Contrasting impacts of grass species on nitrogen cycling in a grazed Sudanian savanna

We investigated the impact of perennial and annuals grass species on nitrogen cycling in a Sudanian savanna of Burkina Faso. We also analysed how the local context in terms of grazing and soil properties modifies these impacts. We selected four plots differing both by the intensity of grazing by cattle and soil depth, and used soil and grass biomass ¹⁵N as integrative indicators of N cycle. If perennials are able to foster a more efficient nitrogen cycling there should be lower ¹⁵N abundances in their biomass and soil. If soil depth and cattle pressure significantly modify nitrogen fluxes, soil depth and cattle pressure should influence ¹⁵N signatures. Our results suggest that perennial grasses are more conservative for nitrogen (inhibition of nitrification, less leaching via a perennial root system, slower cycling). The increase in leaf δ¹⁵N with N concentration is steeper in Loudetia togoensis than in the three other grasses. No significant difference was found between the ¹⁵N signatures of the four plots. Our results on ¹⁵N signatures and the fact that perennial grasses are much more abundant in the plots that are less grazed and have deeper soils, confirm that the switch from perennial to annual grasses is linked to a degradation in soil fertility and pasture quality. This suggests that ¹⁵N signatures can be used as indicators of fertility.