硅对污染土壤-植物体系中砷环境行为的调控机制研究

Arsenic contamination in soil has been concerned recently, especially for arsenic harmful impact on human health directly or indirectly. Most researches on arsenic biogeochemistry and the remediation of As-contaminated soils have been conducted in recent years, which provided more and more evidences to understand arsenic bio-availabilities in soils and its behaviors from soils to plants, even to edible parts. Silicon is a beneficial nutrient element for plant growth and development, especially for rice. The previous studies have proved that Si addition decreased the accumulation of As in rice straws and grains. The mechanisms of Si reducing arsenic levels in rice plants have been explored around the world. Our previous studies illustrated that Si could compete with DMA in adsorption sites of soil particles and increased the levels of DMA in soil solution, finally increased the partition of DMA in rice grains. However, the effects of Si addition on As bioavailability, mitigation and translocation in the system of contamination soil and crops when grow in arsenic contaminated soils with different As levels are not very clear, especially for plants growing in flooded soils, upland soils or soils with the rotation of flooded and non-flooded. Therefore, it is necessary to support the projects to investigate Si influences on Arsenic bioavailability in soils, and arsenic uptake, translocation and accumulation in plants cultivated in farmland contaminated by arsenic. We suppose that different Si application rates have the different effects on arsenic biogeochemistry in soil-plant system. A serial of studies will be conducted to find the mechanisms of Si how to control arsenic behaviors in the system of soil-plant (including rice, wheat) and As bioavailability in soils. Finally, the importance of Si/As ratio in soil solution of paddy soil or available Si/As in wheat soil on arsenic biogeochemistry need to be proved, and to verify the relationship between Si/As ratio in soil solution or available Si/As in wheat soil and As accumulation in edible parts of crops. Meanwhile，the suitable pathway, period and level of Si application in As-contaminated soils with different arsenic concentrations will be screened, which will provide the practical pathways to reduce As levels in edible parts of crops in As-contaminated areas.

土壤砷污染直接或间接危害着人体健康，砷的生物毒性与其形态密切相关，甲基砷毒性明显小于无机砷。前期研究表明外源硅可明显降低稻米中无机砷的累积并增加了籽粒中甲基砷的比例，降低了其人体健康风险。然而就砷污染程度不同的土壤而言，硅对污染土壤-植物体系中砷环境行为的调控机制尚缺乏系统研究。本项目拟以砷污染区轻/中/重度的污染土壤和主要粮食作物-水稻和小麦为研究对象，采用土壤-植物互作的生物学模拟方法，结合高效液相-电感耦合等离子体质谱（HPLC-ICP-MS）和同步辐射分析技术，重点研究硅的施用方式、施用时期和施用量对土壤中砷形态转化及其生物有效性的影响机制，以及对水稻和小麦吸收、转化和累积砷的调控机制；探究稻田土壤溶液中Si/As比值和麦田土壤中有效Si/有效As比值与植物吸收转运和富集砷的关系；最终构建适用于砷污染区农田的硅合理施用技术体系，为砷污染土壤修复及粮食安全提供可行的技术措施。

外源硅可明显降低稻米总砷和无机砷的累积，从而降低其人体健康风险。然而在砷污染程度不同的土壤上，硅对污染土壤-植物体系中砷环境行为的调控机制尚缺乏系统研究。本项目针对此科学问题，在砷污染程度不同的土壤上展开了系统研究。首先，揭示水稻生长介质中Si/As调控了水稻对砷的吸收和累积的作用机制：① 种子萌发：Si/As（12.5:1~125:1）硅处理液浸种可促进水稻种子萌发、幼苗生长、降低幼苗砷累积和缓解砷对幼苗的毒性；②苗期水稻：无论是As(III) 或As(V)处理，在12 h-48 h的暴露时间内Si/As为200:1显著抑制了砷的吸收、三价砷的外排和三价砷在水稻体内的转运。因为在亚细胞水平上Si/As 200:1显著降低了根胞液和细胞器组分中的砷含量；③全生育期水稻：轻、中、重度砷污染土中，施硅后土壤溶液中Si/As最大为150:1、100:1和50:1，该比例调控了水稻秸秆、颖壳和籽粒中砷含量；其次，探明了不同砷污染土壤硅的施用方式、时期和用量：自然土中基施硅肥后在拔节期追施硅肥、污染土中在拔节期和扬花期分别追施硅肥可增产并降低水稻对砷的吸收和转运；在轻度砷污染区分蘖期土施硅肥、中和重度砷污染区拔节期施硅是降低砷转运及其在水稻体内累积的最佳时期。施用量以土壤溶液Si/As>200:1为宜。第三，阐明了叶面施硅对水稻吸收和累积砷的影响效应，水稻喷施的硅经叶片吸收后可通过韧皮部下运至根系，在一定程度上降低水稻对砷的吸收和累积，以表面活性剂鼠李糖脂和吐温80配制的硅叶面调理剂降低水稻籽粒中砷累积的效果最佳。第四，干湿交替水分管理模式下接种胶质芽孢杆菌和巨大芽孢杆菌可促进硅从土壤固相释放到土壤溶液，从而降低了土壤中砷有效性。最终构建了轻中度砷污染水稻种植区冲施液体硅肥和微生物菌剂+喷施叶面硅调理剂的立体阻控砷进入水稻可食部位的技术体系。