How well does ramped thermal oxidation quantify the age distribution of soil carbon? Assessing thermal stability of physically and chemically fractionated soil organic matter

Abstract

Carbon (C) in soils persists on a range of timescales depending on physical, chemical, and biological processes that interact with soil organic matter (SOM) and affect its rate of decomposition. Together these processes determine the age distribution of soil C. Most attempts to measure this age distribution have relied on operationally defined fractions using properties like density, aggregate stability, solubility, or chemical reactivity. Recently, thermal fractionation, which relies on the activation energy needed to combust SOM, has shown promise for separating young from old C by applying increasing heat to decompose SOM. Here, we investigated radiocarbon (14C) and 13C of C released during thermal fractionation to link activation energy to the age distribution of C in bulk soil and components previously separated by density and chemical properties. While physically and chemically isolated fractions had very distinct mean 14C values, they contributed C across the full temperature range during thermal analysis. Thus, each thermal fraction collected during combustion of bulk soil integrates contributions from younger and older C derived from components having different physical and chemical properties but the same activation energy. Bulk soil and all density and chemical fractions released progressively older and more 13C-enriched C with increasing activation energy, indicating that each operationally defined fraction itself was not homogeneous but contained a mix of C with different ages and degrees of microbial processing. Overall, we found that defining the full age distribution of C in bulk soil is best quantified by first separating particulate C prior to thermal fractionation of mineral-associated SOM. For the Podzol analyzed here, thermal fractions confirmed that ∼ 95 % of the mineral-associated organic matter (MOM) had a relatively narrow 14C distribution, while 5 % was very low in 14C and likely reflected C from the < 2 mm parent shale material in the soil matrix. After first removing particulate C using density or size separation, thermal fractionation can provide a rapid technique to study the age structure of MOM and how it is influenced by different OM–mineral interactions.