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

Carbon (C) in soils persists on a range of timescales that depend 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 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, shows promise in separating young from old C by applying increasing heat to decompose SOM and equate this with biochemical stability in soil. Here, we investigated radiocarbon (14C) released during thermal fractionation to reconstruct thermal stabilities and age distributions of C released from bulk soil as well as its component density and chemical fractions. We found that bulk soil and all density and chemical fractions released progressively older C as temperatures increased. In addition, all fractions released some C across the entire temperature range, indicating that bulk soil thermal fractionations integrate young and old C at all temperatures. In the bulk soil, age distribution could be identifed by isolating 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 < 2mm parent shale material in the soil matrix. By 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.