Increasing biological nitrogen fixation by white clover-rhizobia symbiosis

Biological nitrogen fixation (BNF) resulting from the legume-rhizobia symbiosis converts atmospheric nitrogen (N) into mineral forms for plant use. This is considered a sustainable approach for improving N fertility on farm. Field surveys across New Zealand (NZ), within a farm and within paddocks, have revealed large spatial variability of rhizobial population size and symbiotic effectiveness with white clover. These results indicate that naturalised rhizobia may not be supporting optimal BNF. Over 500 strains of clover-nodulating rhizobia were isolated from NZ pasture soils, with more than 90 demonstrating greater N-fixation capacity with white clover than the commercial inoculant strain TA1. All seven NZ isolates tested had significantly higher nodule occupancy rates compared to TA1 in an in vitro assay, indicating increased competitiveness of those strains. In addition, novel seed coating technology improved the survival of TA1 from one month to more than four months. The potential to increase the symbiotic capacity of white clover is indicated by the availability of more effective and competitive rhizobial strains, along with their improved survival on seed provided by a new coating technology.