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−1y−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. Ratios

DON/TDN and PN/TN both are related to watershed area rather than runoff; DON/TDN

decreases and PN/TN increases toward higher stream orders. The analysis suggests for tropical

watersheds the existence of mechanisms promoting strong homeostasis in the yield of N and

its fractions for a given moisture regime, as well as predictable downstream change in propor-

tionate representation N fractions. Yields and concentrations for small tropical watersheds are

much larger than for the few temperate ones with which comparisons are possible.

VL - 46
UR - http://cires1.colorado.edu/limnology/pubs/pdfs/Pub144.pdf
ER -