Mycorrhizal functioning of Phialocephala fortinii with Pinus contorta on glacier forefront soil: interactions with soil nitrogen and organic matter.

Plants growing on an environmentally stressed glacier forefront on soil low in N and organic matter have abundant root colonizations by dark-septate fungi. As the plants appeared fit for this severe habitat, it was hypothesized that the dark-septate endophytes were neutral or beneficial rather than detrimental to the plants. To test this hypothesis, we designed a growth-room experiment with Pinus contorta grown on forefront soil inoculated with the dark-septate fungus Phialocephala fortinii in the absence of climatic stress. N and organic matter treatments were included to explore their interaction with the fungal inoculation. P. fortinii colonized roots inter- and intracellularly and occasionally formed microsclerotia. Inoculated plants absorbed significantly more P than noninoculated plants in all combinations of N and organic matter. Without added N, neither inoculation nor organic matter addition improved plant growth or N uptake, showing that N indeed limits plant growth in this substrate. With added N, however, both organic matter addition and inoculation significantly increased total pine biomass and N uptake. The enhanced P uptake by the P. fortinii-inoculated pine as well as the increased pine growth and N uptake in the treatment combining P. fortinii and N appear as typical mycorrhizal responses.