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ELECTRONS, X-RAYS, AND SOIL AUGERS: PROBING SOIL ORGANO-MINERAL INTERACTIONS WITH A CROSS-SCALE ANALYTICAL TOOLBOX
Possinger, Angela Ruth
The ability of soils to serve as a carbon (C) reservoir depends on the processes that drive the accumulation and persistence of soil organic matter (SOM). The suite of stabilizing interactions between SOM and soil minerals includes surface adsorption, complexation, and co-precipitation with mineral phases. However, organo-mineral associations are not static or indefinitely persistent, and depend on mineralogy, SOM composition, and environmental variables. Here, the differences in both SOM and mineral structure and composition between surface adsorption and co-precipitation of SOM were investigated in a semi-crystalline iron (Fe) oxide–dissolved OM (DOM) synthesis experiment. The addition of DOM during mineral co-precipitation induced changes in both DOM and Fe composition, with a bi-directional set of reactions that ultimately resulted in reduced Fe aggregation, Fe(III) reduction to Fe(II), and spatial separation of aromatic- and non-aromatic C forms. The study of redox and associated processes on organo-mineral interactions was expanded to the landscape scale by testing the effect of legacy soil saturation frequency on composition and mineralizability of mineral-associated SOM. From low to high saturation frequency, a shift from Fe(III)-organic to Fe(II) and Al-dominated SOM interactions was detected using bulk X-ray adsorption and nuclear magnetic resonance (NMR) spectroscopy. At high saturation frequency, an order of magnitude increase in mineralizability of dissolved organic carbon (DOC) mobilized during cyclic aerobic-anaerobic incubation was identified. In addition to field-scale studies and synthesis experiments, direct observation and characterization of both organo-mineral and organo-organic interfaces in natural soil samples was conducted at the <5 nm nanometer scale with cryogenic thin-sectioning and analytical electron microscopy. Enrichment of nitrogen (N) at both organo-organic and organo-mineral interfaces was detected. Alkyl C was enriched by 4.1% at an organo-organic interface, in contrast to enrichment of N-substituted carboxylic C (by 32%) at an organo-mineral interface. Identification of unique organo-organic in comparison to organo-mineral interfaces provides novel insights into previously unknown mechanisms of SOM stabilization, and challenges the assumption of OM accumulation in ordered gradients as a function of distance to mineral surfaces. Collectively, these observations provide motivation to shift the one-dimensional conceptual model of SOM organo-mineral association to account for organo-organic interactions and other spatially complex, multidimensional interaction processes.
soil organic carbon; Co-precipitation; Organo-mineral interactions; Organo-organic interactions; STEM-EELS; Variable saturation frequency; Soil sciences
Lehmann, C. Johannes
Yavitt, Joseph B.; Derry, Louis A.
Soil and Crop Sciences
Ph.D., Soil and Crop Sciences
Doctor of Philosophy
dissertation or thesis