水稻秸秆还田对淹水还原条件下土壤甲基汞迁移转化过程的影响研究

Rice straw disposal has been the focus of agricultural and environmental field. Rice straw incorporation can not only make full use of resources and reduce environmental pollution, but also can change soil properties and improve soil fertility. The harm of methylmercury in soil and rice system can not be ignored. The methylmercury toxicity is closely related to soil properties. Therefore, rice straw incorporation is certain to affect soil methylmercury environmental chemical behaviors. Currently, the scientists pay little attention to the relationships between rice straw incorporation and methylmercury migration, transformation and bioavailability in soils. On basis of early our research results of mercury in soil and its bioavailability, this study will carry out impacts of rice straw incorporation on migration, transformation of soil methylmercury and its mechanism. Chemical analysis and biological simulation experiments, and the mercury speciation extraction and MERX-automatic total mercury and methylmercury analyzer were used in this study. Firstly， this study will reveal the impact of rice straw incorporation on mercury speciation transformation in submerged soils under reductive conditions with analysis of soil properties, especially the transformation mechanism between inorganic mercury and methylmercury. Secondly, this study will indentify the migration, transformation process of methylmercury among soil, rice rhizosphere and rice grain and clarify its function mechanism. Thirdly, biogeochemical processes and ecological effects of mercury in rice straw incorporation soil will be studied. Finally, we will be tried to explore the rice straw incorporation pattern which can reduce bioavailability of mercury in soil. This study may provide scientific basis for the management and control of rice straw incorporation in mercury pollution farmland.

水稻秸秆处置一直是农业与环境领域关注的焦点，秸秆还田不仅能充分利用资源降低环境污染，而且能改变土壤性质提高土壤肥力。甲基汞在土壤水稻系统中的危害已不容忽视，其毒性与土壤性质密切相关。因此，水稻秸秆还田势必会对土壤甲基汞的环境化学行为产生影响，当前关于二者间的研究尚显薄弱。基于前期对土壤汞及其生物有效性的研究，开展水稻秸秆还田对淹水还原条件下土壤甲基汞迁移转化的影响及其作用机制研究。运用化学分析和生物学模拟试验，结合汞形态分级提取及自动测汞仪等手段，在剖析土壤特性的基础上，揭示水稻秸秆还田在淹水还原条件下影响土壤汞转化的作用机制，尤其是无机汞与甲基汞间的转化；明确在水稻秸秆还田中，甲基汞在土壤-水稻根际-水稻籽粒系统中的运移、转化过程及其影响机制；阐明水稻秸秆还田中土壤汞的生物地球化学过程及其生态效应，探索降低土壤汞生物有效性的水稻秸秆还田模式，为汞污染地区秸秆还田的管理与调控提供科学依据。

采用室内盆栽实验研究了水稻秸秆还田对淹水还原条件下土壤甲基汞迁移转化过程的影响。结果表明，土壤添加外源Hg明显抑制了水稻的生长，导致生物量与产量均出现下降，秸秆还田对其影响不明显。外源Hg处理条件下，明显增加了水稻各器官对Hg的吸收，秸秆还田没有降低水稻各器官对Hg的富集。土壤pH、CEC、有机质和碳酸钙均可在一定程度上抑制稻米对土壤外源Hg的吸收。添加秸秆可改变土壤好氧环境和水份情况，进而对Eh、土壤硫还原菌（dsrAB基因）、铁还原菌（Geobacteraceae的16S rRNA）等土壤性状产生影响。随着土壤无机汞添加量的增加盆栽水稻淹水条件下土壤中甲基汞的含量明显提高。在淹水还原条件下土壤硫还原菌（dsrAB基因）、铁还原菌（Geobacteraceae的16S rRNA）是土壤中无机汞向甲基汞转化的重要影响因素。随着秸秆添加量的增加，盆栽水稻收获期土壤中甲基汞含量逐渐提高。与收获期土壤中甲基汞含量变化规律相一致，随着秸秆添加量的增加，盆栽水稻稻米甲基汞含量逐渐提高，且与收获期土壤甲基汞呈极显著正相关，稻米中甲基汞主要来源于土壤。适当的秸秆还田可对土壤甲基化产生作用，进而影响水稻稻米对甲基汞富集。建议在汞污染土壤上种植水稻，应加强水份管理与控制。