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https://doi.org/10.5194/bg-2020-176
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/bg-2020-176
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  26 Oct 2020

26 Oct 2020

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This preprint is currently under review for the journal BG.

Long-term bare fallow soil fractions reveal thermo-chemical properties controlling soil organic carbon dynamics

Mathieu Chassé1, Suzanne Luftalla2,3, Lauric Cécillon2,4, François Baudin5, Samuel Abiven2,6,7, Claire Chenu3, and Pierre Barré2 Mathieu Chassé et al.
  • 1Sorbonne Université, Muséum national d’Histoire naturelle, CNRS, UMR 7590, Institut de minéralogie, de physique des matériaux et de cosmochimie (IMPMC), 75005 Paris, France
  • 2Laboratoire de géologie de l’ENS, Université PSL, CNRS, UMR 8538, 75005 Paris, France
  • 3Université Paris–Saclay, INRAE, AgroParisTech, UMR 1402 ECOSYS, 78850 Thiverval-Grignon, France
  • 4Normandie University, UNIROUEN, INRAE, ECODIV, 76000 Rouen, France
  • 5Sorbonne Université, CNRS UMR 7193, Institut des Sciences de la Terre de Paris (ISTeP), 75005 Paris, France
  • 6Department of Geography, University of Zurich, 8057 Zurich, Switzerland
  • 7Centre de recherche en écologie expérimentale et prédictive (CEREEP–Ecotron IleDeFrance), Département de biologie de l’ENS, Université PSL, CNRS, 77140 St-Pierre-lès-Nemours, France

Abstract. Evolution of organic carbon content in soils has the potential to be a major driver of atmospheric greenhouse gas concentrations over the next century. Understanding soil carbon dynamics is a challenge due to a wide range of residence time of soil organic matter and limited constraints on the mechanisms influencing its persistence. In particular, large uncertainties exist on the persistence of pyrogenic organic carbon in soils. In order to characterise organic matter with varying degrees of persistence and distinguish pyrogenic organic carbon, we combined Rock-Eval analysis, a thermo-chemical method, with the benzene polycarboxylic acid molecular marker method and Raman spectroscopy, to characterise samples from long-term bare fallow experiments, progressively depleted in the most labile organic carbon over time. Considering the heterogeneity of soil samples, size fractions have been separated to distinguish pools of organic carbon with distinct properties. We observe that organic carbon dynamics is dependent on granulometry. A pool of organic carbon with intermediate residence time, from years to a few decades, representing ca 65 % of the bulk soil organic carbon stock, is mainly associated to fine fractions (< 20 µm). With time under bare fallow, this organic carbon is progressively transferred towards finer fractions through the breakdown of organic matter. Coarse fractions (> 20 µm) are rich in centennially-persistent organic carbon, representing ca 20 % of the initial organic carbon stock, due to the chemical recalcitrance of organic matter in these fractions, dominated by pyrogenic organic carbon. A second pool of persistent organic carbon, representing ca 15 % of the initial organic carbon stock, is associated with the clay fraction, indicating mechanisms of protection occurring at the submicron scale (< 2 µm). This persistent organic carbon only represents 30 % of the organic carbon initially present in the clay fraction. Persistent organic carbon exhibits heterogeneous chemical signatures depending on the considered pool but a consistent thermal signature demonstrating the relationship between thermal stability and biogeochemical stability of soil organic carbon. This gives the possibility to assess the size of the persistent organic carbon pool in soils using thermal parameters. The persistence of pyrogenic organic carbon in the clay fraction is similar to the one of total organic carbon. The different persistence of coarse and fine pyrogenic organic carbon reside in the highly condensed nature of sand-sized pyrogenic carbon which may result from burning temperature over 700 °C. Pyrogenic organic carbon is not inert but exhibits a consistent increase in the level of condensation with time in all size fractions, showing the role of chemical quality in pyrogenic organic carbon persistence. Overall, this study helps improve the separation, evaluation and characterisation of carbon pools with distinct residence time in soils and gives insight on the mechanistic origin of soil organic carbon dynamics.

Mathieu Chassé et al.

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Evolution of organic carbon content in soils could be a major driver of atmospheric greenhouse gas concentrations over the next century. Understanding the factors controlling carbon persistence in soil is a challenge. Our study of unique long-term bare fallow samples, depleted in labile organic carbon, helps improve the separation, evaluation and characterisation of carbon pools with distinct residence time in soils and gives insight on the mechanisms explaining soil organic carbon persistence.
Evolution of organic carbon content in soils could be a major driver of atmospheric greenhouse...
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