硒影响污染水稻土中汞形态转化的机制研究

Methylmercury (MeHg) accumulation in rice has great implications for food safety and human health. Accordingly, there is increasing interest in investigating Hg-Se interactions in soil-rice systems, which could provide a promising way to mitigate risk of Hg. Much of the focus in the past decade has been on elucidating the inorganic mercury (IHg)-Se interactions in plants; however, the effects of Se on MeHg biogeochemistry in soils and its bioaccumulation are less understood. Our recent studies demonstrate for the first time that Se-induced reduction in soil MeHg levels (Hg-Se antagonism in soil) rather than MeHg-Se interactions within plant could be the key process triggering the decreased grain MeHg levels.However, its underlying mechanisms and influencing factors are far from clear. Here, the potential effects of Se on Hg speciation, on microbial community structure and biomass of Hg methylating bacteria, and on demethylation of MeHg and evaporation of Hg, will be explored under laboratory conditions, with the help of XANES analysis and stable isotope tracing technique. Meanwhile, factors controlling Hg-Se antagonism in soil will be examined. Based on the results of these laboratory studies, effects of Se amendment on MeHg accumulation in rice plants will be investigated in pot and field experiments, in order to develop practical measures to reduce MeHg phytoavailability. Results gained in the present study would provide theoretical basis for reducing risk of Hg in contaminated soil-rice systems.

稻米甲基汞累积威胁粮食安全和人体健康。鉴于此，稻田环境中硒对于汞风险的抑制作用日渐受到关注。尽管已有大量研究关注植物的无机汞-硒作用，目前对于土壤环境中硒如何调控甲基汞生物化学行为仍所知甚少。我们最新的研究表明施硒可以降低土壤甲基汞含量（即“土壤硒-汞作用”），并率先表明该作用可能是导致稻米甲基汞累积降低的主要原因。然而“土壤硒-汞作用”的内在机制与影响因子仍不清楚。本项目拟在实验室条件下借助汞形态的原位分析技术、同位素示踪技术等现代分析测试技术，从硒影响汞的化学形态，影响汞甲基化微生物的群落与数量及影响甲基汞的去甲基化或还原挥发等角度，揭示“土壤硒-汞作用”的内在机制，并探索其影响因子。在此基础上通过盆栽及大田试验，明确“土壤硒-汞作用”对于水稻甲基汞累积的影响，并建立科学施硒减少水稻甲基汞累积的集成技术。

稻米甲基汞累积威胁粮食安全和人体健康。硒-汞拮抗作用被认为是降低无机汞累积和毒性的天然解毒机制。我们较早提出了土壤环境中硒-汞拮抗作用可能是稻米甲基汞累积降低的主要原因。然而，作为土壤中甲基汞生物累积的重要及初始环节，硒如何影响汞的微生物甲基化过程尚不清楚。本项目通过野外采样分析、室内培养和大田试验，通过现代分析技术，系统深入地研究汞污染水稻土中硒-汞作用的关键机理和影响因子，在此基础上建立了汞污染农田修复技术。主要研究结果包括（1）借助同步辐射和电镜等技术，揭示了施加外源硒后，土壤中汞以纳米及微米级硒化汞形式存在；（2）基于化学提取-电感耦合等离子体质谱技术开发了土壤中汞纳米颗粒的定量表征方法，实现了对纳米汞的尺寸和数量浓度的定量表征，突破了复杂基体中汞纳米颗粒识别的技术难度；（3）发现土壤硒-汞作用的内在机制，包括无机汞生物活性的降低、汞甲基化微生物活性改变、甲基汞的去甲基化和挥发共同作用降低了土壤甲基汞含量。尤其是，从纳米尺度揭示了硒化汞的老化过程会导致无机汞的生物活性降低，为硒修复汞污染土壤提供了理论依据，即施加硒修复污染土壤时，应尽可能形成硒化汞的微米颗粒；（4）揭示了土壤性质尤其是土壤有机质和微生物群落机构与数量差异、土壤背景硒含量、土壤氧化还原电位、生物炭输入、硒的剂量和种类对水稻土中硒-汞作用的影响及机制，为汞污染土壤修复集成技术提供了重要思路；（5）在南京市某区域建立了19亩的大田试验示范基地，明确了施硒的最佳剂量范围，发现硒对稻米总汞和甲基汞抑制作用的长效性可达2年。该集成技术降低稻米甲基汞累积提供理论与技术支持，也为重金属高风险农田的安全利用与修复研究提供新的研究思路与方法。共发表SCI论文12篇。