Changes in the rhizospheric pH induced by arbuscular mycorrhiza formation in onion (Allium cepa L.)

Rhizospheric pH changes induced by arbuscular mycorrhiza formation in onion plants fertilized either with NO₃^? or NH₄⁺ were studied. The pH changes promoted by either mycorrhizal or non-mycorrhizal roots were studied by means of a non-destructive technique using the pH indicator bromocresol purple. Results showed that the pH changes observed depended on i) the symbiotic status of the root and ii) the N form amended to the soil. When growing in a NH₄⁺-supplied soil, mycorrhizal onion roots produced more intense and wider acidification halos than non-mycorrhizal plants did. These differences were maintained throughout the whole experiment (60 days). NO₃^?-supplied mycorrhizal roots initially promoted a more intense alkalinization on their surface, compared to the control roots (30 days); however, at the end of the experiment (60 days), intense acidification halos were observed in the mycorrhizosphere, whereas this acidification was almost absent in the non-mycorrhizal rhizosphere. The link between these mycorrhiza-induced pH changes in the soil and the higher efficiency in the exploitation of nitrogen in the rhizosphere by the arbuscular-mycorrhizal plants is discussed.