Exponential fertilization increases nutrient uptake and ectomycorrhizal development of black spruce seedlings

Intensive fertilization may inhibit adequate mycorrhizal development for forest nursery stock production. Containerized black spruce (Picea mariana (Mill.) BSP) seedlings exposed to four fertilization regimes (one conventional and three exponential at 12.5, 12.5, 25, and 50 mg N/seedling, respectively) and two ectomycorrhizal inoculations (Hebeloma crustuliniforme (Bull. ex St-Amans) Quel. and Laccaria bicolor (R. Mre.) Orton) were grown from seed to assess mycorrhizal formation and nutrition of young trees under intensive greenhouse culture. Exponentially increasing fertilizer additions stimulated ectomycorrhizal development (49-85%) compared with conventional constant-rate fertilization (22-26%). Exponential fertilization also increased seedling N (13-34%) and P (5-18%) uptake, although dry matter production was reduced (17-25%) at the lowest exponential addition rate. Ectomycorrhizal inoculation did not affect seedling biomass but increased uptake of N (6-17%), P (5-20%), and K (4-18%), demonstrating a potential for nutrient loading by fungi. Higher ectomycorrhizal formation found under exponential fertilization regimes was attributed to lower initial nutrient levels maintained in the growing media. Results indicate that high exponential fertilization combined with fungal inoculation may be effective for producing both nutrient-loaded and ectomycorrhizally infected planting stock.