Carbon exchange associated with accelerated erosion following land cover change is an important component of the global C cycle. In current assessments, however, this component is not accounted for. Here, we integrate the effects of accelerated C erosion across point, hillslope, and catchment scale for the 780-km 2 Dijle River catchment over the period 4000 B.C. to A.D. 2000 to demonstrate that accelerated erosion results in a net C sink. We found this longterm C sink to be equivalent to 43% of the eroded C and to have offset 39% (17-66%) of the C emissions due to anthropogenic land cover change since the advent of agriculture. Nevertheless, the erosion-induced C sink strength is limited by a significant loss of buried C in terrestrial depositional stores, which lagged the burial. The time lag between burial and subsequent loss at this study site implies that the C buried in eroded terrestrial deposits during the agricultural expansion of the last 150 y cannot be assumed to be inert to further destabilization, and indeed might become a significant C source. Our analysis exemplifies that accounting for the nonsteady-state C dynamics in geomorphic active systems is pertinent to understanding both past and future anthropogenic global change.
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.
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