SOIL FLUX (LOSS AND GAIN) IN SOUTHWESTERN NIGER AND ITS AGRICULTURAL IMPACT

It is widely believed that wind and water erosion are widespread in th e Sahel, but there is little evidence either for the rate of soil loss or for its agricultural impact. In the present study the radionuclide caesium-137 (Cs-137) was used to make net time-integrated (30-year) m easurements of soil flux (loss and gain) at a site in southwestern Nig er. Accelerating soil gains occurred where the surface is protected by woody vegetation. The source of this material may be secondary entrai nment of Harmattan dust following the removal of vegetation elsewhere. The accumulation rate at these sites for the last 30 years was +3.5 /- 0.2 t ha(-1) yr(-1). This compares well with data from monitoring t he dust in the region over the last 8 years (Drees, et al., 1993). How ever, the net soil flux for the study area was -48.5 t ha(-1) yr(-1), which is four times as large as Lal's (1993) estimate for this region. Point samples in the main agricultural fields have revealed soil loss es of between 26 t ha(-1) yr(-1) and 46 t ha(-1) yr(-1). Yet when the agricultural system in these fields is examined, it is found to be so complex that it is difficult to assess the impact of these rates of lo ss. (C) 1998 John Wiley & Sons, Ltd.