Vertical partitioning and controlling factors of gradient-based soil carbon dioxide fluxes in two contrasted soil profiles along a loamy hillslope

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.