© 1999 Kluwer Academic Publishers. Printed in the Netherlands. Nitrogen yields from undisturbed watersheds in the Americas

yields, nitrate Abstract. Yields of total fixed nitrogen and nitrogen fractions are summarized for thirty-one watersheds in which anthropogenic disturbance of the nitrogen cycle, either through land use or atmospheric deposition, is negligible or slight. These yields are taken as representative of background conditions over a broad range of watershed areas, elevations, and vegetation types. The data set focuses on watersheds of the American tropics, but also includes information on the Gambia River (Africa) and some small watersheds in the Sierra Nevada of California. For the tropical watersheds, total nitrogen yield averages 5.1 kg ha −1 y−1. On average, 30 % of the total is particulate and 70 % is dissolved. Of the dissolved fraction, an average of 50% is organic and 50 % is inorganic, of which 20 % is ammonium and 80 % is nitrate. Yields are substantially lower than previously estimated for background conditions. Yields of all nitrogen fractions are strongly related to runoff, which also explains a large percentage of variance in yield of total nitrogen (r2 = 0.85). For total nitrogen and nitrogen fractions, yield increases at about two-thirds the rate of runoff; concentration decreases as runoff increases. There is a secondary but significant positive relationship between elevation and yield of DIN. Ratio