Stxm-Nexafs Investigation of Organo-Mineral Interactions in Ultisols and Andosols with Contrasting Mineralogy.

Organo-mineral interaction is increasingly considered as the major mechanism for soil C stabilization. To refine our understanding of the specific mineral phases, major cations or the mechanisms responsible for organo-mineral interactions in different soil types, we applied STXM-NEXAFS spectroscopy at C, N, Al and Si K-edges, as well as Ca and Fe L-edges, to conduct submicron-level investigations of the associations of C and N and their functional groups with Ca, Fe, Al and Si in soil clay fraction of Ultisols and Andosols with contrasting mineralogy. Ultisols were under agricultural use receiving an input of limestone and mushroom compost, whereas Andosols were under natural vegetation. The poorly crystalline minerals such as allophane and ferrihydrite were rich in Andosols, while Ultisols were dominated by vermiculite, illite and kaolinite. STXM analysis demonstrated a high C-Ca spatial correlation in the Ultisols with the major Ca forms as organic Ca. A variety of C forms such as phenols, heterocyclic compounds, carboxylamides were found in Ca-rich “hot spots” in Ultisols. In contrast, a relatively uniform distribution of C and C forms on mineral surfaces and a good C-Al/Fe spatial correlation was observed in Andosols. The major C forms associated with minerals in Andosols were aromatic and carboxyl C. These results demonstrates the importance of Ca containing organic compounds in soil C sequestration in agricultural Ultisols, and highlights the important role of the poorly-crystalline soil minerals in soil organic matter sequestration in Andosols.