Symbiont nitrogenase, alder growth, and soil nitrate response to phosphorus addition in alder ( Alnus incana ssp. rugosa) wetlands of the Adirondack Mountains, New York State, USA

Speckled alder ( Alnus incana ssp. rugosa) is a characteristic species of scrub-shrub 1-type wetlands, the second most common wetland type in major watersheds of the Adirondack Mountains in New York State. Speckled alder is an actinorhizal nitrogen fixer that relies heavily on N 2 over soil N and fixes substantial amounts of nitrogen in wetlands, resulting in little vegetation processing of anthropogenic N between alder-shrub wetlands and streams. Phosphorus (P) is an element that limits nitrogen fixation and plant growth. However, studies testing this hypothesis in the field, especially for actinorhizal plants, are very few. The objectives of this study were to evaluate the potential limitation of N fixation and growth in speckled alder by P, and to determine interactions between P fertilization and nitrate levels in riparian alder stands in a region that receives elevated N in atmospheric deposition. P fertilization significantly increased specific nitrogenase activity during the seasonal peak in early August. Nitrate concentrations were greater in reference plots compared to treatment plots, and phosphate concentrations were lower in reference plots compared to treatment plots over a period of 6 weeks in the growing season. There was a significant twig and foliar biomass response to P fertilization in the second year after fertilization, but no significant change in individual biomass or relative numbers of different sized nodules. Response of nitrogen fixation to P appears limited to a brief but significant increase in specific activity of nitrogenase late in the growing season, but P stimulated growth of above ground tissues 1 year following fertilizer application, and decreased resin-captured nitrate beneath riparian speckled alder. These results suggest that growth of alder and growth or activity of soil microbes, rather than nitrogen fixation, is P limited in riparian wetlands dominated by speckled alder, and that P controls nitrate leaching in these near-stream systems.