How well does ramped thermal oxidation quantify the age distribution of soil carbon? Assessing thermal stability of physically and chemically fractionated soil organic matter
<p>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...
| Main Authors: | , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2023-08-01
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| Series: | Biogeosciences |
| Online Access: | https://bg.copernicus.org/articles/20/3151/2023/bg-20-3151-2023.pdf |
| Summary: | <p>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
(<span class="inline-formula"><sup>14</sup></span>C) and <span class="inline-formula"><sup>13</sup></span>C 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 <span class="inline-formula"><sup>14</sup></span>C 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 <span class="inline-formula"><sup>13</sup></span>C-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 <span class="inline-formula">∼</span> 95 % of the mineral-associated organic matter (MOM) had a relatively
narrow <span class="inline-formula"><sup>14</sup></span>C distribution, while 5 % was very low in <span class="inline-formula"><sup>14</sup></span>C and
likely reflected C from the <span class="inline-formula"><i><</i></span> 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.</p> |
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| ISSN: | 1726-4170 1726-4189 |