-This paper deals with the study of proton and oxalate release by two ectomycorrhizal fungi, Hebeloma cylindrosporum and Rhizopogon roseolus, grown alone or in association with Pinus pinaster and supplied with nitrate. An original culture device made it possible to observe the growth of the ectomycorrhizal roots and to measure pH and oxalate content at different levels: roots, ectomycorrhizae, external hyphae and rhizosphere soil. The results showed differences between the two species grown in vitro. These differences were strictly maintained in association with the plant. R. roseolus strongly acidified the rhizosphere and released oxalate. This oxalate excretion was enhanced by CaCO 3 in the soil. A strong correlation appeared between pH and oxalate contents in rhizosphere soil, indicating that oxalate ions and protons were released simultaneously. H. cylindrosporum did not release these ions and tended to alkalinise rhizosphere soil. Such chemicals, released by R. roseolus, could contribute to the mobilisation of P from soil.

Summary• 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 CaCO 3 . 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, CaCO 3 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.

-The release of oxalate and H + by six ectomycorrhizal isolates (Hebeloma cylindrosporum 1 and 9, Paxillus involutus, Suillus collinitus 2 and 22, Rhizopogon roseolus), able to grow in vitro with NO 3 -as the sole source of N, was measured in response to orthophosphate (Pi) concentration (20, 100 and 500 µM Pi) and CaCO 3 (+500 µM Pi) in the solution. Without CaCO 3 , all isolates excepted H. cylindrosporum ones released oxalate. For each isolate, oxalate production was not related to P-deficiency but was strongly enhanced by CaCO 3 . Whatever the medium composition, H. cylindrosporum alkalinised the solution. Among oxalate-producing fungi, S. collinitus 22 and R. roseolus exhibited an important proton efflux that could be related to transport phenomena for oxalate excretion. CaCO 3 enhanced both oxalate and proton efflux, increasing the chemical action exerted by the fungi on the mineral.

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