NITROGEN SOURCES IN ADIRONDACK WETLANDS DOMINATED BY NITROGEN-FIXING SHRUBS

In the Adirondack region of northern New York, USA, Alnus incana ssp. rugosa and Myrica gale often dominate wetland shrub communities and fix nitrogen in symbiosis with actinomycetes of the genus Frankia. The objective of this study was to examine the contribution of these shrubs to the N economies of whole wetlands in the Adirondacks where N has been considered a potential pollutant and contributor to low acid-neutralizing capacity, and where N deposition may reduce rates of nitrogen fixation in actinorhizal plants. Nitrogen chemistry of plant foliage was examined, and density and foliar biomass of nitrogen fixing shrubs were estimated in plots or belt transects in six shurb wetlands near atmospheric deposition monitroing stations in order to estimate the fraction of N derived from fixation in A. incana ssp. rugosa and M. gale tissues. Lake-inlet-wetlands were dominated by alder that derives >-85% foliar N from fixation, but M. gale was most abundant in lake outlet wetlands and seemed to rely less on fixed N, although results for Myrica were more uncertain. Substantial N is therefore added to lake inlet systems dominated by alder (7–18 kg ha−1 yr−1), while N fixed from M. gale does not appear to exceed 3 kg ha−1 yr−1, except in localized patches at smaller spatial scales. Similarity in σ15N between non-fixing field shurbs and reference values for fixed N at some sites suggests that fixed N is being recycled in the plant community. Wet atmospheric N deposition is 3–6 kg ha−1 yr−1 and does not decrease N fixation substantially in alder. Overall, shurbs in wetlands dominated by these actinorhizal N2—fixing plants are not taking up substantial quantities of anthropogenic N, suggesting that nitrogen is processed microbially or transferred along with some fixed N to downstream ecosystems.