Assessment of field situations for the feasibility of vesicular-arbuscular mycorrhizal inoculation,using a forage legume as test plant

This paper reports experiments aimed at predicting ecological situations where vesicular-arbuscular mycorrhizal (VAM) inoculation would be beneficial. Twelve unsterile soils were assayed and Hedysarum coronarium, a forage legume, was the test plant. Three vesicular-arbuscular endophytes, namely Glomus mosseae, G. fasciculatus and a Glomus sp. were tested, and in all cases G. mosseae was the more efficient, with indications of some plant-fungus specificity. A considerable degree of dependency on mycorrhizas, for suitable growth and N₂-fixation, was also demonstrated for the tested legume-Rhizobium sp. system. The studied soils varied in their concentration of plant-available phosphorus and in the amount, infectivity and effectiveness of indigenous VAM propagules; however, no correlation between these parameters could be found. The level of success of the introduction of G. mosseae in the presence of naturally existing endophytes was evaluated by the extent of the plant response in terms of growth, nodulation and N and P uptake in each soil. The inoculation of G. mosseae was effective in 7 out of the 12 soils; however, it was not possible to find a correlation between any of the soil parameters studied (i.e. soluble P concentration, amount of mycorrhizal root pieces and VAM mycelium and spores) and the plant response to G. mosseae inoculation. Thus, it is necessary to emphasize the need for simple techniques to screen situations where it would be worth attempting VAM inoculation, since each particular soil must be checked for each given plant-fungus combination.