Long-term effects of daily grazing orbits on nutrient availability in Sahelian West Africa: I. Gradients in the chemical composition of rangeland soils and vegetation

An investigation of the long-term effects of livestock grazing on the spatial distribution of plant nutrients was conducted in the Sahelian region of West Africa. The study area was located along the transhumance corridor used for at least the past century by a long-established population of agropastoralists in central Mall. Spatial Variation of plant and soil chemistry across eighteen sites was investigated with respect to a grid, whose axes are defined by distance from a pan from which livestock disperse to graze primarily during the early dry season (D1) and by distance from the nearest of three pans used primarily during the rainy season (D2). Particle size distribution, pH, total C, N, and P of surface soil (0-10 cm) was determined for four composite samples collected at eleven of the eighteen ecological sites. Elemental analysis (N, P, Ca, Mg, K, Mn, Fe, Zn, Cu) was performed on above-ground tissue samples (excluding seeds) of the three most common grasses and the most common legume collected at all eighteen sites at the end of the rainy season. Results demonstrate that long-term grazing movements can, over the long-term, affect nutrient availability over wider spatial scales (5 km radii) than previously observed by shorter-term studies. Soil and plant chemistry were found in general to be most affected by the historic gradient of cattle presence during the rainy season (D2). Controlling for soil C and coarse sand fraction, soil pH was found to decline with D2 (-0.5 pH units per km D2) and to a lesser extent DI (-0.15 pH units per km D1), a finding that cannot be explained solely by a consideration of local geomorphology. The pH gradient was found to significantly affect micronutrient concentrations in plant tissues. Phosphorus (P) concentrations of plant tissue were found to decrease with D2. The increase in P-availability near rainy-season encampments could result from P-release from Al-complexes at higher pH and/or by the redistribution of P from outlying zones towards rainy-season encampments.