Interactions Between Rhizobia Nodulating Trifolium ambiguum and Trifolium repens in the Field

Trifolium ambiguum (Caucasian clover) is being released in New Zealand for use in regions where T. repens (white clover) growth is marginal. T. ambiguum has specific nodulation requirements and seed must be inoculated as appropriate strains of rhizobia are not found in NZ soils. T. ambiguum rhizobial strains (eg. ICC105) also rapidly nodulate T. repens but, although they contain structurally normal bacteroids, the nodules do not fix nitrogen. Large scale planting will lead to the establishment of high numbers of T. ambiguum rhizobia in soil and potentially to a rhizobial population that causes ineffective or mixed nodulation of T. repens, resulting in poor nitrogen fixation. Experience has shown that once rhizobia for a particular legume become established in the soil, it is difficult to replace them with improved strains.