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-14-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 delta C-13 and delta N-15. 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 (similar to 1.8-2.6 g cm(-3)), 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 OM composition in these intermediate fractions. As the separation density was increased beyond similar to 2.6 g cm(-3), mineralogy shifted markedly: aluminosilicate clays gave way first to light primary minerals including quartz, then at even higher densities to various Fe-bearing primary minerals. Correspondingly, we observed a marked drop in delta N-15, a weaker decrease in extent of microbial processing of lignin phenols, and some evidence of a rise in C/N ratio. At the same time, however, C-14-based MRT time continued its increase. The increase in MRT, despite decreases in degree of microbial alteration, suggests that mineral surface composition (especially Fe concentration) plays a strong role in determining OM composition across these two densest fractions.