Factors controlling soil organic carbon persistence along an eroding hillslope on the loess belt

Abstract

The integration of hydrologic, geomorphic and biogeochemical approaches is required to determine organic carbon (OC) persistence and dynamics within landscapes. Here, we studied the persistence of OC along an eroding hillslope using soil in situ surface heterotrophic respiration measurement as an indicator. Along this topographical gradient, we quantified the space-time distribution of soil water and temperature as well as the amount of labile OC (i.e. NaOCl-non-resistant) in relation to CO2 fluxes. We used a Generalized Least Square (GLS) regression model (i) to identify the role of each abiotic factor as well as their interactions, and (ii) to quantify spatial differences in CO2 fluxes along the hillslope.We observed significant differences in respiration along the topographical gradient, i.e. 30% more at the downslope and 50% more at the backslope, relative to the uneroded summit position. Soil OC persistence along the hillslope was mainly controlled by the labile OC stock and moisture content. Our in-situ measurements also indicated that the temperature sensitivity of soil OC (as expressed by Q10 values) was strongly related to soil water content. The large stock of OC stored in colluvial soils has a higher temperature sensitivity (Q10 = 3.72 ± 0.17) than in non-depositional slope positions (Q10 = 1.77 ± 0.18 to 2.59 ± 0.13). Given the large stock of labile OC, its high temperature sensitivity and the high water contents that are needed to stabilize OC, we conclude that the OC stored in colluvial soils is particularly vulnerable to OC mineralization under drier/warmer conditions. When considering the large amount of OC that has been buried globally as a result of agricultural erosion during the last decades, the long-term stability of this pool under future land use and/or climate disturbance is an area of concern.