Effects of enhanced nitrogen deposition on foliar chemistry, gas exchange, and forest growth at the Bear Brook watershed in Maine

Effects of chronic nitrogen (N) additions on foliar chemistry, gas exchange and forest growth of tree species were investigated at the Bear Brook Watershed in Maine (BBWM). The BBWM is a paired-watershed forest ecosystem study with one watershed, West Bear, treated since 1989 with 26.6 kg N ha -1 yr-1 applied bimonthly as ammonium sulfate ((NH4)2SO4), while the other watershed, East Bear, serves as a reference. Tree species richness, forest structure, basal area, aboveground biomass and leaf area index were similar between watersheds.; The (NH4)2SO4 treatment significantly increased foliar N concentrations in three species studied, sugar maple ( Acer saccharum), American beech (Fagus grandifolia), and red spruce (Picea rubens). Foliar calcium (Ca) and magnesium (Mg) concentrations were significantly lower in treated American beech and red spruce. No treatment effects on foliar Ca and Mg concentrations for sugar maple were observed. Other foliar nutrient concentrations were similar between treated and reference trees.; Despite higher foliar N concentrations on the treated watershed, only treated sugar maple trees showed significantly higher photosynthetic rates compared to reference trees. The non-responses in photosynthesis to higher foliar N in American beech and red spruce could indicate nutrient imbalances induced by reduced foliar Ca and Mg. Elevated N deposition enhances soil Ca and Mg leaching, making these nutrients less available for plant uptake. The enhanced N availability and the ability to maintain an adequate supply of Ca and Mg to its foliage might explain the increased photosynthesis in sugar maple. In the long run, the treatment could induce forest composition changes by favoring sugar maple over American beech and red spruce.; In a separate greenhouse study, red spruce seedlings grown in organic soils were treated with N at a rate of 8.4, 34.6, 60.8 and 113.2 kg ha -1 yr-1 applied as (NH4) 2SO4 or sodium nitrate (NaNO3) during one growing season. Compared to reference trees, no significant treatment effects on photosynthetic rates and foliar chemistry were observed except for foliar sodium concentrations that were higher in trees treated with NaNO3.