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Discussion papers
https://doi.org/10.5194/soil-2019-44
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/soil-2019-44
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: original research article 21 Aug 2019

Submitted as: original research article | 21 Aug 2019

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal SOIL (SOIL).

Ramped thermal analysis for isolating biologically meaningful soil organic matter fractions with distinct residence times

Jonathan Sanderman1 and A. Stuart Grandy2 Jonathan Sanderman and A. Stuart Grandy
  • 1Woods Hole Research Center, Falmouth 02540, USA
  • 2Department of Natural Resources and the Environment, University of New Hampshire, Durham 03824, USA

Abstract. In this work, we provide a preliminary assessment of whether or not ramped thermal oxidation coupled with determination of the radiocarbon content of the evolved CO2 can be used to isolate biologically meaningful fractions of SOM along with direct information on the turnover rate of each fraction. Using a 30 year time-series of soil samples from a well characterized agronomic trial, we found that the incorporation of the bomb-spike in atmospheric 14CO2 into thermal fractions could be successfully modelled. With increasing activation energy of the fraction, the mean residence time of the fraction increased from 10 to 400 years. Importantly, the first four of five thermal fractions appeared to be a mixture of fast and increasingly slower cycling SOM. To further understand the composition of different thermal fractions, stepped pyrolysis-gas chromatography-mass spectrometry (py-GC/MS) experiments were performed at five temperatures ranging from 330 to 735 °C. The py-GC/MS data showed a reproducible shift in chemistry across the temperature gradient trending from polysaccharides and lipids at low temperature to lignin and microbial-derived compounds at middle temperatures to aromatic and unknown compounds at the highest temperatures. Integrating the 14C and Py-GC-MS data suggests the organic compounds, with the exception of aromatic moieties likely derived from wildfire, with centennial residence times are not more complex but may be protected from pyrolysis, and likely also from biological mineralization, by interactions with mineral surfaces.

Jonathan Sanderman and A. Stuart Grandy
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Status: open (until 09 Oct 2019)
Status: open (until 09 Oct 2019)
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Jonathan Sanderman and A. Stuart Grandy
Jonathan Sanderman and A. Stuart Grandy
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Latest update: 17 Sep 2019
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Short summary
Soils contain one of the largest and most dynamic pools of carbon on Earth yet scientists still struggle to understand the reactivity and fate of soil organic matter upon disturbance. In this study, we found that with increasing thermal stability, the turnover time of organic matter increased from decades to centuries with a concurrent shift in chemical composition. In this proof-of-concept study, we found that ramped thermal analyses can provide new insights for understanding soil carbon.
Soils contain one of the largest and most dynamic pools of carbon on Earth yet scientists still...
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