THE INFLUENCE OF PHOSPHORUS CONCENTRATION AND FREQUENCY OF FERTILIZATION ON ECTOMYCORRHIZAL DEVELOPMENT IN CONTAINERIZED BLACK SPRUCE AND JACK PINE-SEEDLINGS

The growth response of jack pine (Pinus banksiana Lamb.) and black spruce (Picea mariana (Mill.) B.S.P.) seedlings was tested in growth chambers at two levels of P, with or without inoculation of the mycorrhizal fungus Laccaria bicolor (Maire) Orton and with weekly or thrice-weekly fertilizer application. While keeping N and K constant, an increase of P from 1.5 to 7.2 mg per seedling severely reduced formation of L. bicolor ectomycorrhizae on both jack pine and black spruce 17 weeks after inoculation. Inoculation of black spruce with L. bicolor was more successful (75%) than inoculation of jack pine (35%). Inoculated black spruce seedlings were 34% taller and 44% heavier than uninoculated controls, but only at the low P level; they were also 39% heavier and had more abundant L. bicolor ectomycorrhizae (180% of the weekly treatment) when fertilized thrice weekly than with weekly fertilization that supplied the same total nutrients. Dry weights of jack pine inoculated with L. bicolor were 26 and 33% larger than those of uninoculated seedlings at high and low P levels, respectively. Fertilizing thrice weekly also produced 23% larger dry weights of jack pine, but fewer ectomycorrhizae were formed than with weekly fertilization (65% of the weekly value). The results indicate that inoculation of black spruce seedlings with L. bicolor will produce larger seedlings, but only where low P fertilizer is applied, preferably thrice weekly. Jack pine may benefit from artificial inoculation with L. bicolor, especially in a higher fertility growing medium.