The relationship of Trifolium repens and T. ambiguum with host-specific Rhizobium bacteria for potential incorporation into sustainable, low N input pastures

Research on efficient management systems to optimise Trifolium repens-Rhizobium symbiosis, is lacking in South Africa. The amount of nitrogen (N) fixed by symbiotic rhizobia in root nodules of T. repens is ultimately determined by health of the soil environment. Soil organic matter (SOM) is the main attribute that will sustain soil health as it affects the chemical, physical and biological aspects of soil. The aim of this study was to determine the potential of T. repens and T. ambiguum to sustain low N input pastures. The hypotheses of this study is that SOM can play an important role in ensuring good soil health, which supports the optimum growth and production of Trifolium spp. in low N input pastures. Four Trifolium repens cultivars were evaluated in field conditions to determine the effect of Rhizobium bacteria on the potential of the cultivars to nodulate. The cultivars Grasslands Huia, Haifa, Ladino and Regal were selected for assessment. The thousand-seed-mass (TSM) of each cultivar was measured to determine the quality and viability of the seed; and to determine the interaction between seed mass and nodulation. Mean TSM values of T. repens cultivars differed significantly, with Huia having the highest TSM followed by Haifa, Ladino and Regal. Biomass production was also measured as an indicator of efficiency of nitrogen fixation. The cultivar Huia, with the heaviest seed, showed the highest biomass production. After eight weeks of growth, the nodulation index was determined from the size, number and colour of the bacterially associated root nodules. All plants, regardless of cultivar, formed nodules within eight weeks. It was concluded that TSM had no notable effect on nodulation. Planting date with associated temperature effects and the intrinsic cultivar effect also had no influence on nodulation. It was therefore concluded that nodulation potential of the four cultivars tested was similar in the specific environmental conditions. The total number of symbiotic rhizobial cells per gram of soil as affected by soil C content and the host plant was also determined. Inoculated and un-inoculated seeds were planted on five soil treatments, each with a different level of soil C. The plant infection technique (most-probable-number/MPN technique) was used to quantify the rhizobial numbers in soil as affected by soil C content and the host plant. The mean MPN-value ranged from 8907 to 78 Rhizobium cells per gram of soil for T. repens treatments, and 0 to 436 for T. ambiguum treatment. Soil C had no effect on the number of Rhizobium bacteria present in the soil. Inoculation however, had a significant effect on the MPN value of T. ambiguum, but not for T. repens. Most symbiotic Rhizobium was detected between a soil C content of 2.03% to 3.80% in both inoculated and non-inoculated soils. The spread plate count was used to determine the total number of symbiotic and saprophytic rhizobia. This method was used to quantify both symbiotic and free-living rhizobia. The effect of different levels of soil C on the amount of atmospheric N fixed was assessed by the N difference technique. Arctotheca calendula (cape weed) served as the reference plant in this study, to determine what percentage of N is derived from the atmosphere (%Ndfa). Biomass production was determined and served as the parameter to establish the efficiency of the Rhizobium bacteria in the soil. Inoculating seed with host specific rhizobia had no effect on the amount of N fixed. The mean %Ndfa differed significantly between soil organic C treatments with the species T. repens but did not differ significantly between soil organic C treatments with T. ambiguum. It was concluded from this study that symbiotic rhizobia introduced by inoculant was much more efficient in higher C content soils than free-living rhizobia, which highlights the importance of inoculation in improving the sustainable production of T. repens pastures. Although the amount of N fixed increased as the level of soil organic matter decreased, the efficiency of N fixation decreased proportionally. This explains the bigger change in soil N content on soil with a high C content. This study has thus highlighted the importance of soil organic carbon in the host specific Rhizobium inoculation sucsess, of T. repens low N input pasture systems.