水稻根表铁膜的微观结构表征及其固定铬的分子机制

Iron (Fe) plaque on rice root, as an important sorbent for the sequestration of heavy metals (HMs), is a critical and natural barrier to control HM transfer from soil to rice through root absorption. Chromium (Cr) is one of the major contaminant in agricultural lands and currently exceeds the soil quality standards in China. Elucidating molecular structure of Fe plaque on rice root and its impact on Cr sequestration is of great significance to manipulate the properties of Fe plaque and its binding capacity to Cr, and to guarantee the safe use of contaminated paddy soils for rice production as well as the high quality of rice products. Synchrotron-based techniques could characterize the molecular structure of Fe plaque and its associated Cr, which provides a good platform to reveal the impact of Fe plaque on Cr sequestration. In this project, multiple synchrotron-based soft and hard X-ray spectroscopic techniques (XANES/EXAFS/XRF/STXM) will be applied and focus on Cr contaminant associated with Fe plaques which forms on the different region of rice root, during various growth stages of rice or under different sulfur nutrition conditions. We will characterize the distribution and molecular structure of Fe plaques on the different regions of rice roots at the micro spatial scale, and understand the transformation mechanisms on the molecular structure of Fe plaque and its associated Cr during different growth stages of rice, and elucidate the impact of sulfur nutrition on the composition and structure of Fe plaque and its impact on Cr sequestration. This project should provide theoretical bases for guaranteeing the safe use of contaminated paddy soils for rice production as well as the high quality of rice products.

水稻根表铁膜是重金属的重要固定载体，是控制土壤重金属经根系向水稻体内迁移的重要天然屏障；重金属铬是当前我国农田土壤的主要超标污染物。阐明水稻根表铁膜微观结构及其固定铬的分子机制是调控根表铁膜性质并提升其固铬能力，保障铬污染农田安全利用以及稻米品质的关键。同步辐技术可在分子水平上表征根表铁膜的微观结构和铬的分子形态，是从本质上揭示根表铁膜固定铬的分子机制的强力手段。本项目主要利用多种同步辐软硬X射线射光谱技术（XANES/EXAFS/XRF/STXM），以铬为目标污染物，针对水稻不同根系部位、生育期和硫营养状况下形成的根表铁膜，表征铁膜在不同根区表面的微域分布和微观结构及其结合态铬的分子形态，弄清不同生育期铁膜的微观结构变化及其结合态铬的形态转化机制，阐明不同硫营养状况对铁膜的组成和结构变化及其固铬作用的影响机制，最终为保障铬污染水稻土的安全利用和农产品安全提供前瞻性理论依据。

农田土壤重金属污染是中国目前面临的重大环境问题，其中铬（Cr）是我国八种主要重金属超标元素之一。水稻是我国最重要的粮食作物，因此，有效控制重金属在土壤-水稻系统中的迁移对于重金属超标农田的安全利用和防止食物链污染具有重要意义。水稻根表铁膜是重金属的重要固定载体，是控制土壤重金属经根系向水稻体内迁移的重要天然屏障；阐明水稻根表铁膜微观结构及其固定铬的分子机制是调控根表铁膜性质并提升其固铬能力，保障铬污染农田安全利用以及稻米品质的关键。本项目通过室内模拟实验与同步辐射技术相结合，首先从分子水平上揭示了不同水稻根系部位的铁膜分布特征、铁膜中Cr含量差异、典型元素Fe/Cr分布及其Cr的分子形态，发现水稻根系成熟区铁膜较根尖部位富集，根系成熟区Fe/Cr相关性高于根尖部位，有力证实了水稻根系成熟区铁膜是Cr的重要结合载体和含量库；阐明了硫素添加下，两种水稻品种根表铁膜含量及其对Cr（III）和Cr（VI）的固定存在差异，硫素处理显著促进了水稻根表铁膜生成，并提升了水稻根表铁膜中还原态硫含量，进而很可能是驱动铁膜固定Cr的主控因子；最后，阐明了高、低硫处理下，水稻不同生育期对根表铁膜含量及固Cr能力与硫素添加量的交互作用关系，发现高硫处理下水稻在成熟后期根表铁膜中铁、硫含量和固Cr量均显著高于水稻成熟早期，而对于低硫处理，水稻在成熟后期根表铁膜中铁、硫含量和固Cr量与水稻成熟早期比则无显著变化。上述研究结果为有效调控超标稻田铬的生物有效性，保障铬污染水稻土的安全利用和稻米品质提供前瞻性理论依据。