Abstract: Nitrogen fixing Alnus incana spp. rugosa
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.