Our understanding of the marine and terrestrial nitrogen budgets is severely limited. For instance, atmospheric data indicate a residence time of N2O in the atmosphere of about 10 years and consequently sources and sinks of atmospheric N2O of the order of 130¿106 ton N/year. Such large sources and especially sinks for atmospheric N2O have not been satisfactorily identified and it may not be excluded, a priori, that not only the N2O source, but also its sink is affected by man. The limited knowledge about the earth's N2O budget so far obtained is in favor of the view that there is no imminent danger of large ozone reductions following the predicted increase in the input of fixed nitrogen in the soil by the year 2000. However, it seems likely that, with unchanged agricultural practices, there will develop serious long term problems, certainly in a world of exponential growth, and, for best action, it is essential that a well balanced, interdisciplinary research effort now be designed to increase our knowledge of all aspects of the earth's nutrient cycles.