Dual mycorrhizae interact with soil nutrients to influence <i>Pinus strobus</i> establishment in northern hardwood forests

Many tree species form dual mycorrhizal symbioses, arbuscular mycorrhizae (AM) and ectomycorrhizae (EM), especially at the seedling stage, but the effect of soil nutrient availability in shaping these mycorrhizal relationships is not well understood. We quantified the effects of N and P fertilization on survival, growth, and mycorrhizal associations of white pine ( Pinus strobus) seedlings planted into a long-term nutrient addition experiment (MELNHE) in four northern hardwood forests in New Hampshire, USA. Seedling survival was significantly reduced by N addition, potentially due to changes in soil pathogen communities. In contrast, seedling growth indicated N limitation but was unaffected by mycorrhizal colonization. Both microscopic and metagenomic evidence indicated the formation of dual mycorrhizae across all the stands and treatments. The two types of mycorrhizal colonization were equally abundant and were neither positively nor negatively correlated. Addition of P alone significantly reduced AM colonization relative to controls and increased EM colonization. The EM fungal community composition was strongly affected by N and P additions, and P addition caused increases in abundance of common EM fungi with widely differing functional traits (e.g., exploration type, nitrogen affinity).