Symbiotic associations between Alnus maritima (Marsh.) Muhl. ex Nutt. (seaside alder) and actinomycetes in the genus Frankia Brunchorst result in root nodules in which atmospheric nitrogen (N) is fixed. The economic and environmental benefits of N fixation have led to interest in inducing root nodules during production of A. maritima. Because woody plants produced in nurseries typically are provided N fertilizer, our objectives were to determine how applied N influences nodulation of A. maritima and to characterize how short-term changes in root-zone N affect the function of nodules. Potted seedlings were grown in perlite that was inoculated with 30 mL of soil from the root zones of mature plants in their native habitat on the Delmarva Peninsula. Each pot was drenched once daily for 10 weeks with nutrient solution that contained ammonium nitrate at 10 concentrations from 0 to 8 mm. Plants that received no ammonium nitrate formed the most nodules, and nodulation decreased linearly as ammonium nitrate increased from 0.25 to 4 mm. Plants treated with ammonium nitrate at 4 or 8 mm formed nearly no nodules, while ammonium nitrate at 0.5 mm resulted in vigorous plants with an average nodule count of 70. In a second experiment, a population of nodulated seaside alders was established by irrigating seedlings in inoculated perlite once daily with 0.5-mm ammonium nitrate for 6 weeks. Plants then were provided ammonium nitrate at 0.5, 2, or 4 mm for 2 weeks. Acetylene-reduction assays showed suppressed nodule activity among plants provided 2- and 4-mm ammonium nitrate. Daily irrigation of those plants with N-free solution subsequently led to a rapid depletion of root-zone N and to a concomitant resurgence of nodule activity. These results demonstrate that N fertilization can be managed to promote nodulation of A. maritima and show that decreased nodule activity caused by short-term increases in root-zone N is reversible.