石灰岩地区水稻土铁锰化学过程对稻米镉累积的影响机制

The chemical processes of ferro-manganese and mobilization and transformation of pollutants have become key issues in the areas of soil sciences and environments. Researches have studied the chemical process of Fe-Mn and the Cadmium morphological transformation elsewhere.The soil in limestone areas in Guangxi provience was severely contaminated by heavy metals, however, less attention is paid to the relationship between the chemical process of ferro-manganese and cadmium accumulation in rice.In this proposal,the rice and the paddy soils which were developed from different were studied. The relation of the forms of Fe-Mn , Cd and the Cd concentration in different parts of rice plant will be studied. Pot experiments will be conducted to investigate the relation between different soil parents materials and different Fe-Mn forms in different rice stages and the Cd form transformation; the relation between Fe-Mn forms and formation of iron plaque on rice root surface and the Cd concentration in different parts of rice. The aim was to disclose the impacts of Fe-Mn chemical processes on cadmium accumulation in rice from iron plaque on the root surface. The study will provide theoretical bases for cadmium reduction in rice in limestone areas.

铁锰化学过程与污染物的迁移转化是土壤环境化学的热点与难点科学问题。铁锰化学过程与土壤镉形态转化的关系已有相关报道，广西石灰岩地区由于矿区开采导致土壤重金属污染严重，但石灰岩地区水稻土铁锰化学过程对稻米镉累积的影响及其机制未见报道。本项目以石灰岩地区不同类型土壤发育的水稻土和水稻为研究对象，研究铁锰形态、土壤镉形态与水稻不同部位镉含量的关系，阐明铁锰形态与土壤镉形态转化及稻米镉累积的相关性。在此基础上，通过盆栽试验，研究不同类型土壤发育的水稻土不同生育期铁锰形态与土壤镉形态转化、水稻不同部位镉含量及稻米镉累积的关系，从土壤镉形态转化角度揭示铁锰化学行为对稻米镉累积的影响机制；研究土壤铁锰形态变化与水稻根表铁膜形成及稻米镉累积的关系，从根表铁膜角度揭示铁锰化学过程对稻米镉累积的影响机制。为削减石灰岩地区稻米镉累积提供理论参考依据。

以石灰岩地区第四纪红土、石灰岩、结合区发育水稻土研究对象，在水稻成熟期采集有代表性的土壤样品共60个及相应水稻植株样品，分析土壤基本性质、铁锰氧化物、铁锰结核与镉形态及稻米镉累积的相关性。在此基础上，设置盆栽试验，分析在水稻栽培过程中铁锰形态变化、土壤胶体对镉形态变化和稻米镉累积的影响。主要研究结果如下：.1）不同母质发育土壤镉形态和稻米对镉富集能力存在差异，石灰岩、结合区、第四纪红土水稻土残渣态镉/总镉百分比分别为45.09%、49.40%和39.28%，第四纪红土酸溶态镉含量最高。第四纪红土、石灰岩、结合区稻米镉富集系数分别为0.36、0.16和0.21，差异显著。相关分析表明，不同母质发育区残渣态镉与水稻镉富集呈显著或极显著负相关，结合区可还原态镉与水稻镉富集呈显著负相关。.2）对土壤性质与镉形态的相关分析结果表明，第四纪红土区镉形态主要受pH、CEC、锰形态、总铁、无定形铁影响；石灰岩区镉形态主要受交换性钙、总锰、络合态铁影响；pH、络合态铁、络合态锰显著影响结合区镉形态。.3）铁锰结核对镉的富集大小主要由铁锰结核中锰含量来决定，相关系数为0.717，达到极显著水平。铁锰结核对镉的富集系数最高达32.48，平均为6.38，说明铁锰结核减少了镉向水稻迁移。.4）水稻对镉的富集受到根表铁锰膜组分的影响，第四纪红土和结合区根表铁锰膜锰含量与水稻壳对镉的富集呈极显著负相关；石灰岩区根表铁锰膜铁与茎叶镉富集能力存在显著负相关关系。.5）逐步回归分析结果表明，第四纪红土区稻米镉的回归方程为：米镉富集系数=0.512-0.637残渣态镉；石灰岩区为：米镉富集系数=-1.335 -0.043交换性钙；结合区为：米镉富集系数=0.853 -0.099pH。影响稻米镉吸收的土壤因子主要是pH和交换性钙。.6)土壤胶体对镉富集随着水稻生长期的延长而增加，胶体锰高增加镉的富集。第四纪红土发育土壤游离度和结晶度比较高，活化度和络合度低于石灰岩和结合区，导致土壤中可还原态/总镉高；石灰岩发育土壤酸溶态/总镉显著高于第四纪红土和结合区，主要是铁氧化物还原释放镉和碳酸盐结合态镉高造成。.7）在水稻生长前期，第四纪红土对镉富集能力强，成熟期显著下降；石灰岩水稻在生长前期对镉富集能力低，成熟期显著上升，富集差异的变化主要受到Eh、酸溶态/总镉、铁活化度以及根表铁锰膜的影响。