An investigation of soil erosion and redistribution in a Mediterranean lowland agricultural catchment using caesium-137

摘要

This study sought to contribute to an improved understanding of soil erosion and redistribution on Mediterranean agricultural land, where traditional soil conservation practices have been applied over millennia to provide effective protection of cultivated land. The study was undertaken in the Na Borges catchment, a groundwater-dominated lowland limestone basin (319 km2), located in the northeastern part of Mallorca, Spain. The average sediment yield from the basin, based on river sediment load data, is < 1 t/km2·yr. The 137Cs technique was used to quantify soil redistribution rates over the past 40 years and to identify the key factors involved in soil erosion and redistribution processes. To estimate erosion and deposition rates and to elucidate the main factors affecting soil redistribution, samples were collected from six slope transects representative of the local land use and slope gradients and the presence or absence of soil conservation practices. A mass balance and a profile distribution conversion model were used for cultivated areas and areas of natural vegetation, respectively, to derive point estimates of the soil redistribution rates from the 137Cs inventories measured for individual soil bulk cores. In areas without soil conservation practices, the estimated mean soil erosion rates ranged from 12.7 to 26.4 t/ha·yr, which correspond to the slight and moderate erosion classes. The erosivity of Mediterranean climatic conditions combined with the influence of agricultural practices and slope gradient on soil erosion, represent the main factors responsible for the variation of soil losses documented for the cultivated land located in downslope areas, in the absence of soil conservation practices. Deposition dominated for those transects affected by soil conservation practices, with rates ranging between 18.8 and 96.6 t/ha·yr. However, this situation does not mean that soil conservation measures retain all the sediment, but rather that agriculture and urbanization (i.e. new rural paths and stone boundaries) modified the micro-topography and diverted sediment from other upslope zones towards the slopes where sampling transects were located.