Nitrogen fixation in Alnus incana ssp. rugosa: Field estimates, response to nitrogen additions, and effects on riparian nitrogen cycling

Nitrogen fixing Alnus incana ssp. rugosa dominates many Adirondack wetlands, but N inputs to Adirondack ecosystems other than from atmospheric N deposition (approximately 8 kg N ha--1 yr--1) have not been quantified. Research objectives were to determine: (1) the contribution of N2 fixation to N economies of alder shrubs and wetlands; (2) Nif increases in NO 3-- 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--1 yr--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 of N2 fixation to elevated NO3 -- and NH4-- was measured in 10-month-old alders, grown in bottom-watered vermiculite culture in the growth chamber. Specific activity of nitrogenase increased with NH4+ at 250 mueq/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 mueq/L greater in the alder than reference reach in the first dormant season. Near-stream NO3 -- at 25 cin 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 mueq/L) occurred at 75 cm in the alder reach during a January thaw, within 1 m of the stream. Stream NO3-- was 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 only 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-- or NH 4-- 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.