The effects of changing land use on soil erosion and sediment mobilisation in two small catchments in Southern Italy.

Soil erosion is a major concern for the sustainable management of agricultural land because it reduces soil productivity, degrades stream water quality and increases water pollution risk for humans and aquatic ecosystems. In Southern Italy, where soil loss can exceed 100-150 t ha-1 year-1, there is a drive to minimize erosion by implementing improved management practices and erosion control measures. Among these practices, a change in land use involving reafforestation of large areas aimed at providing increased protection of the soil from rainfall and runoff has proved to be highly effective in terms of reducing erosion risk. In order to devise meaningful land-use policies and to select effective soil conservation practices, it is important to evaluate the importance of forest cover in reducing soil erosion. In this context, the use of environmental radionuclides, and particularly 137Cs and excess 210Pb, to estimate soil erosion has proved to possess several important advantages over traditional techniques, if reliable conversion models are available for estimating soil redistribution rates. The study reported focuses on two small catchments (W2 and W3) located in Calabria, Southern Italy, for which measurements of suspended sediment yield are available. Both the catchments originally supported a rangeland vegetation cover and they are subsequently reafforested with Eucalyptus trees in 1968. Currently, only catchment W3 shows a continuous forest cover whereas catchment W2 shows some bare areas that preserve the natural vegetation. Within catchment W2 two additional erosion plots were established in 1994 in order to explore the effect of the tree cover on soil erosion. Measurements of sediment yield from the two catchments and the plots for several storm events and associated information on the 137Cs and excess 210Pb of the sediment have been used to evaluate the effects of the conversion from rangeland to forest cover on sediment mobilisation and soil loss. The analysis showed that the areas with the highest soil loss are associated with the slopes where the tree cover is discontinuous, emphasizing the importance of vegetation cover in influencing rates of soil loss in the study catchments. The results have also been used to validate some of the basic assumptions commonly associated with the use of mass balance models for estimating rates of soil loss from radionuclide measurements.