Hurd, T.M. 1999. Nitrogen fixation in Alnus incana spp. rugosa: Field estimates, response to N additions, and effects on riparian N cycling.  M.S. Thesis, SUNY College of Environmental Science and Forestry, Syracuse, 138pp.
 
 

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.