Quantitative estimation and vertical partitioning of the soil carbon dioxide fluxes at the hillslope scale on a loess soil

Abstract

Both modelling and experimental approaches have been applied to assess C exchange fluxes at large spatial scales. Yet, these approaches are subjected to substantial limitations and uncertainties. Here, we aim to highlight two key mechanisms able to improve the estimation of the hillslope aggregated CO2 fluxes: (i) the persistence of soil organic carbon (OC) in deep colluvium deposits; and (ii) the physical controls on CO2 fluxes along soil profiles. This study focuses on a sloping cropland in the central loess belt of Belgium. On two contrasted soil types along the studied hillslope, we recorded time-series of CO2 concentration, water content and temperature along 1 m long soil profiles during two periods of 6 months. Then, we calculated profiles of CO2 fluxes using the gradient method. To extrapolate these fluxes to entire yearly periods (2011–2013), we performed simulation using the SOILCO2RothC model. The vertical partitioning of the soil CO2 fluxes shows that ca. 90 to ca. 95% of the surface CO2 fluxes originates from the 10 first centimeters of the soil profile at the footslope. We show that high water filled pore space at this slope position disables the transfer of biotic CO2 along the soil profile. However, the total annual flux averaged along 3 years of simulation show that the top soil layer (0–10 cm) of the footslope generates CO2 fluxes (870 ± 64 CO2-C m−2 year−1 which exceed those observed at the summit position (583 ± 61 CO2-C m−2 year−1. Hence, our results reconcile two seemingly contradictory hypotheses, i.e. (i) these support that soil OC at such a footslope is stored along the main part of the soil profile and submitted to a long-term stabilization, and (ii) at the same time these support that the depositional footslope profile emits more CO2 than the summit, due to its high amount and quality of OC. Our results support the need to consider slopes when modeling soil-atmosphere C exchanges. If landscapes dynamic processes are not accounted for, we pointed out a risk to under-estimate annual soil-atmosphere CO2 exchanges by ca. 20%.