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Phosphorus mobilization from forest soils by
ectomycorrhizal fungi: recent data and actual role
Claude Plassard
To cite this version:
Claude Plassard. Phosphorus mobilization from forest soils by ectomycorrhizal fungi: recent data and actual role. 2. Conference of the DFG Priority Program SPP 1685 ”Ecosystem Nutrition 2015”, Jul 2015, Dipperz, Germany. �hal-02739036�
Communication invitée Annual meeting Scientific Priority Program (SPP) 1685 and « Ecosystem Nutrition 2015 », 2-4 July 2015, Dipperz (Germany) Title : Phosphorus mobilization from forest soils by ectomycorrhizal fungi: recent data and actual role. Claude Plassard, INRA Montpellier France plassard@supagro.inra.fr Abstract: It is assumed that the most important role of (ecto)mycorrhizal symbiosis is to improve the P nutrition of the host-plant. Several hypothesis have been proposed to explain this positive role of the symbiosis that are (1) the increase of soil exploitation and P uptake by the fungus developing outside the roots, (2) the capacity to produce organic anions to mobilise adsorbed mineral P and (3) the capacity to produce enzymes, especially acid phosphatases able to mineralize organic P forms, to release free inorganic P (Pi) that could benefit to the host plant. The talk will present the results we obtained regarding the possible role of ectomycorrhizal symbiosis to mobilize soil mineral and organic P from forest soils. We worked mainly with Pinus pinaster as a model species. We used different approaches either in the field or in the laboratory. In the field, we explored the diversity of the ectomycorrhizal species associated with 14 year-old P. pinaster plantations as a function of fertilization (no fertilization, P or NPK fertilization) and irrigation. We also measured the enzyme activities produced by the ectomycorrhizal tips to assess their variability as a function of the fertilization treatment and/or the fungal species. In the laboratory, we cultivated young seedlings in rhizoboxes containing soil collected either near Montpellier (chromic cambisol) or from the same P. pinaster plantation, mentioned above. We cultivated young P. pinaster seedlings, whether or not associated with “laboratory” fungal species such as Rhizopogon roseolus (strong oxalate producer) or Hebeloma cylindrosporum (strong acid phosphatase producer), or with native fungal species from field soil samples. We studied also the interactions between ECM plants and other actors of the P cycle that are bacteria able to mineralize phytate, a recalcitrant organic P source, and their predators that are bacteria-feeding nematodes. In the field, we detected ≈ 20 species using DNA extracted from individual ECM tips and ITS sequencing. As expected, the fertilization regime modified the diversity of the ECM species associated with the roots with the maximum level of diversity found in irrigated, not fertilized plots (18 species) and the lowest one in non irrigated, fully fertilized plots (5 species). We found that N-acetlyl-glucosaminidase (NAG) and acid phosphatase (AcPAse) activities were the highest ones among the 8 activities assayed. Contrary to the NAG that did not vary much between treatments, the AcPAse could vary with fertilization but the variations seemed to depend first on the species. In the laboratory experiments, if our results demonstrated the importance of the fungal oxalate release to mobilize mineral P towards the host plant, our results are more in favour of an indirect role of the ectomycorrhizal fungi to mobilize organic P present in the medium. Therefore a new model of ectomycorrhizal functioning will be proposed.
