Looking underground for revegetation solutions: re-introduction of beneficial organisms helps plants to withstand environmental stresses like soil salinity and competition with exotic weeds by Sophie Clayton

Comparison of inoculated (right) and uninoculated acacia

ONE OF the biggest challenges that comes with revegetation is knowing how to replant native trees and bushes quickly, cheaply and, above all, effectively.

A closer look at the long-neglected field of soil organisms and native vegetation, which could help farmers with broadscale revegetation projects, is underway with Peter Thrall and colleagues at CSIRO Plant Industry. Their research has the potential to greatly increase survival, vigour and health of reintroduced trees and understorey.

Looking at a cleared agricultural landscape, it's easy to see there is often little or no natural vegetation left. But what is not so obvious is that there is a huge diversity of soil organisms under the ground that may also be lost.

Many plants - including Australian natives such as acacias - grow in ' symbiotic' or mutually beneficial relationships with a range of soil organisms. Dr Thrall and his team have identified significant benefits associated with growing trees from seeds coated, or ' inoculated' , with a peat-based substance containing specific strains of naturally occurring soil bacteria.

"In preliminary glasshouse and field trials, we have found that acacias inoculated with beneficial strains of a nitrogenfixing bacteria called Bradyrhizobium survive better and grow faster than un-inoculated acacias," Dr Thrall says.

"The Bradyrhizobium helps acacias form nodules on their roots. The bacteria in these nodules take nitrogen from the air, which plants cannot use, and ' fix ' it into a form they can use. It is this extra nitrogen, 'fixed' in the soil by the acacias, that helps them grow better by effectively fertilising them.

"Aside from seeing the inoculated plants survive and perform better, we also see other plants, such as eucalypts, growing next to the inoculated acacias also growing better as they can take advantage of the extra nitrogen available in the soil too."

But not all strains of Bradyrhizobium promote plant growth - some may have a negative effect. Also, different strains of Bradyrhizobium will have different effects depending on the species of acacia they are associated with.

Dr Thrall and his team have been analysing soil samples collected from native remnant vegetation patches to see what types of bacteria are present and sort the helpful strains from the harmful ones. As a result, a number of 'elite' strains of bacteria have been identified which have been used to inoculate a range of acacia species as part of a large-scale field trial.

With the assistance of Greening Australia Victoria, eight sites on different farms in the Bendigo region have been directly sown with acacia seed inoculated with Bradyrhizobium to assess its effect on tree health and performance.

Each site has also been planted with a range of other local native species, such as eucalypts and casuarinas, to assess their performance.

Of particular interest is the extent to which re-introduction of beneficial organisms helps plants to withstand environmental stresses like soil salinity and competition with exotic weeds. These trials will improve the understanding of the relationships between plants and bacteria and help identify the best strains of bacteria to inoculate different seed with.

This project is a collaboration between CSIRO Plant Industry as part of the Centre for Plant Biodiversity Research and Greening Australia Victoria with substantial on-ground support from the North Central Catchment Management Authority and the Department of Sustainability and Environment, Victoria.