Vertical partitioning of CO<sub>2</sub> production in a forest soil

Abstract

Abstract. Large amounts of total organic carbon are temporarily stored in soils, which makes soil respiration one of the major sources of terrestrial CO2 fluxes within the global carbon cycle. More than half of global soil organic carbon (SOC) is stored in subsoils (below 30 cm), which represent a significant carbon (C) pool. Although several studies and models have investigated soil respiration, little is known about the quantitative contribution of subsoils to total soil respiration or about the sources of CO2 production in subsoils. In a 2-year field study in a European beech forest in northern Germany, vertical CO2 concentration profiles were continuously measured at three locations, and CO2 production was quantified in the topsoil and the subsoil. To determine the contribution of fresh litter-derived C to CO2 production in the three soil profiles, an isotopic labelling experiment, using 13C-enriched leaf litter, was performed. Additionally, radiocarbon measurements of CO2 in the soil atmosphere were used to obtain information about the age of the C source in the CO2 production. At the study site, it was found that 90 % of total soil respiration was produced in the first 30 cm of the soil profile, where 53 % of the SOC stock is stored. Freshly labelled litter inputs in the form of dissolved organic matter were only a minor source for CO2 production below a depth of 10 cm. In the first 2 months after litter application, fresh litter-derived C contributed, on average, 1 % at 10 cm depth and 0.1 % at 150 cm depth to CO2 in the soil profile. Thereafter, its contribution was less than 0.3 % and 0.05 % at 10 and 150 cm depths, respectively. Furthermore CO2 in the soil profile had the same modern radiocarbon signature at all depths, indicating that CO2 in the subsoil originated from young C sources despite a radiocarbon age bulk SOC in the subsoil. This suggests that fresh C inputs in subsoils, in the form of roots and root exudates, are rapidly respired, and that other subsoil SOC seems to be relatively stable. The field labelling experiment also revealed a downward diffusion of 13CO2 in the soil profile against the total CO2 gradient. This isotopic dependency should be taken into account when using labelled 13C and 14C isotope data as an age proxy for CO2 sources in the soil.

Biogeochemical limitations of carbon stabilization in forest subsoils#

“…Three subsoil monitoring observatories (distance about 60 m) were established in 2013 in Grinderwald (Leinemann et al., 2016). They are equipped with multi‐segment suction plates for soil solution sampling (25 × 25 cm, ecoTech Umwelt‐Meßsysteme GmbH, Bonn, Germany), with 16 squared segments (36 cm 2 ) each, installed at 10, 50, and 150 cm soil depth (Leinemann et al., 2016), suction cups, and CO 2 sensors at 10, 30, 50, and 90 cm soil depth for analysis of soil CO 2 concentration profiles in combination with closed chambers for measurement of total soil CO 2 efflux (Wordell‐Dietrich et al., 2020) (Figure S1A). A field labeling approach was performed on the 13.2 m 2 circular catchment area of each of the three monitoring observatories by replacing the natural litter layer on 50% (i.e., 6.6 m 2 per observatory) of the area with 13 C‐enriched litter (Figure S1B), the other 50% remained unlabeled.…”

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

“…The incubation temperature of 20 • C should, therefore, be considered to be above the typical mean temperatures in the subsoils. For subsoils with comparable climatic conditions, Wordell-Dietrich et al (2020) found temperatures at 150 cm depth ranging from 4 to 14.4 • C over a 2-year period. Assuming a Q 10 value of around two for the assumable difference (Hamdi et al, 2013), the respiration rate at typical subsoil temperatures would be roughly half that.…”

“…To be able to compare OC stocks and contributions from GOC, it was decided to set the borders between the topsoils and subsoils at 0.3 m and the transition from subsoils to the sediments at 1.5 m. According to Richter and Markewitz (1995), this represents a common border for the transition from soil to sediment. The sediments were further subdivided into an upper and a lower part at a depth of 4 m. (Eckelmann et al, 2006) and later transferred to the World Reference Base (WRB, 2006). Depth transitions to the starting sediment were 82 cm for loess (Al + Bt + elCv), 72 cm for Miocene sand (Bv − ilCv), and 68 cm for Red Sandstone (ilCv 1 ).…”

“…Figure 1.Respective soil profiles at the loess (a), Miocene sand (b), and Red Sandstone (c) sites. Soil classification was conducted using the German classification system for soil horizons(Eckelmann et al, 2006) and later transferred to the World Reference Base(WRB, 2006). Depth transitions to the starting sediment were 82 cm for loess (Al + Bt + elCv), 72 cm for Miocene sand (Bv − ilCv), and 68 cm for Red Sandstone (ilCv 1 ).…”

“…Solling Formation and Triassic) under European beech forest (Fagus sylvatica) 11.5 km northeast of Göttingen (51 • 35.012 N, 10 • 3.960 E) and is referred to below as Red Sandstone. The soil is classified as a Folic Brunic Arenosol according to the World Reference Base for Soil Resources(WRB, 2006). The sediments were loessic de-…”