Can we understand and predict the regulation of biological N2 fixation in grassland ecosystems

We discuss results from controlled environment studies including mesocosms, grazing experiments and long term field experiments which show how biological N2 fixation in legume based systems is tightly coupled to the N demand at scales ranging from the individual plant to the grassland ecosystem. We further test the consequences of this hypothesis of a feedback regulation of biological N2 fixation by N demand with a mechanistic model linking plant community dynamics and ecosystem functioning. Results confirm the heuristic power of this hypothesis which accounts for a number of observations concerning changes in the relative abundance and N2 fixation rate of legumes in managed grasslands. Then we show how nitrogen and carbon fluxes are affected by plant-plant (e.g. competition and facilitation), plant-soil and plant-herbivore interactions and by climate and management changes.