%）籽粒中的 MeHg 水平升高，这可能是由于秸秆添加后土壤中 MeHg 水平升高所致。微宇宙实验进一步证实，新鲜秸秆衍生的有机质通过提高硫酸盐还原菌（SRB）的活性，从而整体上增加土壤中 MeHg 的净生成，特别是在缺氧条件下。本研究清楚地表明，秸秆添加会增加小麦和水稻籽粒中 MeHg 的积累，并强调了在受 Hg 污染的土壤中进行秸秆还田可能会通过作物食用增加当地居民接触 MeHg 的健康风险。因此，在稻麦轮作系统的农业实践中，应提供一些关于作物秸秆还田的指导。
