Symbiotic N2 fixation of Alnus incana ssp. rugosa in shrub wetlands of the Adirondack Mountains, New York, USA.

Surface waters in forested watersheds in the Adirondack Mountains and northern New York State are susceptible to nitrogen (N) saturation. Atmospheric deposition of N to watersheds in this region has been measured but the extent of internal N inputs from symbiotic N2 fixation in alder-dominated wetlands is not known. We estimated N2 fixation by speckled alder in these wetlands by the 15N natural abundance method and by acetylene reduction using a flow-through system. Foliar N derived from fixation (%Ndfa) was estimated for five wetlands. The δ15N of speckled alder foliage from four of the five sites did not differ significantly (P≤0.05) from that of nodulated speckled alders grown in N-free water culture (-1.2±0.1‰). Estimates from the 15N natural abundance method indicated that alders at these sites derive 85-100% of their foliar N from N2 fixation. At one of the sites, we also measured biomass and N content and estimated that the alder foliage contained 43 kg N ha-1 of fixed N in 1997. This estimate was based on a foliar N content of 55.4±7 kg N ha-1 (mean±SE), 86±4%Ndfa, and an assumption that 10% of foliar N was derived from reserves in woody tissues. At this site, we further estimated via acetylene reduction that 37±10 kg N ha-1 was fixed by speckled alders in 1998. This estimate used the theoretical 4:1 C2H2 reduction to N2 fixation ratio and assumed no night-time fixation late in the season. Nitrogen inputs in wet and dry deposition at this site are approximately 8 kg N ha-1 year-1. We conclude that speckled alder in wetlands of northern New York State relies heavily on N2 fixation to meet N demands, and symbiotic N2 fixation in speckled alders adds substantial amounts of N to alder-dominated wetlands in the Adirondack Mountains. These additions may be important for watershed N budgets, where alder-dominated wetlands occupy a large proportion of watershed area.