Nitrogen Inhibits Nodulation and Reversibly Suppresses Nitrogen Fixation in Nodules of Alnus maritima

ADDITIONAL INDEX WORDS. seaside alder, actinorhizal plants, Frankia, ammonium nitrate, acetylene reduction assay ABSTRACT. 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 fi xed. The economic and environmental benefi ts of N fi xation 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 infl uences 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. Alnus maritima is an attractive shrub or small tree native to three small, disjunct provenances in the United States (Schrader and Graves, 2000). Like other alders, A. maritima is an actinorhizal species that forms root nodules in which Frankia fi x gaseous N (Schrader and Graves, 2000; Stibolt, 1978). Unlike most woody plants that cannot benefi t directly from atmospheric N, A. maritima with functional root nodules might perform well if planted in N- poor soils. We seek to understand how the symbiosis between A. maritima and Frankia can be established and optimized during the culture of plants in nurseries. Commercial production of plants with functional root nod- ules is important for two reasons. First, growers of nodulated A. maritima might apply less N fertilizer, which would reduce production costs and the potential for environmental damage caused by run-off of irrigation water. Second, the soils in which A. maritima is planted may not contain compatible Frankia; installation of nodulated plants would overcome this possible barrier to N fi xation in the landscape. Protocols for producing nodulated A. maritima are needed. In addition, data are needed on how N fertilization affects the N fi xation of nodulated plants, and comparisons of the growth of plants reliant on fi xed N and those supplied N fertilizer would help producers assess the practicality of reducing N fertilization. Formation of functional nodules on actinorhizal plants requires the presence of compatible Frankia in edaphic environments con- ducive to bacterial infection, nodule development, and the activity of nitrogenase. Although numerous physical and chemical factors may govern the nodulation of A. maritima and the function of its