秸秆还田稻田中汞活化的机理与影响因素研究

Incorporation of straw into soils plays a key role in carbon cycling in farmland ecosystem. While much attention has been paid to the impacts of straw return on greenhouse gas emission, soil properties, and microbial communities in soils, very limited information has been gained about the potential effects of straw amendment on the biogeochemistry and risk of mercury (Hg) in soils. Our recent studies revealed that straw return could possibly result in mobilization of mercury in contaminated paddy soils and enhance methylmercury (MeHg) accumulation in rice. Therefore, it is of great importance to investigate the effects of straw amendment on the biogeochemistry and risk of mercury in soil-rice systems, especially considering that straw return is being largely encouraged in China. In this study, field experiments will be carried out in Hg-contaminated paddy soils (Wanshan, Guizhou Province; Xunyang, Shaanxi Province; Nanjing, Jiangsu Province), in order to investigate mercury mobilization and MeHg accumulation in rice under straw amendment. Meanwhile, batch and pot experiments, with the help of stable isotope tracing technique and analysis of Hg speciation by XANES, will be arranged to understand the underlying mechanisms and controlling biogeochemical factors. Furthermore, methods to decrease MeHg accumulation in rice under straw amendment will be explored. Results gained in the study would improve our understanding of Hg dynamics in soil-rice systems, and provide theoretical and technical support for controlling food safety risk of Hg in straw amended paddy soils.

秸秆还田是农田生态系统碳循环的重要环节，也是全球气候变化、农业等领域的研究热点。大量研究报道了秸秆还田对温室气体排放、土壤性质、微生物群落等的影响，然而秸秆还田对汞行为与风险的潜在影响却鲜有关注。我们最新的研究初步揭示秸秆还田可活化污染土壤中的汞，促进水稻甲基汞累积。因此，在我国全面推广秸秆还田的背景下，系统研究及评估还田对稻田汞的行为与风险的影响具有重要意义。本项目拟通过三处典型汞污染稻田（贵州万山、陕西旬阳、江苏南京）田间实验，从宏观角度探索还田条件下土壤汞活化与水稻甲基汞累积的现象与规律；通过室内培养及温室盆栽实验，结合稳定同位素示踪、X射线吸收近边结构光谱等现代分析检测技术，从微观角度阐释汞活化与累积的内在机理与外在影响因素；在此基础上，探索还田条件下调控水稻甲基汞累积的措施。通过上述研究，进一步揭示稻田汞的行为特征，并为控制秸秆还田过程中汞的风险、保障食品安全提供理论与技术支撑。

土壤中有机质与金属的交互作用是一种降低陆生系统金属生物累积的重要天然机制。秸秆还田是农田土壤中有机质的重要来源，也是一项全球广泛推广以提高土壤肥力的农业活动。本项目研究基于水稻盆栽实验及全国尺度野外调研首次揭示：秸秆还田可导致土壤中甲基汞浓度上升11-1043%， 稻米中甲基汞浓度上升高达95%；尤为重要的是，秸秆还田还可导致非汞矿区稻田土壤中甲基汞浓度接近或达到汞污染矿区土壤水平。我们进一步借助微生物汞甲基化基因hgcA分析以表征土壤中甲基化微生物的变化，并运用同步辐射技术以表征土壤中汞形态的变化，从而全面揭示了秸秆还田诱发汞风险上升的内在机制：（1）秸秆还田导致的碳源供给上升可促进硫酸盐还原菌等甲基化微生物的生长，并活化土壤中惰性的硫化汞，从而导致土壤甲基汞浓度的显著上升；（2）秸秆有机质可通过提升土壤中甲基汞的活性、促进水稻根系生长，从而提高水稻根系对土壤中甲基汞的吸收能力；（3）秸秆有机质中的生物活性物质可促进水稻生长，导致更多甲基汞运输到水稻籽粒中，并最终导致籽粒甲基汞浓度的显著上升。本项目研究的理论与现实意义如下：在理论角度，秸秆有机质输入导致的汞微生物甲基化及植物生理方面的变化，应在未来探究土壤有机质-汞交互作用中予以充分考虑；在现实角度，本研究结果显示活性有机质大量输入可诱导土壤中甲基汞风险的提高，这一风险变化应在土壤修复实践中予以规避。此外，在中国等以水稻为主食的亚洲国家推广秸秆还田政策应充分考虑其可能诱发的汞风险提升。