Are ectomycorrhizal fungi alleviating or aggravating nitrogen limitation of tree growth in boreal forests?

Symbioses between plant roots and mycorrhizal fungi are thought to enhance plant uptake of nutrients through a favourable exchange for photosynthates. Ectomycorrhizal fungi are considered to play this vital role for trees in nitrogen (N)-limited boreal forests. We followed symbiotic carbon (C)-N exchange in a large-scale boreal pine forest experiment by tracing (13) CO(2) absorbed through tree photosynthesis and (15) N injected into a soil layer in which ectomycorrhizal fungi dominate the microbial community. We detected little (15) N in tree canopies, but high levels in soil microbes and in mycorrhizal root tips, illustrating effective soil N immobilization, especially in late summer, when tree belowground C allocation was high. Additions of N fertilizer to the soil before labelling shifted the incorporation of (15) N from soil microbes and root tips to tree foliage. These results were tested in a model for C-N exchange between trees and mycorrhizal fungi, suggesting that ectomycorrhizal fungi transfer small fractions of absorbed N to trees under N-limited conditions, but larger fractions if more N is available. We suggest that greater allocation of C from trees to ectomycorrhizal fungi increases N retention in soil mycelium, driving boreal forests towards more severe N limitation at low N supply.