This paper will present selected examples of applications of the natural abundance method to ecological studies. The emphasis will be on results from ongoing ecosystem level studies examining the role of woody legumes in N cycling in warm deserts... Data from other systems (tropical forest, chaparral) where significant sustained inputs of N fixed by woody plants may affect ecosystem processes are also discussed to demonstrate the utility of the natural abundance method in a diversity of systems. The natural 15N abundance approach in an important advancement for the study of deeply rooted plants. Our studies have show that contrary to previous speculation, deeply rooted plants in arid ecosystems can support root symbioses at depths not easily studied using other techniques.