Speckled alder (Alnus incana ssp. rugosa) is a characteristic species of scrub-shrub type wetlands in the Adirondack Mountains region of New York state.It is an actinorhizal nitrogen (N) fixer and fixes significant amounts of N in wetlands where it dominates the shrub vegetation.Phosphorus (P) is an element that limits N fixation and plant growth.However, studies testing this hypothesis in the field especially for actinorhizal plants are very limited.This research was undertaken i) to determine the limiting effect of low P availability on nitrogenase activity of speckled alder using a field P fertilization experiment, ii) to detect the amount of nitrate and phosphate accumulated in the substrate of treated and reference plots over two week intervals using ion exchange resins, and iii) to measure the growth response of speckled alder twigs, foliage and root nodules to P fertilization.P fertilization significantly increased specific nitrogenase activity.Nitrate concentrations were greater and phosphate concentrations were lower in reference plots compared to treatment plots and there was a significant biomass response to P fertilization in the second year after fertilization.