Natural abundance of 15N in nitrogen-limited forests and tundra can estimate nitrogen cycling through mycorrhizal fungi:: A review

The hyphae of ectomycorrhizal and ericoid mycorrhizal fungi proliferate in nitrogen (N)-limited forests and tundra where the availability of inorganic N is low; under these conditions the most common fungal species are those capable of protein degradation that can supply their host plants with organic N. Although it is widely understood that these symbiotic fungi supply N to their host plants, the transfer is difficult to quantify in the field. A novel approach uses the natural N-15:N-14 ratios (expressed as delta N-15 values) in plants, soils, and mycorrhizal fungi to estimate the fraction of N in symbiotic trees and shrubs that enters through mycorrhizal fungi. This calculation is possible because mycorrhizal fungi discriminate against N-15 when they create compounds for transfer to plants; host plants are depleted in N-15, whereas mycorrhizal fungi are enriched in N-15. The amount of carbon (C) supplied to these fungi can be stoichiometrically calculated from the fraction of plant N derived from the symbiosis, the N demand of the plants, the fungal C:N ratio, and the fraction of N retained in the fungi. Up to a third of C allocated belowground, or 20% of net primary production, is used to support ectomycorrhizal fungi. As anthropogenic N inputs increase, the C allocation to fungi decreases and plant delta N-15 increases. Careful analyses of delta N-15 patterns in systems dominated by ectomycorrhizal and ericoid mycorrhizal symbioses may reveal the ecosystem-scale effects of alterations in the plant-mycorrhizal symbioses caused by shifts in climate and N deposition.