Rhizobia form nodules on plant roots which fix nitrogen

Rhizobia form nodules on plant roots which fix nitrogen

Investigating the role of soil microbes in native ecosystem function

CSIRO scientists and international colleagues have examined the way microbes interact with plant species to discover if their genetic diversity determines what roles they play in their native ecosystem.

  • 2 November 2010 | Updated 14 October 2011


Soil microorganisms play fundamental roles in ecosystem performance. Advances in molecular biology have helped reveal the tremendous diversity of microbial species in soil.

For example, just one gram of soil can contain as many as 10 000 discrete species. However, there is still little evidence showing concrete links between soil biodiversity and the ecological function of microbial communities (e.g. in nutrient cycling or plant growth).

CSIRO scientists and their international collaborators have been exploring whether differences in soil microbial diversity might translate into differences in functional diversity.

In other words do different microbial species play different roles in native ecosystems?

The project

The team used two approaches to investigate the relationship between genetic and functional diversity using rhizobia.

These are root-nodule forming nitrogen fixing bacteria which are present in soils associated with native Australian legumes (e.g. Acacia). Rhizobia promote plant growth, and thus can be used as a direct measure of how bacterial diversity relates to ecological function.

The first study was funded by the National Action Plant for Salinity and Water Quality. Using comprehensive glasshouse trials, the team analysed the growth of nine Acacia species treated with 40 different rhizobial strains which were isolated from 22 locations across south-eastern Australia.

An examination of plant growth in relation to the genetic identity of the rhizobial strains showed that much of the variation in plant performance could be ascribed to variation among rhizobial species.

The second part of the project was part of a broader study of geographic variation in interactions between Acacia species and rhizobia with the broad goal of improving the re-establishment of native vegetation in agricultural landscapes. This study was funded by the NSW Environmental Trust.

"The extent to which genetic variation in soil microbial composition affects key ecosystem services is poorly understood. Our results clearly demonstrate that soil community structure is an important determinant of ecological function."
Dr Pete Thrall, CSIRO

Soil samples were collected from beneath adult plants of two Acacia species – A. stenophylla and A. salicina from nearly 60 sites across New South Wales.

The team used molecular approaches to characterise the diversity of rhizobia in these soils, and also conducted an extensive glasshouse inoculation study to evaluate how well the two plant species grew in soil associated with their own species and soil associated with the other species.

Key results 

The first glasshouse study revealed that the variation seen in growth of a diverse range of Acacia species

could be linked to variation among the strains of rhizobia the Acacia were treated with. In particular, analysis showed that the 40 strains represented 13 genetically different ‘species’ which varied in their ability to promote growth of the different plant species.

The second study showed that when plants of A. stenophylla and A. salicina were inoculated with small amounts of ‘live’ soil from 60 sites there were considerable differences in how well plants grew.

Not only did plants perform better in soils which had higher densities of rhizobia, but A. stenophylla clearly preferred its own soils to those from A. salicina while A. salicina grew equally well in both. Of even greater interest was the finding that the diversity of rhizobial species at a given site played an important role in determining plant responses.

Future directions

These researchers are now building on the results found in these studies by focusing on the interactions between rhizobia and a broader range of native legumes.

The goal is to better understand how plant diversity in natural communities might influence the diversity and function of the associated rhizobial communities.

The research is delivered through CSIRO’s Sustainable Agriculture Flagship where researchers are working to identify opportunities and pathways to increase productivity whilst maintaining natural resource function and ecosystem health.

About the scientists

This work was jointly led by Dr Pete Thrall and Dr Linda Broadhurst.

Pete is a senior principal research scientist at CSIRO Plant Industry and theme leader of Advancing Productivity and Environmental Health within the Sustainable Agriculture Flagship.

His expertise in ecological genetics and coevolutionary dynamics of host-pathogen interactions have led him to focus on broader issues surrounding plant-soil interactions in relation to restoration of native plant communities.

Linda is a research scientist and Group Leader for Molecular Genetics and Evolutional Ecology with CSIRO Plant Industry whose research includes improving the restoration of Australia's highly degraded landscapes by identifying high quality seed and appropriate soil symbionts to improve planting success.

Also part of this project is Dr James Bever (Indiana University, USA), who has conducted extensive research on plant-soil interactions over many years, particularly to do with those involving mycorrhizal fungi. He became involved with this project during his time at CSIRO Plant Industry as a Fulbright Fellow.

Read more about CSIRO Plant Industry.