INOCULATION OF ACACIA-MANGIUM WITH ALGINATE BEADS CONTAINING SELECTEDBRADYRHIZOBIUM STRAINS UNDER FIELD CONDITIONS - LONG-TERM EFFECT ON PLANT-GROWTH AND PERSISTENCE OF THE INTRODUCED STRAINS IN SOIL

The growth response of Acacia mangium Willd. to inoculation with selec ted Bradyrhizobium strains was investigated in two field trials in the Ivory Coast (West Africa). In the first trial (Anguededou), four prov enances (i.e., trees originating from seeds harvested in different geo graphical areas) of A. mangium were inoculated with four Bradyrhizobiu m strains from different origins. Six months after being transplanted in the field, the heights of all inoculated trees showed a statistical ly significant increase of 9 to 26% compared with those of uninoculate d trees, with the most effective strain being Aust 13c. After 19 month s, the positive effect of inoculation on tree growth was confirmed. Th e effect of A. mangium provenance on tree growth was also highly signi ficant. Trees from the Oriomo provenance of Papua New Guinea had a mea n height that was 25% greater than those of other provenances. Analysi s of variance showed a highly significant effect of interaction betwee n strain and host provenance factors. Thus, most effective strain x pr ovenance combinations could be proposed. Immunological identification of strains clearly showed that 90 to 100% of nodules from trees inocul ated with three of the four Bradyrhizobium strains or from uninoculate d trees contained exclusively Aust 13c 23 months after tree transplant ation. This predominance of Aust 13c in nodules was still observed 42 months after tree transplantation. The second experiment (Port-Bouet), performed with a different soil, confirmed the long-term positive eff ect of Aust 13c on plant growth, its high competitive ability against indigenous strains, and its persistence in soil. Strain Aust 13c shoul d thus be of great interest for inoculating A. mangium under a wide ra nge of field conditions.