Effects of forest decline on uptake and leaching of deposited nitrate determined from 15N and 18O measurements

Abstract

ATTEMPTS to understand how atmospheric nitrogen deposition affects forest ecosystems1'2 have been hampered by the lack of a direct method to trace the fate of the deposited nitrogen. Nitrate originating in the atmosphere has natural abundances of nitrogen and oxygen isotopes that differ measurably from those of soil nitrate3. Here we present an analysis of the isotope ratios of nitrate in spring waters from eight forested watersheds, ranging from apparently healthy spruce plantations to those in decline owing to acidification. We find that for the healthy, slightly declining and limed sites, only 16–30% of the nitrate in spring water originates directly from the atmosphere without being processed in the soil, whereas for more severely damaged sites almost all of the atmos-pheric nitrate finds its way directly into the spring water. This suggests that acid-induced forest decline significantly inhibits nitrate consumption by soil microorganisms and trees, and that liming to ameliorate soil acidification restores the consumption of atmospheric nitrate. Nevertheless, in limed ecosystems total nitrate output remains high because of internal nitrate production by the ecosystem.