Nursery production of native plants has increased tremendously because of increasing interest in using native plants in urban gardens and landscapes (Thomas and Schrock, 2004). The growing interest in native plants is attributable to their ornamental potential, including aesthetic appearance, bio-diversity, and, most importantly, water conservation (Hooper et al., 2008). In the Intermountain West, native plants are used in low-water-use landscapes with sustainable and low-water-use features (Mee et al., 2003). However, it is difficult to establish native plants in disturbed and poorly drained soils (Edmondson et al., 2011; Mee et al., 2003). Native plants in the Intermountain West, such as Arctostaphylos patula (greenleaf manzanita), Artemisia nova (black sagebrush), Ceratoides lanata (syn. Krascheninnikovia lanata) (winterfat), and Cercocarpus montanus (alder-leaf mountain mahogany), are susceptible to overwatering and wet rooting substrates (Mee et al., 2003). Parkinson et al. (2003) also reported that adequate drainage is essential for growing native plants in the Intermountain West, citing Agave parryi (Parry’s agave), Aquilegia caerulea (Colorado blue columbine), Eriogonum niveum (snow buckwheat), and Eriophyllum lanatum (woolly sunflower) as examples.
A similar scenario was discovered for Shepherdia species (buffaloberry). Shepherdia argentea (silver buffaloberry) and Shepherdia rotundifolia (roundleaf buffaloberry) are both native plants in the Intermountain West (Mee et al., 2003). Shepherdia rotundifolia is an evergreen shrub with a tidy, rounded form and outstanding drought tolerance, but it is highly sensitive to excessive landscape irrigation (Sriladda, 2011; Sriladda et al., 2014). However, S. argentea is a fast-growing, deciduous shrub that adapts to a wide range of soil conditions, but it is less aesthetically acceptable due to its thorny and unkempt appearance (Sriladda et al., 2016). To enhance the adaptability of Shepherdia to wet and poorly drained soils while maintaining drought tolerance and aesthetic appearance, S. ×utahensis Torrey, an interspecific hybrid cultivar of S. argentea and S. rotundifolia, was created by Sriladda et al. (2016). This hybrid cultivar has the potential for use in low-water landscapes because of its tolerance to drought conditions as well as the occasionally wet soils found in residential landscape environments (Sriladda et al., 2016). Furthermore, S. argentea and S. rotundifolia are actinorhizal plants (Benson and Silvester, 1993), and S ×utahensis ‘Torrey’ can form a symbiotic association with Frankia to fix N2 in its root nodules (J. Chen, unpublished data). This biological N2-fixing capacity may reduce the need for nitrogenous fertilization of nodulated actinorhizal plants, thus solving two primary concerns in the nursery industry: mineral N runoff and leaching to groundwater (Urbano, 1989). For example, nodulated Alnus maritima (seaside alder) had better fertilizer use efficiency when inoculated with soils containing Frankia than uninoculated plants (Beddes and Kratsch, 2010). In another study, Frankia-induced nodulation improved plant performance and N use efficiency of Alnus incana (gray alder) (Sellstedt and Huss-Danell, 1986). Laws and Graves (2005) reported that nodulated A. maritima sustained plant vigor and quality with a lower NH4NO3 concentration than uninoculated plants. Therefore, actinorhizal plants with symbiotic nodules have commercial potential in nursery production and urban landscapes (Kratsch and Graves, 2004).
Slow-release fertilizer and CRF gradually deliver mineral nutrients (mainly N) to plants and have been widely used in nursery production (Adams et al., 2013; Beddes and Kratsch, 2010), but excessive N fertilization reduces nodule formation of actinorhizal plants inoculated with Frankia (Huss-Danell, 1997). For example, A. maritima exhibited a decreased nodule number when NH4NO3 increased from 0 to 8.0 mm (Laws and Graves, 2005). In addition, Beddes and Kratsch (2010) reported that A. maritima plants had a decreased nodule number when CRF (15N–3.9P–10K) levels increased from 0 to 1.8 g·L−1, and 3.6 g·L−1 completely inhibited nodule formation. Unfortunately, although N fertilization significantly influences nodule formation, the impact of fertilizers on plant growth and nodule development of S. ×utahensis ‘Torrey’ is largely unknown. Research investigating the effects of CRF and its application rate on nodulation is needed to inform best practices for nurseries.
The objectives of this research were to 1) investigate the impacts of NH4NO3 and CRF on nodule number and plant growth of S. ×utahensis ‘Torrey’, and 2) to determine CRF application rates that maintain acceptable plant quality with minimal nitrate–nitrogen (NO3-N) leaching. In addition, the effects of inoculation on growth and gas exchange parameters were studied by comparing inoculated S. ×utahensis ‘Torrey’ plants treated with 0 to 8.4 g·L−1 CRF with uninoculated plants treated with the manufacturer’s prescribed rate of 3.2 g·L−1.
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