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stable isotopes, differential scanning calorimetry (DSC), scanning transmission X-ray microscopy (STXM), near edge x-ray absorption fine structure spectroscopy (NEXAFS) 2 AbstractSoil carbon turnover models generally divide soil carbon into pools with varying intrinsic decomposition rates. While these decomposition rates are modified by factors such as temperature, texture, and moisture, they are rationalized by assuming chemical structure is a primary controller of decomposition. In the current work, we use Near Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy in combination with differential scanning calorimetry (DSC) and alkaline CuO oxidation to explore this assumption. Specifically, we examined material from the 2.3-2.6 kg L Therefore, our results demonstrate that C age is not necessarily related to molecular structure or thermodynamic stability, and we suggest that soil carbon models would benefit from viewing turnover rate as co-determined by the interaction between substrates, microbial actors and abiotic driving variables. Furthermore, assuming that old carbon is composed of complex or "recalcitrant" compounds will erroneously attribute a greater temperature sensitivity to those materials than they may actually possess.

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