Sequential density fractionation across soils of contrasting mineralogy: evidence for both microbial- and mineral- controlled soil organic matter stabilization

Sequential density fractionation separated soil particles into ‘‘light’’ predominantly mineral-free organic matter vs. increasingly ‘‘heavy’’ organo-mineral particles in four soils of widely differing mineralogy. With increasing particle density C concentration decreased, implying that the soil organic matter (OM) accumulations were thinner. With thinner accumulations we saw evidence for both an increase in C-based mean residence time (MRT) of the OM and a shift from plant to microbial origin.Evidence for the latter included: (1) a decrease in C/N, (2) a decrease in lignin phenols and an increase in their oxidation state, and (3) an increase in dC and dN. Although bulk-soil OM levels varied substantially across the four soils, trends in OM composition and MRT across the density fractions were similar. In the intermediate density fractions (*1.8–2.6 g cm), most of the reactive sites available for interaction with organic molecules were provided by aluminosilicate clays, and OM characteristics were consistent with a layered mode of OM accumulation. With increasing density (lower OM loading) within this range, OM showed evidence of an increasingly microbial origin. We hypothesize that this microbially derived OM was young at the time of attachment to the mineral surfaces but that it persisted due to both binding with mineral surfaces and protection beneath layers of younger, less microbially processed C. As a result of these processes, the OM increased in MRT, oxidation state, and degree of microbial processing in the sequentially denser intermediate fractions. Thus mineral surface chemistry is assumed to play little role in determining P. Sollins (&) Forest Science Department, Oregon State University, Corvallis, OR 97331, USA e-mail: [email protected] M. G. Kramer Department of Earth and Planetary Sciences, University of California Santa Cruz, Santa Cruz, CA 95064, USA C. Swanston US Forest Service Northern Research Station, 410 MacInnes Dr., Houghton, MI 49931, USA K. Lajtha Department of Botany & Plant Pathology, Oregon State University, Corvallis, OR 97331, USA T. Filley Department of Earth & Atmospheric Sciences, Purdue University, West Lafayette, IN 47907, USA A. K. Aufdenkampe Stroud Water Research Center, 970 Spencer Road, Avondale, PA 19311, USA R. Wagai National Institute for Environmental Studies, 16-2 Onogawa, Tsukuba, Ibaragi 305-8506, Japan R. D. Bowden Allegheny College, Meadville, PA 16335, USA 123 Biogeochemistry (2009) 96:209–231 DOI 10.1007/s10533-009-9359-z