Abstract:  Nitrogen fixing Alnus incana spp. rugosa dominates many Adirondack wetlands, but N inputs to Adirondack ecosystems other than from atmospheric N deposition (approximately 8 kg N ha^-1yr^-1) have not been quantified.  Research objectives were to determine 1) the contribution on N₂ fixation to N economies of alder shrubs and wetlands; 2) if increases in NO₃^- and NH₄⁺ to realistic field levels decrease N₂ fixation; and 3) if alder increases N leaching to subsurface and surface waters.
    Natural abundance ¹⁵N dilution showed 80-100% foliar N from fixation in five wetlands, and approximately 47 kg N ha^-1yr^-1 fixed in foliage at one site in 1997.  Fixation inputs of 34 + 4 kg N ha^-1yr^-1 to this wetland were estimated in 1998 using C₂H₂ reduction.
    Response on N₂ fixation to elevated NO₃^- and NH₄⁺ was measured in 10-month-old alders, grown in bottom-water vermiculite culture in the growth chamber.  Specific activity of nitrogenase increased with NH₄⁺ at 250 ueq/L.  Nitrogenase activity per plant was not affected by treatments.  However, high N reduced nodule biomass.
    Nitrogen concentrations and flow patterns of water, and litter fall into the stream, were compared in a riparian alder and upstream reference reach.  Nitrate at 25 cm depth was 23-27 ueq/L greater in the alder than reference reach in the first dormant season.  Near-stream NO₃^- at 25 cm was > two times that of stream or deeper groundwater in the alder reach in June.  Steepest gradients of increasing dissolved inorganic N (DIN) between hill slope and stream occurred in the alder reach.  Greatest subsurface NO₃^- (60 ueq/L) occurred at 75 cm in the alder reach during a January thaw, within 1 m of the stream.  Stream NO₃^- ws greater in the alder reach.  Alders contributed 1.5 g m^-2 N yr ^-1 in litter (C:N=22) to the stream.  Reference vegetation contributed on 0.5 g m^-2 N yr ^-1 (C:N=40-78).
    Alders add substantial N to some Adirondack watersheds, and N₂ fixation is not decreased by NO₃^- of NH₄⁺ at 1-10 x ambient.  Greater NO₃^- production and litter N occurred in an alder shrub vs. coniferous forested wetland, indicating that dominant wetland types function differently in regards to N cycling.