Abstract
Abstract. Geogenic organic carbon (GOC) from sedimentary rocks is an overlooked fraction in soils that has not yet been quantified but influences the composition, age, and stability of total organic carbon (OC) in soils. In this context, GOC is the OC in bedrock deposited during sedimentation. The contribution of GOC to total soil OC may vary, depending on the type of bedrock. However, no studies have been carried out to investigate the contribution of GOC derived from different terrestrial sedimentary rocks to soil OC contents. In order to fill this knowledge gap, 10 m long sediment cores from three sites recovered from Pleistocene loess, Miocene sand, and Triassic Red Sandstone were analysed at 1 m depth intervals, and the amount of GOC was calculated based on 14C measurements. The 14C ages of bulk sedimentary OC revealed that OC is comprised of both biogenic and geogenic components. The biogenic component relates to OC that entered the sediments from plant sources since soil development started. Assuming an average age for this biogenic component ranging from 1000–4000 years BP (before present), we calculated average amounts of GOC in the sediments starting at 1.5 m depth, based on measured 14C ages. The median amount of GOC in the sediments was then taken, and its proportion of soil mass (g GOC per kg−1 fine soil) was calculated in the soil profile. All the sediments contained considerable amounts of GOC (median amounts of 0.10 g kg−1 in Miocene sand, 0.27 g kg−1 in Pleistocene loess, and 0.17 g kg−1 in Red Sandstone) compared with subsoil OC contents (between 0.53 and 15.21 g kg−1). Long-term incubation experiments revealed that the GOC appeared comparatively stable against biodegradation. Its possible contribution to subsoil OC stocks (0.3–1.5 m depth) ranged from 1 % to 26 % in soil developed in the Miocene sand, from 16 % to 21 % in the loess soil, and from 6 % to 36 % at the Red Sandstone site. Thus, GOC with no detectable 14C content influenced the 14C ages of subsoil OC and may partly explain the strong increase in 14C ages observed in many subsoils. This could be particularly important in young soils on terrestrial sediments with comparatively low amounts of OC, where GOC can make a large contribution to total OC stocks.
Highlights
On average, the world’s soils store more than 50 % of organic carbon (OC) in the subsoil below 30 cm depth (Batjes, 2014)
The world’s soils store more than 50 % of organic carbon (OC) in the subsoil below 30 cm depth (Batjes, 2014). This type of carbon is considered a highly stable carbon pool due to its apparently high 14C ages (Mathieu et al, 2015; Schrumpf et al, 2013). Another explanation for this could be the contribution of geogenic organic carbon (GOC), which is defined here as OC that originates from deposition during sedimentation and rock formation and may increasingly influence subsoil OC with depth (Graz et al, 2010; Kögel-Knabner et al, 2008; Schrumpf et al, 2013; Trumbore, 2009)
Using three laboratory replicates per sample and comparing them with the muffled samples, measurable OC contents were detected in all the sediments analysed
Summary
The world’s soils store more than 50 % of organic carbon (OC) in the subsoil below 30 cm depth (Batjes, 2014) This type of carbon is considered a highly stable carbon pool due to its apparently high 14C ages (Mathieu et al, 2015; Schrumpf et al, 2013). Another explanation for this could be the contribution of geogenic organic carbon (GOC), which is defined here as OC that originates from deposition during sedimentation and rock formation and may increasingly influence subsoil OC with depth (Graz et al, 2010; Kögel-Knabner et al, 2008; Schrumpf et al, 2013; Trumbore, 2009). More information about the amount of GOC in sediments is needed
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