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热带地区土壤有机碳的稳定性受有机碳与高岭石、三价铁离子[Fe(Ⅲ)]交互作用的强烈影响.低分子量有机酸(LMWOAs)是土壤有机碳中最易被微生物降解的组分,研究LMWOAs在高岭石-Fe(Ⅲ)-LMWOAs三相系统中的固定机制对于理解热带地区土壤有机碳循环具有重要意义;然而LMWOAs在该三相系统中固定机制仍不清楚,并缺乏分子水平上的直接证据.本研究以柠檬酸(CA)为LMWOAs的典型代表,通过采集Fe/CA初始物质的量比2.0条件下获取的吸附样品,利用同步辐射X射线扫描透射显微术(STXM)在亚微米尺度上表征C,Fe和Si的分布特征,发现C-Fe分布的相关性与C-Si分布相当;并在高C,Fe微区中利用C的K边和Fe的L3边X射线近边吸收精细结构谱(NEXAFS)在分子水平上揭示了Fe在不同富集微区分别以水铁矿和高岭石-Fe(Ⅲ)-CA三元配合体形式存在,从而表明水铁矿诱发的吸附/共沉淀以及以Fe(Ⅲ)为架桥形成高岭石-Fe(Ⅲ)-CA三元配合作用是CA固定的重要机制.本研究成果对于理解热带地区土壤中CA稳定性和低分子量有机酸的碳固定和循环机制提供了重要科学依据.
The stability of soil organic carbon in tropical regions is strongly influenced by the interaction of organic carbon with kaolinite and ferric ion [Fe (Ⅲ)]. Low molecular weight organic acids (LMWOs) are the most easily degradable soil organic carbon The study of the immobilization mechanism of LMWOAs in the kaolinite-Fe (Ⅲ) -LMWOAs three-phase system is of great importance for the understanding of the soil organic carbon cycle in the tropics. However, the immobilization mechanism of LMWOAs in this three-phase system remains unclear. Lack of direct evidence at the molecular level.In this study, citric acid (CA) as a typical representative of LMWOAs, by collecting the Fe / CA initial material ratio of 2.0 obtained under the adsorption sample, using synchrotron radiation X-ray transmission transmission microscopy (STXM) was used to characterize the distribution of C, Fe and Si in the submicron scale. It was found that the correlation between C-Fe distribution and C-Si distribution was similar. In the high C and Fe micro- The near-edge X-ray absorptive fine structure spectrum (NEXAFS) on the L3 side reveals that Fe is present in the form of a triad of ferrihydrite and kaolinite-Fe (III) -CA respectively in different enrichment micro-regions It shows that the iron-iron-induced adsorption / coprecipitation and the formation of kaolins with Fe (Ⅲ) bridging The ternary coordination of stone-Fe (Ⅲ) -CA is an important mechanism for the immobilization of CA. The results of this study provide an important scientific basis for understanding the stability of CA in soil and the carbon fixation and cycling mechanism of low molecular weight organic acids in tropical areas.