Carbon K-Edge Nexafs Spectroscopy of Mineral-Associated Soil Organic Matter During Soil Ecosystem Development.

Up to date, terrestrial C models have focused mainly on total C flux between broadly defined labile and recalcitrant soil pools. Organo-mineral interactions play a key role in the flux of C between the soluble and mineral-associated stabilized organic C pools in soils, and thus regulate the bioavailability of soil organic matter to microorganisms and plants. Mineral-facilitated organic matter accrual with soil development has been observed but further verification of organic C speciation during this accrual process is needed across a number of ecosystems. Different from destructive wet chemical analysis, synchrotron-based carbon K-edge near edge x-ray absorption fine structure (NEXAFS) spectroscopy has been shown to be a selective, sensitive, and nondestructive method capable of providing molecular-scale information on C speciation. In this study, organic C speciation of mineral-associated organic matter in soils from two soil chronosequences (5000-year and 20,000-year soil development, respectively) will be investigated. The mineral-associated organic fraction in each soil was separated from the bulk soil by using the sodium polytungstate extraction method. The non-hydrolysable organic matter associated with mineral phase was isolated using the standard 6 M HCl hydrolysis. Organic C speciation in the bulk mineral-associated organic fraction and the non-hydrolysable fraction was characterized using the carbon K-edge XANES. The C K-edge NEXAFS spectra were collected on the Hermon beamline at the Synchrotron Radiation Center of University of Wisconsin-Madison. Aromatic-C, phenolic-C, aliphatic-C, carboxylic-C, and O-alkyl-C were detected in the samples. The C K-edge NEXAFS results suggested decreasing aromatic-C and increasing carboxylic C in mineral-associated organic matter with increasing soil development. Hydrolyzation significantly decreased aromatic-C.