I am studying the role of legume trees in the nitrogen cycle of tropical dry forests of Costa Rica. Legumes belong to the Fabaceae plant family and differ from other plant species in that because they can form a symbiotic relationship with soil bacteria which fix nitrogen from the atmosphere and convert it to a form that the plants can use. I want to understand to what extent legumes with their nutrient rich lifestyle influence the soil environment around them on a tree basis and at the level of a plant community. It is crucial to understand how the dry forest ecosystems work: they are the most endangered tropical biome because their long dry season has made them favourable has exposed them to for human settlement and for to agricultural uses. Furthermore it is still difficult to establish how humans are altering the global nitrogen cycle through fossil fuel combustion, fertilizer production and the cultivation of N-fixing crops or what the consequences are for ecosystem functions in the future. In order to understand how much to what extend this process has been artificially perturbed, I consider necessary to have robust knowledge of the nitrogen cycle in natural ecosystems to serve as a benchmark for comparison.