Rhizobia survival and new methods to improve nodulation in tropical legumes

Summary

​This project aimed to test the effectiveness of introduced and native rhizobia in two tropical pasture legumes (Stylosanthes seabrana or Caatinga stylo and Desmanthus virgatus) and evaluate new approaches with the potential to improve rhizobia establishment and legume nodulation for improved nitrogen fixation and greater legume growth. Glasshouse trials were conducted on a range of collected pasture soils to assess the legume species compatibility and nodulation potential of native rhizobia. Field trials to measure the impact of introduced rhizobia using new inoculation approaches in the hot, dry environments in which perennial tropical legumes are typically sown were also conducted. The results of this project demonstrated that for both legume species, inoculation using commercial rhizobia strains is a well-founded recommended practice and that methods that deliver inoculum deeper into the soil such as water injection or granules are more likely to be effective than inoculum on seed that is sown shallow. Of the 14 soils assessed in the glasshouse, five soils showed significant increases in nodulation due to the addition of the commercial inoculum strain (CB3481) for Caatinga stylo with five others showing increases. Four of those soils also yielded an increase in plant growth in Caatinga stylo where inoculum was added (an important result given it is not always easy to show responses due to the inherent soil fertility masking or reducing nitrogen fixation in the initial stages of pasture establishment and growth). Three soils had higher nodulation with the addition of the commercial inoculum strain (CB3126) for desmanthus but this did not translate into an increase in plant growth and may indicate that more research is required to identify a more effective rhizobium strain for this species. This study shows that for many of soils of the brigalow bioregion, inoculation with an effective rhizobia strain that is specific to the legume species is critical. Even if native strains do form nodules with either species, there is no commercially available method to test a soil for the presence of compatible and effective rhizobia strains. It is also therefore very important that introduced commercial strains survive the inoculation and planting processes so that enough rhizobia are present when the seed germinates.