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

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.

Heterotrophic soil respiration and carbon cycling in geochemically distinct African tropical forest soils

“…Tropical forests and the soils therein are one of the most important and largest global terrestrial carbon (C) pools and serve as important climate regulators (Cleveland et al, 2011;Kearsley et al, 2013;Lewis et al, 2009;Sayer et al, 2011). They contain about one-third (421 Pg C) of the global soil organic carbon (SOC) stock in the upper 1 m of soil (Köchy 640 B.…”

Probing the exchange of CO2 and O2 in the shallow critical zone during weathering of marl and black shale

“…However, the volume of material that contributes to the measured CO2 fluxes is rarely quantified. If this could be determined, the production of CO2 can be considered in terms of the mass of reactants, allowing comparisons between different field sites and laboratory experiments (e.g., Angert et al, 2015;Kalks et al, 2021;Lefèvre et al, 2014;Soucémarianadin et al, 2018;Tokunaga et al, 2016). In the case of the internal rock chambers used here, quantification of the contributing rock volume would allow us to upscale the fluxes over an outcrop surface area.…”

Global patterns of radiocarbon depletion in subsoil linked to rock-derived organic carbon

“…Within the upper 1 m, there are an estimated 1100 petagrams of carbon (PgC) present as radiocarbon ( 14 C) "dead" rock organic carbon (OC petro ) (Copard et al, 2007), equivalent to the carbon stock in soils derived from the biosphere (Jackson et al, 2017). When soil forms on sedimentary rocks, OC petro is a "bottom up" input of carbon that can contribute to the soil OC pool, especially in deep soils with low OC concentrations (Hemingway et al, 2018;Kalks et al, 2021). While OC petro could be widespread in soils developed on sedimentary rocks, observations come from only a handful of weathering profiles, including OC petro -rich rocks, such as black shales which are not globally representative (Petsch et al, 2001).…”

“…In Himalayan rivers, the river OC also has a linear relationship between these variables (p < 0.05, R 2 = 0.37), albeit with a shallower slope due to lower OC petro contents (Galy et al, 2015). In soils, OC petro has been recognised in only a few studies (Petsch, 2014;Hemingway et al, 2018;Hilton et al, 2021;Kalks et al, 2021). In Taiwan, high erosion rates continuously supply fresh bedrock to the surface and keep surface soils young, and these samples have a linear relationship between Δ 14 C and 1/[OC].…”