The Influence of Decadal-Scale N Enrichment On N Dynamics and 15N Discrimination in a New England Forested Watershed.

Atmospheric deposition of nitrogen (N) and contemporary trends in climate are important considerations for understanding and predicting future influences of N dynamics on forest ecosystem form and function.  The Bear Brook Watershed in Maine (BBWM) offers an opportunity to determine how N dynamics can be altered by experimental N enrichment on a decadal time scale.  BBWM is a paired watershed study, with one watershed serving as a reference, and the other treated with (NH₄)₂SO₄ at the rate of 25.2 kg N ha^-1 yr^-1 since 1989.  Despite being an N-limited forest, applications of NH₄-N resulted in an almost immediate increase in NO₃-N export in the treated stream, with ¹⁵N labeling of treatments from 1989 to 1992 indicating that the initial elevated flux of NO₃-N was not directly from the NH₄-N  inputs.  Since 1989, we have identified N enrichment in soils, soil solutions, streams, microbial biomass, tree roots, and tree foliage.  We report here on the effects of decadal-scale N enrichment on ¹⁵N natural abundances in soils within the watershed as a tool to study altered N dynamics. Results include evidence of altered N dynamics in contrasting coniferous and deciduous forest types that help define the causes of ¹⁵N enrichment evident in the treated stream.  The integrated response to chronic N loading is a 0.5 – 1.5 per mill enrichment in the treated watershed stream, and ecosystem component d¹⁵N is used to define the mechanisms for these changes.