Growth improvement of Ethiopian acacias by addition of vesicular-arbuscular mycorrhizal fungi or roots of native plants to non-sterile nursery soil

The effect of applying vesicular-arbuscular mycorrhizal (VAM) fungi or chopped roots of native plants to a standard, non-sterile nursery soil was investigated in a tree nursery in Ethiopia. Acacia abyssinica and Acacia sieberiana were grown for 3 and 4 months, respectively, in five different soils: standard nursery soil, soil inoculated with a tropical or with a temperate isolate of the VAM fungus Glomus mosseae , and soil inoculated with chopped roots of native forest plants or of grasses. Inoculation with both kinds of native roots and with the tropical G. mosseae isolate increased the shoot dry weight of A. sieberiana significantly, by 48, 33 and 36%, respectively, compared with seedlings in the standard soil. Inoculation with native forest roots and the tropical G. mosseae increased the shoot dry weight of A. abyssinica by 23 and 38%. The temperate isolate of G. mosseae did not improve the growth of either acacia species significantly. Growth stimulation was accompanied by a significantly higher mycorrhizal colonization in all cases except one, whereas shoot phosphorus and magnesium uptakes were generally not significantly affected. The survival of the A. abyssinica seedlings 6 months after their transfer from the nursery to the field was enhanced by all inoculations, although significantly so only with the tropical VAM isolate. The root dry weight after 18 months in the field was also improved in this treatment. The results demonstrate that application of a simple inoculum consisting of chopped roots of native plants with naturally occuring VAM fungi can be equally effective in enhancing seedling growth in tree nurseries in the tropical region as the more elaborate use of pure cultures of VAM fungi, both when seedlings are amply watered and when they are drought-stressed.