Nodulation and Plant Growth of Shepherdia ×utahensis ‘Torrey’ Topdressed with Controlled-release Fertilizer

Shepherdia ×utahensis ‘Torrey’ (‘Torrey’ hybrid buffaloberry) is an actinorhizal plant that can fix atmospheric nitrogen (N₂) in symbiotic root nodules with Frankia. Actinorhizal plants with N₂-fixing capacity are valuable in sustainable nursery production and urban landscape use. However, whether nodule formation occurs in S. ×utahensis ‘Torrey’ and its interaction with nitrogen (N) fertilization remain largely unknown. Increased mineral N in fertilizer or nutrient solution might inhibit nodulation and lead to excessive N leaching. In this study, S. ×utahensis ‘Torrey’ plants inoculated with soils containing Frankia were irrigated with an N-free nutrient solution with or without added 2 mm ammonium nitrate (NH₄NO₃) or with 0.0 to 8.4 g·L⁻¹ controlled-release fertilizer (CRF; 15N–3.9P–10K) to study nodulation and plant morphological and physiological responses. The performance of inoculated plants treated with various amounts of CRF was compared with uninoculated plants treated with the manufacturer’s prescribed rate. Plant growth, gas exchange parameters, and shoot N content increased quadratically or linearly along with increasing CRF application rates (all P < 0.01). No parameters increased significantly at CRF doses greater than 2.1 g·L⁻¹. Furthermore, the number of nodules per plant decreased quadratically (P = 0.0001) with increasing CRF application rates and nodule formation were completely inhibited at 2.9 g·L⁻¹ CRF or by NH₄NO₃ at 2 mm. According to our results, nodulation of S. ×utahensis ‘Torrey’ was sensitive to N in the nutrient solution or in increasing CRF levels. Furthermore, plant growth, number of shoots, leaf area, leaf dry weight, stem dry weight, root dry weight, and N content of shoots of inoculated S. ×utahensis ‘Torrey’ plants treated with 2.1 g·L⁻¹ CRF were similar to those of uninoculated plants treated with the manufacturer’s prescribed rate. Our results show that S. ×utahensis ‘Torrey’ plants inoculated with soil containing Frankia need less CRF than the prescribed rate to maintain plant quality, promote nodulation for N₂ fixation, and reduce N leaching.