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Seaside alder (Alnus maritima) is a rare species adapted for ornamental use. It can be produced easily by commercial growers, and when soil-inoculated in nursery containers, will form root nodules containing nitrogen (N)-fixing bacteria in the genus Frankia. It has been demonstrated that nodulation within this species can be controlled by the amount of N plants receive from a liquid source; however, granular controlled-release fertilizers (CRF) also are commonly used by commercial growers, and information is not available on CRF effects on nodulation. We sought to determine the application rate of CRF that will maximize nodulation of seaside alder while sustaining acceptable plant health and growth. Inoculated containerized plants were topdressed with CRF at eight application rates from 0 to 32.0 g per container. A control group of uninoculated plants received the manufacturer's prescribed rate of 6.0 g per 6-inch-diameter standard round pot. Regression analysis revealed that treatment with 2.0 g of CRF maximized root nodule formation (a mean of 26 nodules), while nodulation was inhibited at CRF rates ≥4.0 g. Plant growth parameters and leaf N content of inoculated plants treated with 2.0 g of CRF were similar to those of uninoculated plants receiving the prescribed rate of CRF (P < 0.05); nitrate-nitrogen leaching from pots containing 2.0-g CRF-treated plants was minimal. We conclude that vigorous nodulated plants of seaside alder can be produced, with minimal nitrate leaching, by providing CRF at lower than prescribed rates. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Although many species of Alnus Miller grow in wet soils, none is as closely associated with low-oxygen, waterlogged soils as Alnus maritima (Marsh.) Muhl. ex Nutt. (seaside alder). An actinorhizal species with promise for use in horticultural landscapes, land reclamation, and sustainable systems, A. maritima associates with Frankia Brunchorst, thereby forming root nodules in which gaseous nitrogen is fixed. Our objective was to determine how root-zone moisture conditions influence the occurrence, location, and anatomy of nodules on A. maritima. Plants of Alnus maritima subsp. maritima Schrader and Graves were established in root zones with compatible Frankia and subjected to four moisture regimens (daily watered/drained, partially flooded, totally flooded, and totally flooded with argon bubbled through the flood water) for 8 weeks. Oxygen content of the root zone, number and location of nodules on root systems, and dry weight and nitrogen content of shoots were determined. Root-zone oxygen content ranged from 17.3 kPa for daily watered/drained plants to 0.9 kPa for argon-treated plants. Across all treatments, 87% of the nodules were within the upper one-third (4 cm) of the root zone. Although shoot dry weights of daily watered/drained and partially flooded plants were not different, daily watered/drained plants had more nitrogen in their leaves (2.53 vs. 2.21 mg·g-1). Nodulation occurred in all treatments, but nodules on totally flooded roots (with or without argon) were limited to a single lobe; in contrast, multilobed nodules were prevalent on partially flooded and daily watered/drained plants. Frankia infection within submerged nodule lobes was limited to one or two layers of cortical cells. Submerged nodules developed large air spaces between cortical cells, and phenolic-containing cells appeared to inhibit Frankia expansion within the nodule. These data suggest that access to root-zone oxygen is critical to the Frankia-A. maritima subsp. maritima symbiosis, and that plants of this subspecies in the drained soils of managed landscapes may benefit more than plants in native wetland habitats from nodulation and nitrogen fixation. [ABSTRACT FROM AUTHOR]