The utility of 137Cs for measuring soil redistribution rates in southwest Niger

Measurable quantities of Cs-137 were found in soil samples from profiles on a toposequence in southwest Niger. Erosional and depositional sites were identified by comparing Cs-137 inventories with a reference site, although, because of the addition of dust, the identification of such a site was difficult. A first approximation of the reference inventory (2066 +/- 125 Bq m(-2)) was achieved by modelling. Marked disparities between the Cs-137 profiles and the reference profile at some sites may be due to the eluviation of Cs-137 fixed to clay, where soils have been exposed to cycles of wetting and drying. This hypothesis is supported by principal components analysis and non-hierarchical multivariate classification. Other divergences from the reference profile were interpreted as the results of the redeposition of material with small Cs-137 content, derived from subsoil and gully walls. These complications created the need for additional modelling to estimate the net soil flux. At some sites the net soil flux was calculated using models that related Cs-137 movement to soil redistribution. It appears that a vegetation canopy protects accumulated dust from water erosion on steep slopes and from wind erosion on gentle slopes. The net soil flux was found to be -16 +/- 2 t ha(-1) yr(-1). (C) 1998 Elsevier Science B.V.