L'inoculum endomycorhizien : composition et méthodes d'inoculation

National audienc

Utilisation de l'offre potentielle du sol par les plantes

National audienc

Applications pratiques des endomycorhizes VA

National audienc

Receptiveness of some tropical soils from banana fields in Martinique to the arbuscular fungus Glomus intraradices

International audienc

Mycorhizes et nutrition phosphatée des plantes

International audienc

Potentiel infectieux mycorhizogene du sol des parcelles du dispositif Deherain

National audienc

Is the isotopically exchangeable phosphate of a loamy soil the plant-available P?

International audienceThis study compared the validity of using the isotopically exchangeable phosphorus (P) as an accurate measurement of plant available P by comparing the specific activity of P, i.e. the 32P/31P ratio, in soil solution (Ss) against the specific activity of P in plants (Sp) growing in a loamy soil after applying a 32P-labelled fertilizer (NaH2PO42H2O) at different rates (F) and specific activities (Sf). Non-mycorrhizal and mycorrhizal (Glomus intraradices) plants of two species (soybean and barley) were grown in greenhouse experiments. Ss values were determined on 1:10 soil suspension after periods of incubation ranged from 1 min to 35 d. At a given rate of P application, the Sp values of both non-mycorrhizal and mycorrhizal soybean and barley did not show significant difference although the plant P uptake varied 18 fold for all the (crop species × mycorrhizal infection) treatments over soil solution P values ranging from 0.02 to 5.46 mg P L-1 (0.6–176 μM). Ss values decreased with time and reached a steady state after 35 d of equilibration period. Both Sp/Sf and Ss/Sf increased with applied P and there is a 1:1 correspondence between Sp/Sf and Ss/Sf values. The identity between the isotopic composition of both P in soil solution and in plant indicates that the isotopically exchangeable P (E=F(Sf/Ss−1)) is the only source of phosphate in solid soil phase which replenishes P of the soil solution after P has been removed by the plant, i.e. the only source of P which participates in plant nutrition. The isotopically exchangeable P of a loamy soil is the P available to growing plants and mycorrhizal fungi increases the P uptake giving plants wider access to isotopically exchangeable P in soil, and not making previously non-exchangeable P available. An immediate application of the 1:1 correspondence between a soil parameter (Ss/Sf) and a plant parameter (Sp/Sf) concerns the agronomic evaluation of P fertilizers

External phosphorus requirement of mycorrhizal and non-mycorrhizal barley and soybean plants

International audienceAn experiment was conducted under greenhouse conditions to evaluate the effects of vesicular arbuscular mycorrhizal (VAM) fungi on the external P requirements of barley and soybeans. The plants were grown in pots containing a P-deficient soil. A range of 10 P levels was obtained by adding 0, 20, 30, 40, 50, 60, 70, 110, 160, or 310 mg P kg-1 as NaH2PO4 2H2O. Half of the pots were inoculated with the VAM fungus Glomus intraradices. The P concentration in the soil solution was determined using an adsorption isotherm and plotted against the relative yield. Barley did not respond to mycorrhizal inoculation and we concluded that P nutrition was not the limiting factor on the growth of this lowmycotrophic plant. In contrast, mycorrhizal inoculation stimulated the growth of soybeans. The external P requirements were 0.110 μg ml-1 for mycorrhizal and 0.148 μg ml-1 for non-mycorrhizal soybeans to obtain 80% of the maximum yield. In terms of P fertilization this corresponds to a saving of 222 kg P2O5 ha-1. The mycorrhizal dependency of the soybean was highly correlated with the P concentration in the soil solution and it is proposed that both values should be displayed together