Nitrogen yields from undisturbed watersheds in the Americas

Yields of total fixed nitrogen and nitrogen fractions are summarized for th irty-one watersheds in which anthropogenic disturbance of the nitrogen cycl e, either through land use or atmospheric deposition, is negligible or slig ht. These yields are taken as representative of background conditions over a broad range of watershed areas, elevations, and vegetation types. The dat a set focuses on watersheds of the American tropics, but also includes info rmation on the Gambia River (Africa) and some small watersheds in the Sierr a Nevada of California. For the tropical watersheds, total nitrogen yield a verages 5.1 kg ha (-1) y(-1). On average, 30% of the total is particulate a nd 70% is dissolved. Of the dissolved fraction, an average of 50% is organi c and 50% is inorganic, of which 20% is ammonium and 80% is nitrate. Yields are substantially lower than previously estimated for background condition s. Yields of all nitrogen fractions are strongly related to runoff, which a lso explains a large percentage of variance in yield of total nitrogen (r(2 ) = 0.85). For total nitrogen and nitrogen fractions, yield increases at ab out two-thirds the rate of runoff; concentration decreases as runoff increa ses. There is a secondary but significant positive relationship between ele vation and yield of DIN. Ratios DON/TDN and PN/TN both are related to water shed area rather than runoff; DON/TDN decreases and PN/TN increases toward higher stream orders. The analysis suggests for tropical watersheds the exi stence of mechanisms promoting strong homeostasis in the yield of N and its fractions for a given moisture regime, as well as predictable downstream c hange in proportionate representation N fractions. Yields and concentration s for small tropical watersheds are much larger than for the few temperate ones with which comparisons are possible.