稻田铵态氮转化耦合铁还原影响镉有效性的效应与机制

Cadmium (Cd) pollution in paddy soils of the red soil regions in China is serious. A lot of urea and ammonium nitrogen (N) are essential for rice growth. However, there is rare report that how N fertilizer affects availability of Cd in paddy soils, and the mechanism is still unclear. Ammonium N exhibits high activity in paddy soils and its transformation is liable to be coupled to iron reduction. Accordingly, the hypothesis for this project is that the process of transformation of ammonium N coupling iron reduction can significantly affect the speciation and availability of Cd in paddy soils. Our proposed research program will focus on (1) verifying the effect of ammonium N application on Cd accumulation in rice and exploring its influencing factors such as soil parent materials and key geochemical factors via soil cultivation experiments and with water cultivation experiments as controls; (2) verifying the effect of ammonium N on the speciation of Cd in paddy soils and investigating its key processes and microbial population via soil microcosm experiments; (3) revealing the microbial and chemical mechanisms of the coupling processes of ammonium N transformation, iron reduction and transformation of Cd speciation via laboratory culture experiments in the presence of iron oxides and microbial consortiums. The microbial consortiums are capable of iron reduction coupling ammonium N transformation and can be obtained via enrichment culture experiments. The innovation for this project is to elucidate the scientific links among transformation of ammonium N, iron reduction and Cd speciation in the perspective of the coupling of N, iron and Cd cycles. We expect that the results of this study can provide a theoretical basis for reducing health threat derived from Cd in rice via N application.

我国红壤区稻田镉污染严重。水稻生产需要施用大量的尿素或铵态氮，但氮素如何影响稻田镉有效性却鲜有报道，机制不明。铵态氮在水稻土中具有较高的活性，并与异化铁还原过程耦合。因此，本课题的科学假设是：稻田铵态氮转化耦合铁还原过程显著影响镉的形态转化与有效性。以典型母质发育的低硫稻田土壤为对象，采用水稻盆栽试验，探明铵态氮与尿素影响水稻积累镉的效应、母质响应与关键地球化学因子，并以水培试验作为其生理效应的对照；采用土壤微宇宙实验，明确铵态氮与尿素影响土壤镉形态转化的效应，探讨可能的关键过程与关键微生物类群；通过多次驯化富集培养获得铵态氮转化耦合铁还原过程的驯化菌液，以驯化菌液+氧化铁培养体系，揭示铵态氮转化—铁还原—镉形态转化的化学与微生物机制。其主要创新在于：从氮铁镉元素循环耦合的视角，深入阐明稻田铵态氮转化、铁还原、镉形态转化的科学联系。可为红壤稻田合理施氮降低稻米镉风险提供理论依据。

农田土壤重金属污染治理契合“土壤污染防治攻坚战”的国家重大战略需求。稻田镉污染尤为突出，水稻是镉超标最为严重的粮食作物。水稻生产离不开大量氮肥的投入，氮素如何影响镉的移动性和有效性有待深入探讨。我国镉污染稻田主要集中在南方红壤区，铁是红壤中最为重要的氧化还原活性元素。土壤中二价铁和三价铁交替的氧化还原反应构成铁循环过程，铁循环可驱动碳氮等养分循环与污染物的迁移转化，在土壤物质循环过程中具有独特的重要性。. 本项目以铵态氮转化与铁还原过程的耦合关系为切入点，基于水稻盆栽和土壤微宇宙实验系统研究了尿素或铵态氮肥影响镉移动性和有效性的主要机理。研究发现施用铵态氮可小幅降低体系pH且显著提高体系铁还原菌丰度促进铁还原过程释放镉，从而提高了镉的移动性和有效性。研究过程中还发现外源施加铵态氮不仅可提高镉的移动性而且会显著提高土壤中砷的移动性及稻米中砷的积累。在实际的环境介质中，重金属多以两种或两种以上的复合污染形式呈现，特别是镉砷复合污染。而稻田干湿交替条件下镉砷的环境行为相反，如何同步钝化镉砷是一大挑战。针对这一难点，项目组从铁-碳循环及铁-氮循环与镉/砷环境行为的耦合关系入手，系统研究了木本泥炭+硝酸铁以及零价铁+生物炭驱动稻田镉砷同步钝化的效应与机制。研究发现外源施用木本泥炭和硝酸铁显著提高土壤pH、降低孔隙水中镉/砷浓度，且促进土壤有效态镉/砷向氧化铁结合态转化及水稻根际铁膜对镉/砷的吸附固定，从而显著降低了镉/砷移动性及水稻籽粒中镉砷的积累。零价铁+生物炭共施对降低稻米中镉及砷积累亦具有协同效应，且铁物种在其中扮演着重要角色。. 基于以上原理，项目组研发了铁改性腐殖质功能材料，申请并授权国家发明专利1项并获得进一步转让和应用。研究成果从土壤多过程耦合角度，阐明了稻田铁-碳与铁-氮循环耦合镉砷环境行为的机制，为农田土壤重金属污染控制技术的研发提供了重要理论依据。