Abstract
Abstract. In this study we aim to elucidate the role of physical conditions and gas transfer mechanism along soil profiles in the decomposition and storage of soil organic carbon (OC) in subsoil layers. We use a qualitative approach showing the temporal evolution and the vertical profile description of CO2 fluxes and abiotic variables. We assessed soil CO2 fluxes throughout two contrasted soil profiles (i.e. summit and footslope positions) along a hillslope in the central loess belt of Belgium. We measured the time series of soil temperature, soil moisture and CO2 concentration at different depths in the soil profiles for two periods of 6 months. We then calculated the CO2 flux at different depths using Fick's diffusion law and horizon specific diffusivity coefficients. The calculated fluxes allowed assessing the contribution of different soil layers to surface CO2 fluxes. We constrained the soil gas diffusivity coefficients using direct observations of soil surface CO2 fluxes from chamber-based measurements and obtained a good prediction power of soil surface CO2 fluxes with an R2 of 92 %. We observed that the temporal evolution of soil CO2 emissions at the summit position is mainly controlled by temperature. In contrast, at the footslope, we found that long periods of CO2 accumulation in the subsoil alternates with short peaks of important CO2 release. This was related to the high water filled pore space that limits the transfer of CO2 along the soil profile at this slope position. Furthermore, the results show that approximately 90 to 95 % of the surface CO2 fluxes originate from the first 10 cm of the soil profile at the footslope. This indicates that soil OC in this depositional context can be stabilized at depth, i.e. below 10 cm. This study highlights the need to consider soil physical properties and their dynamics when assessing and modeling soil CO2 emissions. Finally, changes in the physical environment of depositional soils (e.g. longer dry periods) may affect the long-term stability of the large stock of easily decomposable OC that is currently stored in these environments.
Highlights
Soils play a major role in the global C budget, as they contain 2 to 3 times more C than the atmosphere (Eswaran et al, 1993; Lal, 2003)
While the volumetric water content (VWC) at the footslope remained high throughout the year, we observed that the soil surface CO2 flux dramatically increased when the air-filled pore spaces becomes high enough, which is illustrated by the gas diffusivity exceeding a threshold value of c. 0.1 cm2 d−1
We argue that the significantly higher CO2 concentrations observed at the footslope, especially for deeper soil layers, are related to the large amount of labile organic carbon (OC) that was found at this position, but more likely result from the long term accumulation of the CO2 produced by the mineralization of this large labile OC stock
Summary
Soils play a major role in the global C budget, as they contain 2 to 3 times more C than the atmosphere (Eswaran et al, 1993; Lal, 2003). Berhe et al, 2007; Van Oost et al, 2012; Wang et Published by Copernicus Publications on behalf of the European Geosciences Union. Van Oost et al, 2012). Some studies suggested an erosion-induced C source along hillslope ranging from 0.37 (Jacinthe and Lal, 2001) to 0.8–1.2 petagram C yr−1 (Lal, 2003). This shows that more quantitative information on the contribution of deep C to soil-atmosphere C exchange as well as an increased understanding of the controlling factors is needed
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