Seaside alder is a threatened species native to the United States in Georgia, Oklahoma, and along the Delmarva Peninsula in Delaware and Maryland (Schrader and Graves, 2000a). Efforts have been made to encourage its use by the nursery industry because it has been found to be well suited for landscape use (Graves and Schrader, 2004; Kratsch, 2008), and can be propagated readily from cuttings or seed (Schrader and Graves, 2000a, 2000b). Plants of seaside alder have the capacity to fix atmospheric nitrogen (N) by way of a relationship with Frankia bacteria, which colonize root nodules (Stibolt, 1978). Such N-fixing species can survive, and often thrive, in N-poor soils in the wild (Paschke, 1997); in cultivated situations, they can add significant N to the soil environment (Hansen and Dawson, 1982).
Nursery production of nodulated plants could be potentially valuable because nodulated N-fixing plants might perform better than non-N-fixing taxa in landscape situations as a result of their capacity to thrive in N-deficient soils. Furthermore, the presence of nodules on nursery plants could allow for reduction in N-fertilizer use and more efficient use of applied N. Sellstedt and Huss-Danell (1986) found that nodulated gray alder (Alnus incana) used N more efficiently than did non-nodulated gray alder receiving ammonium nitrate (NH4NO3) in aqueous solution as its N source. Nursery growers view nitrate-N (NO3-N) contamination of groundwater as one of the major challenges for sustainable production (Urbano, 1989). The use of controlled-release fertilizers (CRF) for crop fertilization has been touted as a means to reduce NO3-N leaching from containerized nursery crops (Colangelo and Brand, 2001), and CRF is now widely used in nursery production. However, no studies have investigated effects of CRF on nodule formation in N-fixing plants.
It has been demonstrated with various actinorhizal species that nodulation can be limited when plant roots are exposed to high concentrations of N in aqueous solution (Berry and Torrey, 1985; Gentili and Huss-Danell, 2003; Kohls and Baker, 1989; Martin et al., 2003; Thomas and Berry, 1989). Laws and Graves (2005) investigated the dynamics of this response in seaside alder, and demonstrated N-induced inhibition of nodulation and nodule activity, which was rapidly reversible after removal of N from the fertilizer solution. This is useful information for nurseries desiring to produce nodulated plants because inadvertent, short-term exposure to high N does not destroy the N-fixing capacity of seaside alder (Laws and Graves, 2005).
Our objectives were to examine how NH4NO3 delivered in a controlled-release form would influence nodulation in seaside alder, and to determine the level of CRF that would enhance nodule formation while sustaining acceptable plant health and growth and minimizing NO3-N leaching from containers. Our results will provide growers who rely on CRF for delivery of plant nutrients with guidelines for their use with nodulated seaside alder.
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