Sorptive fractionation of organic matter and formation of organo-hydroxy-aluminum complexes during litter biodegradation in the presence of gibbsite

Solid and aqueous phase Al species are recognized to affect organic matter (OM) stabilization in forest soils. However, little is known about the dynamics of formation, composition and dissolution of organo-Al hydroxide complexes in microbiallyactive soil systems, where plant litter is subject to microbial decomposition in close proximity to mineral weathering reactions. We incubated gibbsite–quartz mineral mixtures in the presence of forest floor material inoculated with a native microbial consortium for periods of 5, 60 and 154 days. At each time step, samples were density separated into light (<1.6 g cm ), intermediate (1.6–2.0 g cm ), and heavy (>2.0 g cm ) fractions. The light fraction was mainly comprised of particulate organic matter, while the intermediate and heavy density fractions contained moderate and large amounts of Al-minerals, respectively. Multi-method interrogation of the fractions indicated the intermediate and heavy fractions differed both in mineral structure and organic compound composition. X-ray diffraction analysis and SEM/EDS of the mineral component of the intermediate fractions indicated some alteration of the original gibbsite structure into less crystalline Al hydroxide and possibly proto-imogolite species, whereas alteration of the gibbsite structure was not evident in the heavy fraction. DRIFT, Py–GC/MS and STXM/NEXAFS results all showed that intermediate fractions were composed mostly of lignin-derived compounds, phenolics, and polysaccharides. Heavy fraction organics were dominated by polysaccharides, and were enriched in proteins, N-bearing compounds, and lipids. The source of organics appeared to differ between the intermediate and heavy fractions. Heavy fractions were enriched in C with lower C/N ratios relative to intermediate fractions, suggesting a microbial origin. The observed differential fractionation of organics among hydroxy-Al mineral types suggests that microbial activity superimposed with abiotic mineral-surface-mediated fractionation leads to strong density differentiation of organo-mineral complex composition even over the short time scales probed in these incubation experiments. The data highlight the strong interdependency of mineral transformation, microbial community activity, and organic matter stabilization during biodegradation. Published by Elsevier Ltd. 0016-7037/$ see front matter Published by Elsevier Ltd. http://dx.doi.org/10.1016/j.gca.2013.07.043 ⇑ Corresponding author at: Northern Research Station, USDA Forest Service, CAMS, L-397, 7000 East Ave, Livermore, CA 94550, USA. Tel.: +1 925 422 9556; fax: +1 925 423 7884. E-mail address: [email protected] (K. Heckman). 1 Tel.: +1 603 862 1700. 2 Tel.: +1 520 621 1646; fax: +1 520 621 1647. 3 Tel.: +1 541 737 2441; fax: +1 541 737 1589. 668 K. Heckman et al. /Geochimica et Cosmochimica Acta 121 (2013) 667–683