Phosphorus availabilities in beech (Fagus sylvatica L.) forests impose habitat filtering on ectomycorrhizal communities and impact tree nutrition

Abstract

Phosphorus (P) is an important nutrient, whose concentrations are declining in many European forest ecosystems. Here, we selected five old-aged temperate beech (Fagus sylvatica) forests that represented a sequence of decreasing soil P resources. We addressed the following hypotheses: (i) root P concentrations correspond to soil P concentrations, when P availability is suboptimal for tree nutrition, (ii) decreasing soil P concentrations, and increasing host P demand foster increasing ectomycorrhizal fungal (EMF) species richness and lead to a shift in the EMF community structure towards increasing soil exploration. We found that the decrease in P concentrations along the geosequence was less steep in the organic layer than that in the mineral topsoil. P concentrations in roots showed a positive relationship with P concentrations in soil, with a stronger correlation in coarse than in fine roots. This finding indicates that low P availability mainly affected P storage of the host. The root tips were completely colonized with EMF. In the organic layer EMF biomass was higher than that of saprophytic fungi, and correlated with inorganic P (Pi). In the mineral topsoil EMF biomass was about 10-fold lower than in the organic layer and biomass of saprophytes and microbial P, but not that of EMF, was correlated with Pi and phosphatase activities. Based on these results, we propose that beech P nutrition was mainly achieved by EMF in the organic layer. Variation in EMF species richness was unrelated to P in soil and decreased with increasing N in the organic layer. The EMF community structures were taxonomically divergent and filtered by habitat soil chemistry in the mineral layer and Pi in the organic layer between the P-rich forest and the P-poor forest. Changes in the taxonomic structures of the EMF did not result in corresponding changes in soil exploration. In conclusion, our results support a relationship between soil P concentrations and P storage in roots, but do not support mono-causal relationships between soil P and EMF species richness or hyphal soil exploration. Our results suggest that the taxonomic dissimilarities of the EMF along the P gradient were mainly driven by Pi concentrations in the organic layer and by the nutrient resources in the mineral layer.