EFFECTS OF NITROGEN FERTILIZATION ON STREAM CHEMISTRY OF JAPANESE FORESTED WATERSHEDS

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Abstract

Steam chemistry was investigated from May 1991 through April 1992 for 13 Japanese forested watersheds and from May 1990 through August 1994 for two of these watersheds. Nine watersheds were treated over different periods (1983–1991) with different amounts of N (nitrogen) fertilizer as urea and ammonium salts. Total N additions ranged from 20 to 375 kg ha−1. There were no distinct seasonal differences in stream NO−3 concentrations in either the treated or untreated watersheds, but concentrations tended to be somewhat higher during periods of high discharge. The annual average NO−3 concentrations in streams had a significant, positive (p < 0.001, r = 0.84) relationship to the total amount of N applied from 1985–1991. The application of 330 kg N ha−1 raised annual average stream NO−3 concentration to about 300 μeq L−1 compared to less than 160 μeq L−1 in untreated watersheds. The concentrations of K+, Ca2+, and Mg2+ in stream water also increased in those watersheds with high rates of N fertilizer as a result of nitrification that increased the generation of the mobile nitrate anion. The lack of seasonality in stream NO−3 concentrations and the large rates of N loss with N addition both suggest that these watersheds were ‘nitrogen saturated’

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