Organo-mineral Interactions in Ultisols and Andosols with Contrasting Mineralogy

Abstract:

Organo-mineral associations play an important role in long-term stabilization of soil organic matter. To understand the role of soil minerals and major cations in organo-mineral interactions, 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 C forms with Ca, Fe, Al and Si in soil clay fraction of Ultisols and Andosols with contrasting mineralogy. Ultisols were under agricultural use receiving 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 showed a high spatial correlation between C and Ca in the Utisols. The major Ca forms in agricultural Ultisols were 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 was observed in Andosols, revolutionizing our view of heterogeneous distribution of OM in natural soil. The C forms associated with minerals 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.