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

Abstract

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 14 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 d 13 C and d 15 N. 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 -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

Comparison of soil organic matter dynamics at five temperate deciduous forests with physical fractionation and radiocarbon measurements

“…Surface soil and density fractions C isotopes (Table 4), a trend observed by others (Sollins et al 2009). This trend was most pronounced at MO-OZ and NH-BF and not statisti- …”

Variation in the Molecular Structure and Radiocarbon Abundance of Mineral-Associated Organic Matter across a Lithosequence of Forest Soils

“…Recently, atomic force microscopy was used to illustrate the strong binding of N to goethite, where ammonia bonds were on averageIn addition to illustrating competitive sorption behavior, bonding to Fe surfaces seems to confer greater stability to the bound organics. In previous work exploring the possibility of an influence of mineral chemistry on organic matter composition and stability, no clear patterns were found aside from a consistent increase in MRT associated with increasing concentration of Fe-rich mineral phases [17]. In the dataset examined here, heavy fraction MRT was explained almost equally by Fe D and SSA (Table 3 and Table S1).…”

Chemical characteristics of particulate organic matter from a small, mountainous river system in the Oregon Coast Range, USA

“…The progressive decrease in the OC content of the suspended load as a function of increasing discharge has been observed in other SMRS (e.g., Coynel et al 2005) although in some cases this trend has been incorrectly explained as a simple dilution by mineral material. A more accurate way to describe the mechanism responsible for this trend is that as Q increases, there is an enhanced entrainment of denser fractions of soil organic matter that are enriched in mineral material and have lower %OC contents (e.g., Sollins et al 2009). In the case of the Santa Clara River (Masiello and Druffel 2001), with a few exceptions, the %OC contents of suspended sediment samples also generally decrease with Q (Fig.…”