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© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  05 Jun 2020

05 Jun 2020

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A revised version of this preprint is currently under review for the journal SOIL.

Geogenic organic carbon in terrestrial sediments and its contribution to total soil carbon

Fabian Kalks1, Gabriel Noren2, Carsten Mueller3,4, Mirjam Helfrich1, Janet Rethemeyer2, and Axel Don1 Fabian Kalks et al.
  • 1Thünen Institute of Climate-Smart Agriculture, Bundesallee 65, 38116 Braunschweig, Germany
  • 2Institute of Geology and Mineralogy, University of Cologne, Institute of Geology and Mineralogy, Zülpicher Str. 49b, 50674 Cologne, Germany
  • 3Technical University of Munich, Chair of Soil Science, Emil-Ramann Strasse 2, 85354 Freising-Weihenstephan
  • 4University of Copenhagen, Department of Geosciences and Natural Resource Management, Øster Voldgade 10, DK-1350 CopenhagenK, Denmark

Abstract. Geogenic organic carbon (GOC) from sedimentary rocks is an overlooked fraction in soils that has not been quantified yet, influencing the composition, age and stability of total organic carbon (OC) in soils. In this context GOC is referred to as the OC in bedrocks deposited during sedimentation. However, the contribution of GOC to total soil OC varies with the type of bedrock. So far studies investigating the contribution of GOC derived from different terrestrial sedimentary rocks to soil OC contents are missing.

In order to fill this gap, we analysed 10 m long sediment cores at three sites recovered from Pleistocene Loess, Miocene Sand and Triassic Red Sandstone and calculated the amount of GOC based on 14C measurements. 14C ages of bulk sedimentary OC revealed that OC represents a mixture of biogenic and geogenic components. Biogenic refers to OC that entered the sediments recently from plant sources. All sediments contain considerable amounts of GOC (median amounts of 0.10 g kg−1 at the Miocene Sand, 0.27 g kg−1 at the Pleistocene Loess and 0.17 at Red Sandstone) in comparison to subsoil OC contents (between 0.53–15.21 g kg−1). Long-term incubation experiments revealed that this GOC seemed to be comparatively stable against biodegradation. Its possible contribution to subsoil OC stocks (0.3–1.5 m depth) is ~ 2.5 % in soil developed in the Miocene Sand, ~ 8 % in the Loess soil and ~ 12 % at the Red Sandstone site. Thus GOC having no detectable 14C contents influences 14C ages of subsoil OC and thus may partly explain the strong 14C ages increase observed in many subsoils. This is particularly important in soils on terrestrial sediments with comparatively low amounts of OC, where GOC can considerably contribute to total OC stocks.

Fabian Kalks et al.

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Fabian Kalks et al.

Fabian Kalks et al.


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Short summary
Sedimentary rocks contain organic carbon that may end up as soil carbon. However, this source of soil carbon is overlooked and has not been quantified sufficiently. In this study we analysed 10 m long sediment cores with three different sedimentary rocks. All sediments contain considerable amounts of geogenic carbon contributing 3 to 12 % to total soil carbon below 30 cm soil depth. The low 14C content of geogenic carbon can result in underestimations of soil carbon turnover derived from 14C data.
Sedimentary rocks contain organic carbon that may end up as soil carbon. However, this source of...