Long-term effects of daily grazing orbits on nutrient availability in Sahelian West Africa: 2. Effects of a phosphorus gradient on spatial patterns of annual grassland production

This paper reports on an investigation of the effect of a livestock-generated gradient in phosphorus-availability on spatial patterns of annual rangeland production around pastoral watering points in the Sahelian region of West Africa. The study was conducted along a major transhumance corridor used for at least the past century by herds managed by agropastoralists of the inland Niger Delta of Mall. Eighteen sites on sandy substrate were established at varying: distances from: (1) a more persistent pan from which livestock disperse to graze primarily during the early dry season (D1); and (2) a line of ephemeral pans from which livestock disperse to graze during the rainy season (D2). Data on the species composition, standing herbaceous mass, and annual herbaceous cover were collected at these sites over the 2 years following a significant drought. These vegetative parameters show considerable interannual variation in their spatial pattern. As reported elsewhere (companion paper), chemical analyses of soil and plant samples collected at these sites show a significant negative correlation between phosphorus availability and D2. This is consistent with livestock-mediated nutrient transfer from outlying zones toward rainy-season pans. The effect of this phosphorus gradient on annual rangeland production was analysed using a regression modelling approach. Two vegetative production parameters, vegetated surface fraction (VSF) and herbaceous above-ground standing mass of vegetated surface (VAGM), were regressed in a two-and three-step process on the following factors: (1) background soil and rainfall data; (2) forb fraction of herbaceous production (for VAGM only), followed by: (3) D1 and D2. These analyses find that while the phosphorus-availability gradient had a significant effect on spatial patterns of herbaceous production during the year immediately following the drought, it had little effect in the subsequent year's production. This difference is explained by the increased importance of nitrogen in limiting herbaceous production during the second year, resulting from its different rainfall history.