Mycorrhizal inoculum potential of soils from alley cropping plots in Senegal

Intact soil cores were sampled from around three leguminous tree species (A cacia nilotica, Acacia tortilis and Prosopis juliflora) in 10-year-old alle y-cropping plots at Thienaba, Senegal. Cores were removed from two depths ( 0-25 and 25-50 cm) and at two distances from the trunk (1 and 5 m). Duplica te soil cores were taken for assessment of root concentration (cm/ 100 cm(3 ) soil), mycorrhizal infection (% of infected root length) and spore concen tration in the soils. In order to determine the mycorrhizal inoculum potent ial (MIP) of the soils, a mycorrhizal bioassay of the soil cores was conduc ted in the greenhouse using millet seedlings. For all plots, seedlings grow n in soils from the surface layer (0-25 cm) were larger and formed higher l evels of infection than those grown in soils from 25 to 50 cm depth. Mycorr hizal infection of the seedlings was greatest in soil from the A. tortilis plots and, unlike the other tree species, also the greatest in soil collect ed near the me. Positive relationships were found between the growth and in fection of the bioassay seedlings and the root and spore concentrations in the field soils. Seedling growth and infection may also have been related t o higher levels of carbon in the field soils. The results indicated that ro ot 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 main taining these sources of inoculum. (C) 2001 Elsevier Science B.V. All right s reserved.