The complexity of soil–plant–animal interactions in grassland ecosystems highlights the importance of studying biological phosphorus (P) cycling in relation with that of nitrogen (N). Several case studies employing original approaches developed by both agronomists and ecologists are presented. The nutrition index approach, based on nutrient dilution in the process of biomass accumulation, is used in order to evaluate the relative response of grassland to P in relation to changes in nutritional status (N and P). In parallel, the functional characterization of grassland vegetation from plant functional type (PFT) definition is presented. It relies on the fact that grassland communities may contain a wide diversity of species that influence nutrient biological cycling and regulate the biogeochemical cycle of nutrients. Finally, the effect of grazing herbivores on the biogeochemical cycle of major elements is examined to give a better insight into the complexity of the processes associated with N and P cycles in grazed grasslands.