长期定位施用有机肥土壤中氧化铁的形态转化特征

Ferric (hydr)oxide, as the most important active component in soil, can form the organic-mineral complex with organic matter. The complex seems as the cell of soil could regulate the environmental behavior of contaminates and nutrient elements in soil. Long-term organic fertilization could improve the physical, chemical and biological characteristics of soil markedly, which would affect the redox process of ferric (hydr)oxide, and simultaneously influence the organic matter forms and the soil aggregate stability. However, there is limited information regarding the operation mechanism of organic matter on ferric (hydr)oxide transformation in farmland with long-term organic fertilization. Red soil in south China was selected for investigation. In this project, we combined the characterization of soil and laboraorial stimulation studies. Eletrochemistry method, modern analytic thechniques and density function theory (DFT) will be applied to 1) definite the chemical form, mineral type and structure of ferric (hydr)oxide and explore the relationship between the transformation of ferric (hydr)oxide and the composition of organic matter; 2) reveal the mechanism by the synthesis analysis of microstructure and surface chemistry properties of ferric (hydr)oxide conducted by typical organic matters and its reponse to soil environmental conditions; 3) clarify the effect of ferric (hydr)oxide transformation on organic carbon composition and soil aggregate stabilization. The results could promote our understanding of ferric (hydr)oxide evolution in the context of human activities, and to provide important scientific basis for rational soil fertilization and soil quality and productivity improvement.

氧化铁是土壤中重要的活性组分，与有机质形成有机无机复合体—构成土壤的“细胞”，调控着土壤中污染物和营养元素的环境行为。长期施用有机肥可显著改善土壤物理、化学、生物学特性，影响土壤氧化铁的氧化还原过程，同时改变土壤有机质的形态和团聚体的稳定性。目前，长期施用有机肥土壤中有机质对氧化铁转化过程的作用机制并不清楚。本项目以长期定位施用有机肥红壤为研究对象，以土壤氧化铁形态转化为核心，利用电化学和穆斯堡尔谱等现代分析技术，结合密度泛函理论计算，研究土壤氧化铁的形态、类型与结构特征，探讨氧化铁的形态转化与有机质组分的关系，揭示有机质催化形成氧化铁的微观机制及其与环境条件的互馈关系，阐明氧化铁形态转化过程中土壤有机质组分、团聚体稳定性的变化。研究成果将有助于理解高强度人为下土壤氧化铁的演化过程，为合理施肥、提高土壤质量与生产力提供科学依据。

本项目以区域土壤（黑土、红壤、砖红壤）为研究对象，以土壤氧化铁形态转化为核心，利用化学溶提、土壤磁学、漫反射光谱、穆斯堡尔谱及高效液相-排阻色谱等分析技术，研究了不同利用方式与耕作施肥土壤中氧化铁形态转化的特征与作用机制及其对团聚体稳定性的影响。取得得主要结果有：1）长期施用有机肥（NPK+OM、OM）红壤中高有机质含量及弱酸性或近中性环境抑制了针铁矿向赤铁矿的转化；耕作与施肥（无机化肥、有机肥）通过影响土壤中OC/Fed的摩尔比值调控土壤氧化铁的活化与晶质化过程。2）分析了区域土壤各形态有机质（溶解性有机质/DOM、颗粒有机物/POM、矿物结合态有机物/MOM）的分布，肯定了矿物在提升土壤有机碳库稳定性方面的重要作用；结合经典DCB溶提法区分了铁氧化物与硅酸盐矿物对土壤固定有机碳的贡献；明确了利用方式和施肥措施影响铁结合态有机碳占比的区域差异。3）Fed、Feo、Fep分别通过吸附（OC:Fed＜1）、共沉淀（OC:Feo＞1）及鳌合作用（OC:Fep＞＞1）结合稳定有机质；游离铁主要结合0.5~2.3 kDa和0.29~0.5 kDa的小分子有机碳；耕作施用无机化肥降低黑土Fex-OC复合物的OC:Fe摩尔比，施用有机肥使黑土铁结合有机碳的能力略有回升；而红壤则相反，长期的耕作与施肥促进铁固定更多的有机碳，但降低了游离铁结合态有机碳的芳构化程度。4）邻苯二酚主要通过还原作用诱导Fe(II) 催化氧化铁的形成转化过程，且碱性条件下更有利于该过程的进行，而水杨酸主要通过酸性溶解方式促进晶质氧化铁的形成与转化；水铁矿向晶型赤铁矿转化时间的延迟或提前以及产物的形貌取决于外源有机分子还原能力与结构特征及反应pH条件。5）耕作后红壤团聚体抵抗外力的能力降低，与撂荒相比，NPK配施土壤团聚体抵抗力约是撂荒的2/3；揭示了不同施肥方式下有机质与氧化铁胶结作用特点及其对团聚体稳定性的影响。项目的执行一方面揭示了氧化铁形态转化与土壤有机碳稳定的内在联系与机制，另一方面加深了对土壤团聚体形成与稳定的宏观表现与微观机制的理解，为合理施肥、提高土壤质量与生产力提供科学依据。