Rhizobia survival and new methods to improve nodulation in tropical legumes

The Rhizobia survival and new methods to improve nodulation in tropical legumes project looked at ways to better establish rhizobia for pasture legumes.

Summary

The Review of productivity decline in sown grass pastures identified legumes as the best long-term solution to the losses of up to 50% in productivity that occur with ‘rundown’ across huge areas of tropical grass pastures in Queensland and northern New South Wales.

When legumes are introduced to these sown pastures they can increase available soil nitrogen to sustain grass production. However, their contribution to soil nitrogen is often inhibited by commercial inoculation practices that limit effective nodulation of legumes, their subsequent growth and therefore their contribution to soil nitrogen. There is a great opportunity to improve inoculation practices and so lift productivity across the northern beef industry.

This project tested the effectiveness of rhizobia in sub-tropical pastures and evaluated new approaches with the potential to improve rhizobia establishment, and legume nodulation for nitrogen fixation and greater legume growth. Glasshouse trials assessed the effectiveness of ‘native’ rhizobia in soils collected from across the region. However, field trials were used to assess these new inoculation approaches in the hot, dry environments in which perennial tropical legumes are typically sown.

The results of this project demonstrated that for both legume species(Stylosanthes seabrana or Caatinga stylo and Desmanthus virgatus), 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.

These trials and their results were discussed with graziers participating in the Improving productivity of rundown sown grass pastures project to determine the most practical methods for commercial properties and develop recommendations for their use across the region. The ‘rhizobia’ project then supported the ‘rundown’ project to demonstrate these recommendations on commercial farms. The results and outputs of the project were also shared with participants of the ‘sub-tropical pastures project in the new MLA Southern Feedbase Initiative which identified rhizobia and inoculation practices as a critical area for future RDE investment.

When: 1 July 2012 to 30 June 2015

Contact: Brian Johnson

Collaborator: Department of Agriculture and Fisheries

More information

For more details please refer to the final report summary and download the final report (B.NBP.0748) (PDF, 1.1 MB) from the Meat & Livestock Australia website.