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

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.