| Ectomycorrhizal inoculated seedlings may improve forest plantation establishment by enhancing growth and nutrition of newly planted seedlings, but successful nursery colonization of planting stock is often incompatible with conventional fertilization practices because of toxic inhibitive effects. A new cultural technique for seedlings is examined, called “mycorrhizal nutrient loading” that integrates exponential high dose fertilization with ectomycorrhizal association without causing serious inhibition. Containerized black spruce (Picea mariana (Mill.) B.S.P.) were inoculated at sowing with Hebeloma crustuliniforme or Laccaria bicolor and fertilized with a complete nutrient solution conventionally at 12.5 mg N/plant and exponentially at 12.5, 25 or 50 mg N/ plant representing conventional, loading and high loading application rates, respectively. At the end of nursery culture, exponential fertilization significantly stimulated ectomycorrhizal formation at higher rates (49–85%) than those of conventionally fertilized (22–26%) seedlings. Improved ectomycorrhizal colonization, even at high loading rates, was attributed to low initial nutrient additions and lower electrical conductivity levels maintained in the growing media under exponential fertilization. The gradual increase in nutrient delivery enabled the fungi to develop tolerance to high fertilizer inputs. Nutrient loading significantly increased N (51–135%), P (29–45%), and K (13–47%) uptake of seedlings, reflecting progressive luxury consumption of nutrients. Mycorrhizal inoculation further elevated loading efficiency by stimulating plant nutrient uptake of N (9–20%), P (7–12%), and K (4–18%), demonstrating accumulation was more efficient with fungal colonization. Vector nutrient diagnosis revealed marked nutrient dilution under conventional fertilization, but induced steady-state nutrition under exponential fertilization that benefited sustained symbiosis. When outplanted on bioassays retrieved from two contrasting boreal forest (Feathermoss and Hardwood-Alnus) sites in northern Ontario, mycorrhizal nutrient loaded seedlings outperformed conventional seedlings by enhancing dry matter production (45–92%), and increasing N (80–124%), P (89–129%), and K (72–106%) assimilation. The new growth drew on greater internal nutrient reserves built up by both exponential fertilization and mycorrhizal colonization in the nursery. Results were integrated into conceptual models demonstrating interactions of fundamental processes involved in mycorrhizal nutrient loading of trees and their resource utilization after planting in relation to different nutritional regimes. Given the current concern about adequate forest regeneration in Canada, nutrient loading in combination with mycorrhizal inoculation practices provides a potentially effective tool to improve early plantation establishment on boreal sites. |
