Variations in seedling growth, nodulation and nitrogen fixation of Acacia nilotica inoculated with eight rhizobial strains

Abstract

Nodulation, nitrogen fixation and growth in seedlings of a Senegalese provenance of Acacia nilotica subsp. tomentosa, inoculated with eight different Rhizobium strains isolated from various African Acacia species, were measured. The Rhizobium strains were markedly different in their ability to induce functional nodules suggesting, in view of previous reports of cross-inoculation studies, that strain specificity at subspecies and/or provenance level may occur in A. nilotica. Three strains isolated from Acacia species other than A. nilotica were non-invasive. Of two strains isolated from A. nilotica subsp. kraussiana, one strain induced formation of ineffective nodules, while the other induced formation of effective nodules displaying a mean acetylene reduction rate, on nodules from 10-week-old seedlings, of 57.7 μmol g −1 h −1. This is more than ten times higher than previously reported for an unspecified subspecies of A. nilotica in a field trial in India and compares favourably with results reported from other nitrogen-fixing tree species widely promoted for use in agroforestry such as Leucaena leucocephala. Three re-inoculated strains induced the formation of partially effective nodules. Seedlings with effective nodules had 50% larger dry mass and more than double the total nitrogen content of non-nitrogen fixing seedlings. Variability in total dry weight, nodule dry weight and total nitrogen content of effectively nodulated seedlings were high and strongly positively correlated ( r=0.88 to 0.99) indicating considerable potential to improve initial growth rates and nitrogen fixation in A. nilotica through selection.