The relative importance of nitrogen inputs from atmospheric depositions and biological fixation is reviewed in a number of diverse, non-agricultural terrestrial ecosystems. Rates of both symbiotic and nonsymbiotic fixation appear to be greater during early succession stages of forest development, where they have major impacts on nitrogen dynamics and ecosystem productivity. Fates and impacts of these nitrogen inputs are important considerations that are inadequately understood. These input processes are highly variable in space and time, and few sites have adequate comparative information on both nitrogen deposition and fixation.