Land use impacts on soil erosion and rejuvenation in Southern Brazil

Abstract

Topography is one of the key factors controlling soil erosion and redistribution of pedogenic material along slope. Land cover change can have an accelerating or retarding impact on topographically-controlled soil erosion rates, depending on the type and intensity of land use and management. In this study, we investigated the combined effect of hillslope gradient and land cover change on soil redistribution and rejuvenation in a subtropical region where Atlantic rain forest was converted to agricultural land. We used a two versus two factorial design, and evaluated the effect of hillslope gradient (steep vs. gentle) and land cover (forest vs. cropland) on the spatial pattern of soil weathering degree along slope. In four soil toposequences, soil weathering indices (Total Reserve in Bases, Chemical Index of Alteration, clay content, iron oxide content) and mineralogical assemblages were used to express genetic and morphological differences among soil profiles. Our data showed that the spatial differentiation in chemical weathering degree along slope is strongly dependent on the hillslope gradient: while the gentle slopes show negligible differences in chemical weathering degree along slope, the steep slopes show clear spatial differences. Besides, there is an interaction effect between hillslope gradient and land cover. Forest conversion to cropland enhances erosion-driven soil redistribution with a marked effect on soil rejuvenation along steep slopes but no clear effect along gentle slopes. The comparative study based on four toposequences highlights that accelerated soil erosion after conversion of forests to cropland has further enhanced lateral soil fluxes and redistribution of topsoil material along steep slopes, and led to soil rejuvenation and exposure of less weathered soil material at the eroding sites.