Nodulation and nitrogen fixation by some acacia species in the Sudan

This study has been carried out to investigate the capability of some species of the genus Acacia (A. seyal, A. senegal, A. nilotica, A. polycantha, A. mellifera, A. nubica and A. tortilies) for nitrogen fixation and to assess the efficiency of isolated and re-inoculated rhizobia for nodulation and N2 fixation. Also the effects of different soil types, (clay and sand), moisture [field capacity (M1), 2/3 field capacity (M2) and 1/3 field capacity (M3) on A. polycantha, A. seyal and A. mellifera] and salinity levels [0.5 (S1), 2 (S2), 4 (S3), 6 (S4) and 8 dS/m (S5) A. polycantha, A. seyal and A. mellifera] on growth performance, nodulation and N2 fixation of Acacia species. The amounts of N2 fixed by some Acacia species (A. seyal, A. senegal, A. mellifera and A. nubica) was estimated using 15N isotope dilution method. Tamarindus indica was used as a reference plant. Fertilization was carried out by adding 20 (5% 15N atom excess) and 100 (1% 15N atom excess) kg N/ha for the fixing plants and the non-fixing plant, respectively (all the experiments were executed in pots). The following parameters were determined: dry biomass, nodule number and dry weight, N% and N- content. In the last experiment, plant tissue analysis was carried out for 15N and 14N. In the two experiments (Testing for Natural Nitrogen Fixation and Rhizobium Inoculation of selected Acacia species),the study proved that the species showed significant differences in the growth parameters. Acacia polycantha, A.seyal and A. mellifera showed the highest means. The inoculated plants gave the highest biomass and nodulation parameters. The most vigorous species (Acacia polycantha and A. mellifera) gave the highest biomass and N-content. In both clay and sandy soils, Acacia species responded positively to inoculation. Generally, in all parameters, the clay soil gave the highest means. The study showed that A. polycantha and the moisture level M2 gave the highest means for the different growth parameters. Plants at M2 were affected positively by inoculation. Inoculation was favored by M2 while it was suppressed by both M1 and M3. Acacia polycantha at all the moisture levels gave the highest nodule number. Inoculation did not affect the N-content of A. mellifera while A. polycantha was affected positively. The dry biomass for all the species increased when inoculated. A. mellifera at M2 gave the highest mean dry biomass. Inoculated A. seyal was not affected by the different moisture levels. The study showed that salinity affected growth and nodulation (natural and inoculated) of the three tested A. species. Nodulation was affected more by salinity rather than by inoculation. Nodule number increased at S2 (2 dS/m) and then decreased with increased salinity. A. mellifera showed tolerance to salinity, A. polycantha low tolerance to salinity while A. seyal was less tolerant to salinity. The study showed differences in N2 fixation among the tested A. species (A. nubica, A. senegal, A. mellifera and A. seyal) both in the % of the need of the plant (60.0%, 58.0%, 53.0% and 50.0%) and total N2 fixed (0.063, 0.104, 0.098 and 0.078 g/plant, respectively)