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Effect of P availability on the diversity and functioning of mycorrhiza and mycorrhizosphere bacteria in maritime pine stands of the Landes forest ecosystem

Domergue O., Plassard C., Prin Y., Guinberteau J., Bakker M., Augusto L., Trichet P., Saur E., Hinsinger P., Jaillard B., Ali M.A., M'Balla-Morgan J., Duponnois R., Castells C., Galiana A.. 2007. In : Second International Rhizosphere Conference, Montpellier, France, 26-31 August 2007. s.l. : s.n., p. 54-54. International Rhizosphere Conference. 2, 2007-08-26/2007-08-31, Montpellier (France).

The maritime pine (Pinus pinaster), distributed from Moroccan Atlantic coast to South of Europe, is the most planted forest species in France with 1million hectares located in the Landes region. Most of these monospecific plantations grow on sandy acidic soils that are deficient in mineral elements, phosphorus in particular. Since mycorrhiza associated to pine roots play an important role in P availability, the different fertilization regimes applied to pine plantations throughout the Landes forest could have a direct effect on the diversity and functioning of mycorrhiza and their associated bacteria. We analyse the diversity of pine ectomycorrhiza and that of their associated mycorrhizosphere bacteria (MB) in thirty representative forest sites ranging from 6 to 93 years-old, along three successive surveys (autumn 2005 to autumn 2006). After intensive mycorrhiza sampling from soil cores combined to a systematic survey of sporophores, mitochondrial DNA of mycorrhiza and sporophores was sequenced (ML5/ML6 primers). MB were isolated on trypcase medium from the surface of mycorrhiza and their ability to solubilize P was assessed after incubation on TCP agar medium. Among the isolated MB, fluorescent pseudomonads, known to include mycorrhization helper bacteria, were also detected using the King's B medium. The first results show a great diversity of mycorrhizal species (~30) wiIh a predominance of Russulaceae species. Clear differences are observed in the proportion of various fungal species according to the site and between belowground fungal micoflora (mycorrhiza) and aboveground one (sporophores). The proportion of MB able to solubilize inorganic phosphate is significantly higher, in non-P.fertilized plots than in those regularly fertilized with P, 70% vs 45% respectively. Among all the MB isolated, the proportion of fluorescent pseudomonads varied dramatically according to the season, i.e. from about 5% in autumn to 45% in spring. 16S gene sequencing of MB will be performed to investigate specific affinities and functional relationships between bacterial and fungal partners. (Texte intégral)

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