Soil organic carbon and nitrogen losses due to soil erosion and cropping in a sloping terrace landscape

Effects of soil erosion and cropping on soil organic carbon (SOC) stocks need to be addressed to better understand the processes of SOC loss following the conversion of natural ecosystems to agriculture. The aims of the present study were to: (1) understand the mechanism of SOC and total nitrogen (TN) losses in a small-scale agricultural landscape with sloping terraces; and (2) quantitatively assess vertical changes in SOC and TN of soil profiles at specific landscape positions and the lateral distribution of SOC and TN in areas with different soil erosion and deposition rates. Soil samples from cultivated land were collected at 5-m intervals along toposequences in different parts of hilly areas of the Sichuan Basin, China; uncultivated land was used as a reference for 137Cs, SOC and TN. The profile shape of SOC and total N depth distribution was markedly different between cultivated and uncultivated soils, with differences in descriptive coefficients of 2.1–3.4- and 2.0–3.2-fold for a, 1.2–2.2- and 1.0–1.8-fold for b, respectively, in the equation y = –aln(x) + b, where y is the depth SOC or TN concentration and x is the depth from the soil surface. SOC and TN concentrations in the surface soil horizon were significantly higher on uncultivated land (17.5 g kg–1) than on cultivated land (7.06–9.81 g kg–1). In particular, the 0–5 cm surface layer of uncultivated soils had 1.3-, 1.7-, and 2.3-fold higher SOC concentrations than that of the depositional, weak erosional and strong erosional areas, respectively, in cultivated soils. However, there were no significant differences in SOC and TN concentrations in subsoil layers between cultivated and uncultivated lands, suggesting that cropping is one of the factors causing SOC and N losses. SOC and TN inventories exhibited an increasing trend from the upper to toe proportions of the cultivated toposequences. In all the cultivated soils, SOC and TN concentrations of the surface soil horizon and inventories of SOC and TN were closely associated with 137Cs inventories (P < 0.001, P < 0.01, P < 0.0001 and P < 0.0001, respectively), suggesting that soil erosion has an important impact on SOC and TN dynamics in the cultivated landscape. The results of this study suggest that soil erosion and cropping result in SOC and N losses, and that soil erosion contributes to marked variations in SOC and N distribution along the slope transect within individual sloping terraces, as well as in the entire landscape.