Towards integrating the influence of erosion on within field variability of C input, stock and stability in regional SOC estimates

Improved management of the SOC pool has become an important issue for policymakers in order to maintain soil quality and reduce climate change. Hence, there exists an increased interest in accurate mapping of SOC at the regional scale. Most of these studies are limited to topsoil, consider only factors present at the landscape scale (e.g. climate, land use, soil type) and are at rather coarse resolution. Consequently, the variability of SOC at smaller scales in complex terrain driven by soil erosion, such as stable subsoil carbon buried in depositional areas, is still rather understudied and is not (well) presented in these estimates. Nevertheless, incorporating this smaller level of spatial detail will most probably have a major influence on regional SOC stock dynamics' calculations and mapping. In the present study we aim to unravel the variation in quantity and quality of SOC depth distributions along typical hillslope transects under cropland (Devon, UK) and relate these to soil redistribution rates and variations in C input, i.e. below and above ground biomass productivity. The radionuclide isotope Caesium-137 (137Cs) was used as proxy for erosion. The results show contrasting vertical patterns in SOC stock and stability depending on the rate and type of erosion. For example, sites characterized by deposition due to water erosion (i.e. foot slope) have much higher SOC values near the surface, but show a fast decline with depth, while sites characterized by deposition due to tillage erosion (i.e. most concave position) have moderated SOC surface values that stay constant until a depth of 50 cm, but with increasing stability with depth. The above ground biomass productivity is most linked to water erosion, since we found lowest above ground biomass at the steepest slope position and the highest above ground biomass at the foot slopes. Furthermore, root biomass in the most concave section is significantly higher as compared to any other topo-position. The present approach allows us to unravel the influence of lateral transport processes (erosion) on the within field vertical heterogeneity in SOC stock and stability and associated variation in C input, and hence can be considered as a first step to refine spatial-temporal mapping of SOC at regional scales.