Quantification of oxalate ions and protons released by ectomycorrhizal fungi in rhizosphere soil

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 CaCO3 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.