Puzzling isotopic patterns included high C and N isotopic signatures for mycorrhizal fungi, and a lack of agreement between mineral N and plant foliage signatures. In computer simulations of N dynamics, an isotopic fractionation during mycorrhizal transfer of N to vegetation provided the best fit to isotopic data on mineral N, plants, soils, and mycorrhizal fungi. Model and field results show that mycorrhizal fungi are less important to N supply of vegetation under high soil N availability. Similar correlations were also observed when individual species were compared across sites. I hypothesize that these correlations reflect patterns of mycorrhizal influence on plant N supply and on the allocation of C below-ground in response to changing N availability.
Isotopic analyses appear to be a powerful probe of the ecosystem role of mycorrhizal fungi.