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

The New phytologist

PubMedID: 23356503

Näsholm T, Högberg P, Franklin O, Metcalfe D, Keel SG, Campbell C, Hurry V, Linder S, Högberg MN. Are ectomycorrhizal fungi alleviating or aggravating nitrogen limitation of tree growth in boreal forests?. New Phytol. 2013;198(1):214-21.
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.