Issues affecting the competitiveness of white clover rhizobia in New Zealand pastures

Research into improving symbiotic nitrogen fixation of white clover in New Zealand pastures through the introduction of effective rhizobia is reviewed. Naturalised populations of rhizobia are usually highly diverse and of reduced effectiveness compared to inoculant strains, and large increases in nitrogen fixed have been found in situations where high nodule occupancy by an inoculant strain was obtained. The likelihood of an inoculant strain initially forming a high proportion of nodules is dependent on the size of the naturalised and inoculant populations, and the strain of rhizobia. Lack of persistence of the inoculant strain in competition with naturalised rhizobia also limits improvement of symbiotic nitrogen fixation in pasture through inoculation. Recent studies suggest that genetic instability of inoculant strains and exchange of symbiotic plasmids contribute to the diversity of naturalised populations and lack of inoculant persistence. Therefore, it is necessary to understand the ecology of naturalised populations, including their genetic interactions with inoculant strains, in order to develop strategies to improve the competitiveness and persistence of inoculant strains. Alternatively it may be possible to increase the effectiveness of indigenous populations through gene transfer from the inoculant strain. The possibility of breeding specific host cultivar/rhizobial strain combinations also merits further research. Keywords: competition, genetic stability, inoculation, nitrogen fixation, rhizobia, white clover