Quantification of ectomycorrhizal fungal effects on the bioavailability and mobilization of soil P in the rhizosphere of Pinus pinaster

A differential effect of ectomycorrhizal symbiosis on soil P mobilization and host P nutrition is shown after culture of Pinus pinaster associated with Hebeloma cylindrosporum and Rhizopogon roseolus, poor and good oxalate/proton producers, respectively. Plants were grown in minirhizoboxes with a thin layer of a Mediterranean soil with a low level of easily available P. This soil was supplemented, or not, with inorganic P and/or CaCO3. The fungal efficiency on P mobilization and host mineral nutrition was quantified after a 3-month culture period. R. roseolus had a strong effect on the mobilization of poorly available P, whereas H. cylindrosporum had no effect. However, CaCO3 suppressed the positive effect of R. roseolus. Hydroxyapatite had the greatest effect on growth and P nutrition of nonmycorrhizal plants. With soluble P addition, both ectomycorrhizal species improved shoot P concentrations. The relationship between soil available P and shoot P concentrations enabled us to separate the fungal effects into two categories, the chemical actions and the soil exploration, explaining the positive effect of ectomycorrhizal symbiosis on P. pinaster P nutrition.