As researchers scurry across the globe collecting exotic legume germplasm from isolated, middle-eastern desert plains to drive the environmental and economic sustainability of Australia’s grains industry, efforts at home are integrating these novel plants into local growing systems.

Western Australia applies an average of 40 kg per hectare of nitrogen every year to maintain cereal production on impoverished southern Australian soils, making the nitrogen fixation of legumes a valuable rotational tool to help alleviate fertiliser input costs.

Meanwhile, deep-rooted legume alternatives to lucerne, with better adaptation to acidic soils, have also been earmarked as high water use crops to help manage recharge.

However, to deliver their full benefit to the farming system, all such legumes’ roots must be colonised with compatible root-nodule bacteria, known as rhizobia, to sustain the symbiotic relationship that allows them to return nitrogen to the soil.

Supported by growers and the Federal Government through the Grains Research & Development Corporation (GRDC), PhD student, Matt Denton surveyed the native rhizobium populations of southern Australia to determine how they affected the performance of root nodulation in pastures and the performance of rhizobia inoculants in aiding nodulation.

The University of Adelaide research was centred in South Australia, but targeted low rainfall, alkaline soils which typically support annual medic pastures right across southern Australia.

Using polymerase chain reaction technology to identify nodule isolates, Mr Denton found that certain rhizobia achieved low occupancy of pasture root systems under prevailing conditions. Because not all existing rhizobia inoculants were able to establish an effective symbiotic relationship with new pasture species, two strategies are recommended for the development of future legume systems.

The first is to concurrently select suitable rhizobia as a compatible commercial inoculant for release with new pasture species, as those species are being developed. The second is for breeders to select ‘rhizobially promiscuous’ legumes which are able to nodulate and fix nitrogen with a range of naturalised rhizobia.

During this GRDC project, researchers identified several dominant, naturalised rhizobia populations that shape as good prospects for helping nodulate new pasture species.

These dominant rhizobia could achieve better occupancy of pasture roots in alkaline soils to outperform existing commercial inoculants and increase the root nodulation and therefore nitrogen fixation of specific pastures across southern Australia.