基于稳定同位素示踪技术研究大气沉降“新汞”在稻田生态系统中的生物地球化学过程

Methylmercury (MeHg) contamination of rice grain (Oryza sativa) have recently focused scientific attention. Rice paddy, as a typical ephemeral wetland, is known to be a significant setting for Hg methylation, which results in the accumulation of MeHg in rice grain. Numerous studies have reported high MeHg concentrations in rice grain collected from mercury (Hg) pollution areas. Rice consumption was the primary pathway of MeHg exposure for local residents in these Hg pollution areas especially in Southwest China. Previous studies confirmed that the newly deposited Hg (“new Hg” hereafter) is more readily transformed to MeHg and accumulated in rice grain than Hg forms with an extended residence time. Unfortunately, the biogeochemical processes of newly deposited Hg in rice paddy ecosystem are extremely complex and remain poorly understood. In this study, in-door pot experiments will be carefully designed based on the stimulating atmospheric wet Hg deposition jointly with the stable Hg isotope addition technique. Concentrations and distributions of spiked inorganic Hg and MeHg isotopes in rice plant tissues, soil, and overlying water will be determined during a full rice growing season to trace the transport, transform, and bio-accumulate processes of newly deposited Hg in rice paddy ecosystem. Current study will 1) elucidate the transformation process of newly deposited Hg in paddy soil and the migration mechanism of this “new Hg” within rice-soil-air surfaces; 2) supply new theory foundation for better understanding the biogeochemical process of newly deposited Hg in the typical Hg sensitive ecosystem-rice paddy ecosystem.

汞污染区稻米富含甲基是一个普遍的现象，食用稻米已成为当地居民甲基汞暴露的主要途径。汞污染区来自大气沉降的“新汞”一旦进入稻田这一典型的“汞敏感”生态系统，很容易被转化为高神经毒性的甲基汞，进而被水稻吸收、富集，对人体健康构成潜在威胁。然而，来自大气沉降的“新汞”在进入到稻田之后所发生的迁移、转化过程及其控制因素并不清楚。本研究拟开展室内盆栽实验，人为添加稳定汞同位素并模拟大气汞湿沉降，通过分析不同生长阶段水稻植株、土壤、上覆水中人为添加的汞同位素含量及形态分布特征，研究大气沉降“新汞”在稻田生态系统中的迁移、转化与生物富集过程。通过本项目的开展，将明确大气沉降“新汞”在稻田土壤中的形态转化过程，阐明“新汞”在水稻-土壤-大气界面间的迁移机制，为典型“汞敏感”生态系统（稻田生态系统）汞的生物地球化学过程认识提供重要理论依据和数据支撑。

大气沉降的“新汞”进入稻田土壤之后，很容易在微生物作用下被转化为甲基汞，进而被水稻吸收富集。但是，大气沉降的“新汞”在稻田土壤中的生物地球化学过程还不清楚。本研究通过开展盆栽实验，人为添加稳定汞同位素（200Hg2+）并模拟大气汞沉降，通过分析水稻植株和土壤中人为添加的汞同位素含量及形态分布特征，研究大气沉降“新汞”在稻田生态系统中的迁移转化过程。结果表明，人为模拟大气降雨添加至土壤中的汞同位素标记物（200Hg2+），可以在微生物作用下被转化为甲基汞，进而被水稻吸收富集。但是，人为添加至土壤中的无机汞同位素（I200Hg）同样也可以通过水稻根部吸收，并向地上部分运移。土壤汞形态分析结果表明，人为添加至土壤中的汞同位素会快速与土壤中的有机质结合，导致绝大部分汞同位素标记物（200Hg2+）以有机结合态形式存在。相反，溶解与可交换态汞整体处在较低水平，且在水稻生长期间呈持续降低的趋势。此外，还有部分的汞同位素标记物（200Hg2+）被转化为生物难以利用的残渣态汞，表现出人为添加的汞同位素在土壤中的老化过程。水稻成熟后，土壤中大部分甲基汞同位素（Me200Hg）被水稻吸收富集，仅有少部分存在于土壤中；不同的是，虽然水稻可以吸收无机汞同位素（I200Hg），但是整个体系中绝大部分无机汞同位素（I200Hg）仍然保存在土壤汞库中。通过本项目的开展，基本明确了大气沉降“新汞”在稻田土壤中迁移、转化过程，为稻田生态系统汞的生物地球化学过程认识提供重要理论依据和数据支撑。