Description and Quantification of the Associations Between Soil Organic Matter and Clay-Sized Minerals in a Mollisol Using Stxm High Resolution Chemical Mapping.

Description and Quantification of the associations between soil organic matter and clay-sized minerals in a Mollisol using STXM high resolution chemical mapping

Sofia OUFQIR^1,2, James J. Dynes³,  Brandy M. TONER², Paul R. BLOOM²

¹ Dept. of Chemistry, University Mohamed V-Agdal, Rabat, Morocco
² Dept. of Soil, Water, and Climate, University of Minnesota, Saint Paul, 55108, MN, USA
³Canadian Light Source Inc., University of Saskatchewan, Saskatoon, Saskatchewan Canada S7N 0X4

Abstract

In this research study we used the scanning transmission x-ray microscopy (STXM) technique, providing high quality imaging and resolved spectroscopy with a spatial resolution of 30nm, to identify and quantify organic matter components distributed across density fractions of a clay (< 2µm) separated from the plow layer of a Minnesota Mollisol soil. The soil was subjected to particle-size fractionation to recover the soil clay particles, followed by density separation combined with low energy ultrasonic dispersion to separate the free carbon from mineral bound organic matter. We determined the organic and aluminosilicate components in the different soil density fractions using STXM and determined the association between the various components using multivariate statistics. The analysis of C-NEXAFS spectra of the STXM data revealed an abundance of proteins, polysaccharides, and black carbon in the whole clay and in the light fraction (density < 1.80 g cm^-3). In the heavy fraction (density > 2.10 g cm^-3) polysaccharide-C was identified as preferentially associated with the clay mineral. Analyses of the Fe, Al, and Si spectra showed FeII, FeIII, tetrahedral Si-O and octahedral Al, mostly associated with the illite-smectite clay that is abundant in the soil. We also observed the presence of hydrated silica probably fragments of phytoliths. Furthermore, we inferred that peptides, black carbon, and phytoliths exist as phases separate from the aluminosilicate minerals. These conclusions expound the stabilization of carbon in soil clays.