Does resource exchange in ectomycorrhizal symbiosis vary with competitive context and nitrogen addition?

Ectomycorrhizal symbiosis is essential for the nutrition of most temperate forest trees and helps regulate the movement of carbon (C) and nitrogen (N) through forested ecosystems. The factors governing the exchange of plant C for fungal N, however, remain obscure. Because competition and soil resources may influence ectomycorrhizal resource movement, we performed a 10-month split-root microcosm study using Pinus muricata seedlings with Thelephora terrestris, Suillus pungens, or no ectomycorrhizal fungus, under two N concentrations in artificial soil. Fungi competed directly with roots and indirectly with each other. We used stable isotope enrichment to track plant photosynthate and fungal N. For T. terrestris, plants received N commensurate with the C given to their fungal partners. Thelephora terrestris was a superior mutualist under high-N conditions. For S. pungens, plant C and fungal N exchange were not coupled. However, in low-N conditions, plants preferentially allocated C to S. pungens rather than T. terrestris. Our results suggest that ectomycorrhizal resource transfer depends on competitive and nutritional context. Plants can exchange C for fungal N, but coupling of these resources can depend on the fungal species and soil N. Understanding the diversity of fungal strategies, and how they change with environmental context, reveals mechanisms driving this important symbiosis.