还原性水稻土中铁-氮循环耦合体系影响镉活性的机理

Recently, there is a significant tendency towards the accumulation of cadmium (Cd) in paddy soils of China. Iron’s fluctuation between the II (ferrous) and III (ferric) oxidation states has been coined as the “FeIII-FeII redox wheel”. At present, numerous studies suggest the coupled relationships between the FeIII-FeII redox wheel and the nitrogen (N) cycle, and their respective geochemical mechanisms controlling the variations of Cd activity in reductive paddy soils. So it is likely that the coupled system of the FeIII-FeII redox wheel and the N cycle drives the variations of Cd activity in reductive paddy soils. As a result of the microbiological mechanisms controlling this coupled system remain ambiguous, few studies have considered the effects on the Cd activity with respect to it. According to the basic theories and research techniques of thermodynamics, kinetics and surface chemistry and regarding the chemical interactions of nitrate (NO3-)/ammonium (NH4+) with the crystalline/amorphous FeIII (hydr)oxides as a starting point, the present study applies the serum bottle incubation experiments, the continuous N2 bubbling experiments and the 15N stable isotope tracer method to investigate the redox processes of the different FeIII (hydr)oxides, the chemical transformations and dynamics of various N species, the chemical equilibria of Fe and N and their coupled relationships, the distribution and transformation of Cd between solid and liquid phases of paddy soils. The chemical interactions of the Fe-N coupled system are discovered firstly, and then their effects on Cd activity and its geochemical mechanisms in reductive paddy soils are elucidated. Finally, the conceptual model is suggested on variations of the Cd activity and its controlling mechanisms in reductive paddy soils. The results of this study can be a precursor mission for the in-depth study on the microbiological mechanisms controlling the coupled system of the FeIII-FeII redox wheel and the N cycle and then the variations of Cd activity in reductive paddy soils.

近年来，我国水稻土中镉呈现出较明显的累积趋势。还原性水稻土中铁循环和氮循环存在耦合关系，且各自都与镉的活性密切相关。所以，铁-氮循环耦合体系势必对镉的活性产生影响。由于目前仍不十分清楚水稻土中铁循环和氮循环耦合的微生物机理，因此，关于铁-氮循环耦合体系对镉活性影响的研究尚鲜见报道。本项目基于热力学、动力学和表面化学的基础理论和研究手段，以硝酸根/铵根与晶形/无定形铁氧化物相互作用的化学过程为切入点，将血清瓶培养实验、淹水通氮气实验和15N示踪法相结合，研究还原性水稻土中铁氧化物还原溶解演替规律、氮形态转化动力学、铁和氮形态转化热力学平衡和镉在固相和液相中的分配规律，试图揭示铁循环和氮循环耦合时的化学过程及其对镉活性影响的机理。其成果不仅可为铁循环和氮循环耦合的微生物机理研究奠定基础，而且可为铁-氮循环耦合体系对镉活性影响的微生物机理研究奠定基础，丰富和拓展水稻土中Cd的生物地球化学理论。

近年来，我国水稻土中镉出现明显的累积趋势。水稻土中铁循环和氮循环存在耦合关系，且各自都与镉的活性密切相关。所以，铁-氮循环耦合体系势必对镉的活性产生影响。关于铁-氮循环耦合体系对镉活性影响的研究目前尚鲜见报道。本项目以南方酸性水稻土为供试土壤，采用淹水厌氧培养试验和15N示踪法，综合运用热力学、动力学和微生物学的基础理论和研究手段，深入研究南方酸性水稻土中铁循环与氮循环耦合时的化学过程，及其影响镉活性的机理。结果表明：硝酸根依赖的厌氧铁氧化（NDFO）和铁还原依赖的厌氧氨氧化（Feammox）是铁循环与氮循环耦合时的主要化学过程；NDFO与Feammox在水稻土经历从中度还原强度向强度还原条件演替时有共存可能；强度还原条件下，有机氮铵化与铁还原发生耦合；中度还原条件下，NDFO生成的铁矿物对镉吸附和与镉共沉淀降低镉活性；中度还原条件下，Feammox提高pH会增大镉在土壤表面的吸附量降低镉活性，但铵根与镉离子对固相表面吸附位点的竞争，会更大程度地减少镉离子在土壤表面的吸附量而提高镉活性。本研究构建了水稻土由中度还原条件向强度还原条件转变过程中铁-氮循环耦合体系、强度还原条件下铁-氮-硫循环耦合体系、以及中度还原条件下铁-氮循环耦合体系影响镉活性机理的概念模型。本研究的成果不仅可为铁循环和氮循环耦合的微生物机理研究奠定基础，而且可为铁-氮循环耦合体系对镉活性影响的微生物机理研究奠定基础，丰富和拓展水稻土中镉的生物地球化学理论。