Recent reports of large nitrate exports from forests to surface waters raise concern that losses of nitrate, together with associated acidity and cations, may result in gradual decrease in forest soil fertility and impoverishment of aquatic ecosystems downstream of forested catchments through eutrophication and acidification. Field experiments will be conducted to: 1) quantify and compare nitrate retention capacities of several different forest ecosystems in the Northeast; 2) compare the importances of nitrate uptake by growing vegetation vs. nitrate immobilization in decomposing organic matter as sinks for excess nitrate; and 3) relate tree species growth rates and N use efficiencies to both nitrate and ammonium availabilities. The work will take advantage of two on-going research projects. One is the EPA's Watershed Manipulation Project (WMP), a large multiple-investigator study of the effects of nitric and sulfuric acid deposition on soil biogeochemical processes and soil solution chemistry at both plot and catchment levels in a mixed-species forest in Maine. The other is a plot-level N enrichment experiment conducted at the Harvard Forest NSF-LTER site in Massachusetts where investigators are now studying the effects of chronic N additions on N cycling rates, primary productivity, and canopy chemistry. The in-place, EPA project provides a unique opportunity to trace the paths and to identify the fate of nitrate additions to forest ecosystems. This project will support N isotope studies that will allow for this. Preliminary 15N analysis of soils and tissues from the WMP plots show that the high sensitivity of techniques proposed here will allow for quantifying nitrate retention in important ecosystem components. This research will improve our basic understanding of forest ecosystem processes that are important in controlling exports of nitrate and acidity to stream, lake, and estuarine ecosystems. It will also improve our ability to predict changes in forest productivity as patterns and rates of N cycling change due to both natural processes and human caused N deposition.
