Effects of land-use change on soil erosion over the last century: Reconstruction from peri-urban sedimentary archives located in the instrumented site OBSCURE (SNO Observil, France)

Numerous publications pointed out that ‘urbanization’ correspond to the expansion of built-up areas at the expense of agricultural and forest land. The ecosystem services provided by soils in such zones change, as they are particularly susceptible to erosion. Understanding these changes over time is crucial for the sustainable management of land and the conservation of the environment, including surface water quality. This study sought to evaluate the effects of recent land use modifications brought about by soil artificialization on the pond sediment budget, from AD1955 to AD2018. Reconstruction of land use changes, based on historical map and aerial photography of the Beulie pond watershed was combined with stratigraphy and sediment-based geochemistry. Specific organic tracers were used to quantify soil fluxes and assess their relationship with catchment artificialization through time. Results show that even a slight increase of built-up land (from 2 to 13%, here) can drastically increase soil erosion and sediment transfer, multiplying per 10 accumulation rate into the pond over the last century. Despite the pond appearing anoxic and eutrophic, this does not lead to algal blooms, as it is linked to increasing allochthonous fluxes of fresh organic matter from soil erosion. Soil fluxes finally reached a very high figure of 3 t.ha⁻¹.year⁻¹, compared with non-urban lowland environments in temperate climates. A significant acceleration in soil erosion occurred after AD1990, finally stabilizing despite the watershed being reforested. This suggests that, in urban contexts, once soil is disturbed, erosion processes involving runoff phenomena and potentially gully dynamics continue to occur.