Impact of land use on runoff, sediment and carbon exports in two small headwater catchments in central Morocco

Soil organic carbon (SOC) is a key property of a wide range of soil functions related to soil stabilization, carbon sequestration, water/nutrients/contaminants retention, etc. As soil erosion is a very active process in the Mediterranean environment, it is likely to have a considerable impact on the stocks and future of SOC. In this study, we explore the impact of land use on water, sediment and carbon exports by considering two small headwater catchments in central Morocco with contrasting land use. Both catchments are located in the same area (less than 2 km) in a carbonate soil environment and semi-arid climate. They have fairly similar sizes (about 4ha), soil types and topsoil SOC stocks, but differ mainly in their land use: one catchment is cultivated with annual cereal crops while the other is a degraded catchment under sparse thuja (tetraclinis articulata) forest cover. The water, sediment and carbon exports were monitored for each flood during 2016-2017 hydrological year. The results indicate that all runoff, sediment and carbon exports occurred within a few days and that the majority of carbon exports (i.e. 82% in the agricultural basin and 78% in the degraded forest basin) were in organic form, with a strong dominance of particulate organic carbon. The export of organic carbon for each flood in the two catchments was closely linked to the quantities of sediment exported (R² > 0.97), the latter being highly variable from one event to another and therefore difficult to predict. Compared to the agricultural catchment, the degraded forest catchment exported more than 5 times as much runoff and between 2 and 3 times as much sediment and carbon annually. The degraded forest catchment therefore lost more carbon than its cultivated neighbour, which could increase its sensitivity to erosion in the form of a negative feedback loop. There is therefore an urgent need to consider better specific soil protection measures in this type of environment.