Biomass, community structure and phosphorus uptake of ectomycorrhizal fungi in response to phosphorus limitation and nitrogen deposition

High levels of nitrogen (N) deposition might result in a transition from N to phosphorus (P) limitation in high latitude forests. This
could have fundamental consequences for forest production, nutrient acquisition and nutrient leaching.
I studied a Norway spruce forest in a region of high N deposition in southwest Sweden and added N, P or N+P to force the
system to N or P limitation. I studied tree growth and foliar nutrient concentration. Also, using ingrowth meshbags, I followed
ectomycorrhizal (EMF) production, foraging for N and P patches (urea and apatite) and community composition.
I found that tree production was limited by P. Furthermore, P fertilization reduced EMF production indicating that EMF biomass
production was stimulated by P-limiting conditions. Apatite had a positive effect on EMF production when the system was Plimited.
P fertilization reduced foraging for nutrients by EMF, also for N rich urea. P had a stronger effect on the composition of
EMF communities than N, suggesting that P nutrition had a larger impact on belowground carbon (C) allocation than N in this
ecosystem. Furthermore, certain EMF species responded positively to apatite under P limiting conditions, which might have
increased mobilization of P from this source.
To enhance my understanding of P mobilization from different P compounds by EMF, I studied one species, Paxillus involutus,
under more controlled conditions in the laboratory. P. involutus is adapted to high N deposition levels and has a documented
capability to take up P from poorly soluble sources. I found that P. involutus was able to take up P from apatite, P bound to
goethite and from phytic acid. Moreover, I found that iron-reducing activity was produced when these sources were provided
but not when the fungus was provided with soluble P (phosphate). One possible interpretation to this result was that iron (Fe)
reduction is a way for the fungus to prevent that newly liberated phosphate ions are captured by Fe3+ and became unavailable
for uptake.
In conclusion, the high production of EMF found in P-limited forest decline when P is added, probably due to reduced
belowground C allocation when less foraging for P is needed. EMF communities are strongly regulated by P in these forests
and species better adapted for P foraging are probably selected for under these conditions.