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

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 CO 2 fluxes and abiotic variables. We assessed soil CO 2 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 CO 2 concentration at different depths in the soil profiles for two periods of 6 months. We then calculated the CO 2 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 CO 2 fluxes. We constrained the soil gas diffusivity coefficients using direct observations of soil surface CO 2 fluxes from chamber-based measurements and obtained a good prediction power of soil surface CO 2 fluxes with an R 2 of 92 %.We observed that the temporal evolution of soil CO 2 emissions at the summit position is mainly controlled by temperature. In contrast, at the footslope, we found that long periods of CO 2 accumulation in the subsoil alternates with short peaks of important CO 2 release. This was related to the high water filled pore space that limits the transfer of CO 2 along the soil profile at this slope position. Furthermore, the results show that approximately 90 to 95 % of the surface CO 2 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 CO 2 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.

Soil erosion affects variations of soil organic carbon and soil respiration along a slope in Northeast China

“…Relatively inconsistent views about the effect of soil redistribution on the spatial variability of CO 2 effluxes exist. Some studies reported higher respiration rates at footslope positions relative to summit and shoulder positions (Reicosky et al 2005;Webster et al 2008;Wiaux et al 2015), but opposite trends were also observed in some regions (Epron et al 2006;Wei et al 2014;Zhang et al 2016).…”

“…These results support that the SOC at such a footslope is stored along the soil profile and involved in a long-term stabilization process and simultaneously support that the depositional profile emits more CO 2 than the summit due to its high amount and quality of SOC. Wiaux et al (2015) reported that in crop soils, approximately 90 to 95% of the fluxes originated from the first 10 cm of the soil profile at the footslope, while at the summit, the first 30 cm of the soil profile significantly contributed to surface fluxes. These results suggest that the depositional topsoil SOC is prone to be mineralized, and the SOC in this depositional context is stabilized at subsoil depth.…”

Soil Respiration in the Profiles of Forest Soils in Inland Dunes

“…Location within the relief also to a large extent defines the soil profile structure. The research of Wiaux [15] showed that carbon dioxide excretion from a soil profile at the peak of a loess hill significantly differs from that at the foot of the hill. This was evidently due to differences in the thermal-humidity conditions, in both the surface and deeper horizons.…”

“…This was evidently due to differences in the thermal-humidity conditions, in both the surface and deeper horizons. The investigations of Wiaux [15] primarily concerned CO 2 excretion from the soil surface, but measurements carried out at different depths showed how the physical characteristics of every soil horizon can impact the intensity of CO 2 excretion.…”

New Sensor Technology for Field‐Scale Quantification of Carbon Dioxide in Soil

“…Compared with basic soil properties (e.g., soil texture, porosity), direct information on O 2 and/or CO 2 concentrations has been determined to be more sensitive for the evaluation of soil aeration and respiration (Klimanek and Greilick, 1981;Ben-Noah and Friedman, 2018). Direct measurements of soil gas concentrations can thus be highly valuable to improve our mechanistic understanding of biological soil processes and their interaction with physical soil properties Šimůnek and Suarez, 1993;Risk et al, 2002;Hashimoto and Komatsu, 2006;Taneva et al, 2006;Riveros-Iregui et al, 2007;Blagodatsky and Smith, 2012;Wiaux et al, 2015). Direct measurements of CO 2 concentrations, together with modeled or estimated diffusivity, provide direct information on depth-dependent respiration and flux as implemented in the soil-CO 2 profile method (Risk et al, 2002;Hirano et al, 2003;Pumpanen et al, 2003a;Tang et al, 2003;DeSutter et al, 2008;Han et al, 2014;Xiao et al, 2015;Wiaux et al, 2015).…”