Combined effect of geomorphic and pedogenic processes on the distribution of soil organic carbon quality along an eroding hillslope on loess soil

Abstract

At the scale of hillslopes, a detailed mechanistic understanding of the processes controlling OC stabilization is still lacking. Here, we aimed to study the impact of geomorphic and pedogenic processes on the distribution of OC quality (ability of OC to release carbon dioxide through metabolic pathways) along an agricultural hillslope in the Belgian loess belt. We collected soil cores at four topographic positions along the hillslope (summit, convex shoulder, backslope and footslope). We assessed (i) cumulative soil erosion using diagnostic soil horizons and the 137Cs techniques, (ii) OC stocks and its quality (NaOCl-resistant OC), and (iii) reactive soil mineral phases (concentration of Fe, Al and Si in specific oxalate and dithionite–citrate–bicarbonate extractants).Our results show that ongoing erosion has resulted in a small amount of reactive soil phases (e.g. Fe and Al-oxyhydroxides) in the upper first meter of the most eroded soil profile (backslope position). The erosion observations show that this is related to the truncation and rejuvenation of the backslope soil profile by bringing unweathered and calcareous loess to the soil surface. As a consequence, the potential of soil to stabilize OC by molecular interactions with soil minerals is substantially reduced by erosion when calcareous loess is reached. This was supported by the observed amount of mineral-protected OC (using NaOCl-resistant OC as an indicator) which was significantly lower at the eroded midslope than at the other slope positions. The combined effect of geomorphic and pedogenic processes thus strongly impacts the distribution of soil OC quality along the hillslope. We observed a spatial differentiation of the labile OC pool (i.e. the OC not resistant to NaOCl) along the hillslope with a significant enrichment at the depositional site. The labile OC pool contributed 64±5%, 69±5%, 40±22% and 49±6% of total OC at the footslope, backslope, convex shoulder and summit, respectively. Despite the fact that a part of this high labile OC stock at the footslope (5.8±0.2kgOCm−2) can be protected from microbial degradation due to specific environmental conditions, our results suggest that a large part of this depositional OC stock has a high potential for mineralization given its quality.