Effects of livestock grazing on physical and chemical properties of sandy soils in Sahelian rangelands

The effects of grazing by livestock on soil surface features, bulk density and chemical properties were studied at the completion of a 4-year grazing experiment carried out in Sadore, Niger. Grazing treatments were a factorial arrangement of two stocking rates (62.5 and 125 kg live weight ha(-1)) and four sheep:goat ratios (0:6, 2:4, 4:2 and 6:0 animals per pasture), with two pastures per treatment and two ungrazed controls. Observations were also made in a fallow subjected to 9 years of intense and uncontrolled mixed grazing, and in a site that had been protected from grazing for 15 years. The topsoil was sampled (at depths of 0-2, 2-6, 6-14 and 14-30 cm) below shrub canopy in herbaceous vegetation and in bare soil patches within each of 20 paddocks for determination of pH, organic C, and total N and P concentrations. Soil bulk density was measured in a subset of soil profiles. The areal extent of different types of soil crusts and other soil surface features was assessed in one-half of the paddocks. Grazing resulted in a reduction (p < 0.01) and fragmentation of the area of crusted soils. However, this trend was partially compensated for by an increase of newly formed crusts. As a result, the soil infiltration index slightly increased with moderate grazing, but decreased at higher stocking rates. Compaction due to trampling was observed ih the topsoil beneath the shrub canopy and also in vegetated patches, but only under intense grazing pressure. Soil bulk density was not affected by grazing except for an increase observed below 10 cm depth at the understorey of shrubs which is therefore unlikely due to trampling. When compared to the ungrazed control, pH, organic C and N concentrations, and to lesser extent P concentration, decreased after 4 years of grazing. Soil P and pH further decreased after 9 years of very high grazing pressure. However, neither N nor organic C decreased further. (C) 1999 Academic Press.