Intact soil cores were sampled from around three leguminous tree species (Acacia nilotica, Acacia tortilis and Prosopis juliflora) in 10-year-old alley-cropping plots at Thiénaba, Sénégal. Cores were removed from two depths (0-25 and 25-50 cm) and at two distances from the trunk (1 and 5 m). Duplicate soil cores were taken for assessment of root concentration (cm/100 cm3soil), mycorrhizal infection (% of infected root length) and spore concentration in the soils. In order to determine the mycorrhizal inoculum potential (MIP) of the soils, a mycorrhizal bioassay of the soil cores was conducted in the greenhouse using millet seedlings. For all plots, seedlings grown in soils from the surface layer (0-25 cm) were larger and formed higher levels of infection than those grown in soils from 25 to 50 cm depth. Mycorrhizal infection of the seedlings was greatest in soil from the A. tortilis plots and, unlike the other tree species, also the greatest in soil collected near the tree. Positive relationships were found between the growth and infection of the bioassay seedlings and the root and spore concentrations in the field soils. Seedling growth and infection may also have been related to higher levels of carbon in the field soils. The results indicated that root and spore concentrations in the field soils were good indicators of MIP, but that levels of root infection were not. The results also indicated the potential benefit to crop yield of maintaining high levels of mycorrhizal propagules in alley-cropping soils and a possible role of the trees in maintaining these sources of inoculum. © 2001 Elsevier Science B.V.
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