Container Production of Actinorhizal and Rhizobiarhizal Woody Plants without Supplemental NPK

Actinorhizal and rhizobiarhizal plants can form mutualistic symbioses with soil-borne bacteria housed in root nodules, a relationship in which the plant receives fixed atmospheric nitrogen while providing energy and a controlled environment for the microsymbiont.  Actinorhizal plants form N₂-fixing symbioses with actinomycetes from the genus Frankia Brunchorst, and rhizobiarhizal plants (mostly legumes) form N₂-fixing symbioses with rhizobial bacteria mainly from two genera, Rhizobium Frank and Bradyrhizobium Jordan.  While a few actinorhizal and rhizobiarhizal plant species are recognized for their outstanding performance in stressful landscapes, the potential to produce these species sustainably in nurseries is poorly understood.  Our objectives were to determine if eight actinorhizal and rhizobiarhizal taxa with ornamental potential could be grown to marketable size and ornamental quality without supplemental fertilizer, to determine the suitability of these species for production without fertilizer, and to compare responses during production without fertilization of symbiotic species and functionally similar non-symbiotic species.  Woody plants of four actinorhizal species and four rhizobiarhizal species, along with a non-symbiotic species functionally similar to each, were produced with either a standard fertilizer treatment or without fertilization.  All plants were grown in a soilless medium that, for plants not fertilized, was inoculated with 150 ml of field soil.  At the end of production, plants were measured for nodulation, greenness of foliage (SPAD units), health (blind ratings by three horticulturists), and size (shoot height and dry weight).  Two of the four actinorhizal species and all four rhizobiarhizal species formed root nodules in the inoculated/non-fertilizer treatment, while no plants in the un-inoculated/fertilizer (control) treatment nodulated.  Among the nodulated taxa, two actinorhizal species (Alnus nitida [Spach] Endl. and Alnus maritima [Marsh.] Muhl. ex Nutt.) and two rhizobiarhizal species (Maackia amurensis Rupr. & Maxim. and Acacia angustissima [Mill.] Kuntze) grew larger, were rated as healthier, and developed greener foliage in the non-fertilizer treatment than did plants of these species that received fertilizer.  Size of Robinia pseudoacacia L. in the two treatments was similar, but nodulated, non-fertilized plants were less healthy and less green, and size, health, and greenness of Laburnum alpinum J.Presl. were reduced by inoculation and no fertilization.  All non-symbiotic species in the non-fertilizer treatment showed marked reductions in size, health, and greenness of foliage.  Responses of A. nitida, A. maritima, M. amurensis, and A. angustissima demonstrate potential to produce certain actinorhizal and rhizobiarhizal nursery crops sustainably with little or no fertilizer input.