Nitrogen (N) is added to watersheds in several forms. N is an essential nutrient for both aquatic and terrestrial organisms, and is a growth-limiting nutrient in most ecosystems. N compounds are found in the atmosphere in reduced and oxidized forms. N leaching losses were highly variable, ranging from less than 5 to about 80% of inputs, and this seemed to depend on the initial N status of the site and the form of deposited N. N retention efficiencies were much higher than expected, ranging from 85% at the high-N pine stand at Harvard Forest to values between 93 and 99% at the other 8 treatments and sites. N output was most highly correlated with input N, but also significantly correlated with input sulphur, soil pH, percent slope, bedrock type, and latitude. If N is limiting for aquatic primary production, the added NO3- will generally result in increased algal productivity that can cause disruption of aquatic community dynamics.