侵蚀作用下红壤铁形态与含量变化对有机碳稳定性的影响

Soil iron is highly related to soil aggregate structures and occurrence status of soil organic carbon (SOC). Meanwhile, the changes in iron content and morphology have great impact on SOC stability. However, the loss, enrichment and morphology transfer of iron and their influence on SOC stability under the impact of water erosion are not fully understood. This project is designed to select red soil as the study object. A typical small watershed in red soil hilly region is to be selected, and soil samples in key eroding positions (i.e. eroding, depositional and control sites) in different seasons are to be collected. Various methods and techniques, including soil aggregation analysis, incubation of SOC, thermogravimetry-differential scanning calorimetry analysis, and solid 13C NMR analysis, are to be used to analysis these soil samples. Based on these analyses, distributions and variations of iron in content and morphology will be explored, and the key factors that impact on soil iron are to be revealed. Meanwhile, the variations of SOC component and chemical structure, soil aggregation distribution and stability, SOC mineralization rate and thermal characteristics will also be studied. Then, the interaction mechanisms between iron and SOC are to be deeply discussed, and the influence of iron changes on SOC stabilization under the effect of water erosion will be revealed. The results of this project are of important meaning to enrich the study of SOC stability mechanisms under soil erosion. Furthermore, the results will also provide theoretical support for the regional management of SOC in red soil region of south China.

土壤铁与土壤团聚结构及有机碳赋存状态关系密切，其形态和含量变化显著影响有机碳稳定性。但水力侵蚀作用下土壤铁流失、富集和铁形态转化规律及其对有机碳稳定性的影响并不明确。本项目通过红壤丘陵区典型流域关键侵蚀部位（侵蚀区和沉积区）土壤的监测分析，结合固态核磁共振、矿化培养、热重-差示量热扫描分析等方法技术，探索长期侵蚀作用下侵蚀区和沉积区不同形态铁含量的分布和变化规律及其关键影响因素，在深入分析不同粒径团聚体分布及稳定特征、有机碳组分和结构变化规律、有机碳矿化速率及热稳定特征的基础上，阐明土壤铁形态和含量与有机碳的相互作用机制，揭示水力侵蚀作用下土壤铁含量和形态变化对有机碳稳定性的影响。本项目的开展将丰富土壤有机碳稳定机制研究，充实侵蚀作用下的土壤碳循环理论，并为南方红壤有机碳的区域化管理提供理论依据。

红壤丘陵区水热条件丰富、土壤侵蚀频繁，水力侵蚀对红壤有机碳循环过程有重要影响。然而，水力侵蚀作用下，土壤铁形态及含量变化对有机碳稳定性的影响机制还存在较大不确定性。本项目以典型红壤为研究对象，通过流域典型侵蚀和沉积区域土壤的季节性监测，分析了长期水力侵蚀驱动下土壤铁形态含量变化对有机碳稳定性的影响。研究结果表明，侵蚀区和沉积区总铁含量分别为46.5±0.05g/kg和31.04±0.05g/kg，但侵蚀区土壤铁的游离率(16%)要远低于沉积土壤(30%)。水力侵蚀仅能导致活性铁随径流泥沙大量迁移并在沉积区富集，土壤总铁含量则因有机质等轻质物质的流失而在侵蚀区表现出积累特征。同时，土壤铁形态含量变化没有体现出显著的季节性变化（P>0.05）。此外，土壤碳铁结合物（Fe-OC）在土壤有机碳中的比例平均值为13.03%（变化范围为0.98~34.99%），整体上表现出随着土壤深度增加而增加的趋势。通过系统性分析发现，在侵蚀环境下，不同形态铁含量、铁结合态有机碳含量等铁相关指标共同解释了土壤有机碳稳定性 80.5%的变异性；而沉积环境下，铁相关指标对有机碳稳定性的解释量低于10%，其他理化性质的解释量显著上升。这一结果与前期研究结果存在逻辑上的一致性。主要原因是侵蚀/沉积区域物质组成在侵蚀作用下存在明显差异，沉积区碳稳定性主要以团聚体的物理保护为主，而侵蚀区碳稳定性则与铁氧化物的存在紧密相关。研究结果从一定程度上明确了非游离态土壤铁对有机碳稳定性的关键作用，对于进一步的有机质调控有一定的实践意义。