Effects of dual mycorrhizal inoculation on Pinus strobus seedlings are influenced by soil resource availability

Mycorrhizal interactions can drive plant productivity, diversity, and ecosystem function; however, gaps remain in our understanding of interactions among plants and different mycorrhizal types. Species in the Pinaceae primarily associate with ectomycorrhizal (ECM) fungi; however, some species can be colonized by arbuscular mycorrhizal (AM) fungi as rudimentary hosts. Interactions between plants and mycorrhizal fungi are expected to change along the mutualism-parasitism spectrum with changes in resource availability; however, the impacts of AM fungi on rudimentary hosts under different resource conditions are unknown. In this study, we assessed the individual and interactive effects of ECM and AM inoculation on Pinus strobus seedlings under factorial combinations of low, medium, or high nitrogen (N) and phosphorus (P) supply. No AM colonization was observed at the end of the experiment; however, seedlings inoculated with AM fungi showed significantly reduced growth. AM inoculation also had the greatest effect on P. strobus tissue nutrient concentrations under low fertilization including increased P and reduced N and N:P ratio. ECM colonization benefited P. strobus growth and P uptake, especially under low P availability, but ECM colonization rate itself was limited by low P. ECM colonization also improved seedling regulation of N:P ratio near optimum compared to uncolonized seedlings. We conclude that the effects of ECM and AM inoculation on P. strobus are context dependent, which is important for understanding mutualistic and rudimentary host responses and may have important implications for regeneration and early growth of P. strobus in natural and managed forests.