26 Sep 2022
26 Sep 2022
Status: this preprint is open for discussion.

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

Shane W. Stoner1,2, Marion Schrumpf1, Alison M. Hoyt3, Carlos A. Sierra1,4, Sebastian Doetterl2, Valier Galy5, and Susan Trumbore1 Shane W. Stoner et al.
  • 1Biogeochemical Processes Department, Max Planck Institute for Biogeochemistry, Jena, 07745, Germany
  • 2Department of Environmental Systems Science, ETH Zürich, Zürich, 8092, Switzerland
  • 3Earth System Science, Stanford University, Stanford, 94305, USA
  • 4Department of Ecology, Swedish University of Agricultural Sciences, Uppsala, SE-750 07, Sweden
  • 5Woods Hole Oceanographic Institution, Woods Hole, 02543, USA

Abstract. 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.

Shane W. Stoner et al.

Status: open (extended)

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  • RC1: 'Comment on egusphere-2022-624', Alain F Plante, 20 Oct 2022 reply

Shane W. Stoner et al.

Shane W. Stoner et al.


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Short summary
Soils store more carbon (C) than any other terrestrial C reservoir, but the processes that control how much C stays in soil, and for how long, are very complex. Here, we used a recent method that involves heating soil in the lab to measure the range of C ages in soil. We found that most C in soil is decades to centuries old, while some stays for much shorter times (days to months), and some is thousands of years old. Such detail helps to estimate how soil C may react to changing climate.