Rhizobiology and nitrogen fixation of some tree legumes native to Sri Lanka

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Rhizobiology of four tree legumes, Abarema bigemina (endemic), Adenanthera bicolor (endemic), Humboldtia laurifolia (indigenous) and Pericopsis mooniana (indigenous), was studied by: (1) observations of their in situ root nodulation, (2) characterization and authentication of rhizobia isolated from them, and (3) an examination of the infectivity and effectivity of the rhizobial isolates using the respective hosts and the broad-spectrum host, siratro (Macroptillium atropurpureum). Investigations were also conducted to evaluate the effect of rhizobial inoculation on growth and N fixation of the valuable timber species, P. mooniana, during its early stages of growth. All the hosts revealed in situ nodulation, except A. bicolor which had nodule-like swellings on its roots. While four rhizobial isolates were obtained from A. bigemina, four from H. laurifolia and 18 from P. mooniana, no authentic rhizobia were isolated from A. bicolor. The nodule-like structures did not show acetylene reduction activity, and the plants failed to nodulate upon inoculation with rhizosphere soil. A. bicolor was therefore tentatively assigned as a non-nodulating species. This report includes the first description of root nodules in the endemic species A. bigemina and the indigenous species H. laurifolia and the isolation and characterization of rhizobia from them. Significant differences were observed among the isolates with regard to their infectivity and effectivity. One isolate from P. mooniana was outstanding in its infectivity and effectivity on the original host, as well as on siratro. Inoculation of P. mooniana with this isolate together with a reference strain significantly improved nodulation and N fixation and gave a 50% increase in plant dry matter under a low level of N fertilizer, during a 12-month growth period. It is suggested that the nursery culture of P. mooniana could be improved by inoculation under low N-input conditions. The ability of these rhizobia to persist in the soil for a 12-month period was observed by the use of strains marked with antibiotics.