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

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Abstract
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.

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