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 mean of a non-destructive technique using the pH indicator bromocresol purple. Resuitsshowed 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 intially 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 the rhizosphere by the arbuscular-mycorrhizal plants is discussed.