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Grasses hungry for diesel

Monday, 21 May 2007
Author: Flinders News and Research Stories

Screening by Flinders researchers tested nine species of native grass, and found three that displayed the capacity to successfully germinate and grow in soil contaminated by diesel and oil.
One species, Cymbopogon ambiguus or lemon-scented grass, actually grew faster in contaminated than clean soil.

PhD student Sharyn Gaskin of the School of Environmental Health and her supervisors, microbiologist Dr Richard Bentham, and Flinders plant physiologist Associate Professor Kathleen Soole, will shortly present papers at local and international conferences that point to a valuable role for the grasses as a cheap and effective means of remediating sites contaminated by the petrol and oil from mine machinery and transport.

The roots of grasses contribute to the breakdown of hydrocarbons in the soil by the secretion of compounds that stimulate the action of micro-organisms in the root zone.

"The grasses act as an accelerant by providing a food source to kick-start the action of the adapted organisms. These micro-organisms then degrade the pollutants," Ms Gaskin said.

Similar studies overseas have identified a range of grasses that demonstrate ability to degrade organic contaminants, but this is the first time local grasses have been tried for the purpose.

Dr Bentham said native varieties have the advantage of avoiding issues associated with introduced species, and harnessing our biodiversity that is better adapted to grow in local conditions.

The project set out to grow native grass seeds in samples of soil sent from a Rio Tinto mine at Weipa. Each grass was tried in an uncontaminated control, as well as in samples contaminated to .5 per cent and 1 per cent concentration with a mix of diesel and oil.

"After we had established that three of the grasses were tolerant to contamination, we measured root and shoot development and, surprisingly, C. ambiguus showed more shoot growth and more root growth, and liked it better in the more contaminated soil," Dr Bentham said.

The ability of the grass root mass by all three species to encourage remediating bacteria was also clearly demonstrated, Dr Bentham said.

"It's one thing to get it to grow in the soil; it's another for it to actually stimulate degradation of the hydrocarbons.

"When we went back and did a microbial count, we found we had a hundred-fold (at 60 days) and a thousand-fold more hydrocarbon-degrading bacteria (at 120 days) in the planted soil than in the unplanted soil," he said.

Dr Bentham said it appears that the roots are excreting organic compounds into the soil that fortuitously select in favour of bacteria which will be able to degrade diesel.

"So, in effect, they are their own little enrichment factories."

While data collection on the rates of hydrocarbon degradation by C. ambiguus is continuing, Dr Bentham said the rate appears to be quicker and the endpoint is lower; that is, the removal of hydrocarbons is both faster and more thorough.

As to the accelerated growth of C. ambiguus, Dr Bentham has a couple of theories. Hydrocarbon limits water uptake, so the rapid root mass growth may be a response to stress: "And once it has more roots, it has better access to water, and grows faster," he said.

Alternatively, he said, the microbes may be breaking down complex molecules and making simple molecules available to the plants as food.

"It's one of those neat little projects where we postulated a theory, and everything worked," Dr Bentham said.

"Because some of these grasses can be mowed, there is a potential for native grasses to provide a quick, green and mowable form of remediation.

"So far as we know, we are the only people in Australia looking at remediation of hydrocarbons by native plant species. We are now in the process of looking at the performance of the grasses in even higher concentrations of diesel."

Contact

Dr Richard Bentham (email)
website
Senior Lecturer
School of Medicine, Environmental Health
Flinders University of South Australia
Business: (08) 8204 5732
Fax: (08) 8204 5226