Successful rhizoremediation of aliphatic hydrocarbon contaminated soil using an Australian native grass

The breakdown of contaminants in soil resulting from microbial activity that is enhanced in the presence of the plant root zone (rhizosphere) has been termed rhizoremediation. Australian native plants have not been assessed for their hydrocarbon rhizoremediation potential. The objective of this study was to evaluate an Australian native grass species that may be a suitable candidate for rhizoremediation application. The grass species, lemon scented grass (Cymbopogon ambiguus) was selected on the basis of previously established essential and desirable growth criteria. The species was then evaluated for seedling emergence and growth in laboratory and greenhouse studies, to assess tolerance of aliphatic hydrocarbon contaminated soil. Seed were sown in soil sourced from a mine site which was artificially contaminated with a 60:40 diesel/ oil mix at concentrations of 10,000mg/Kg, 5000 mg/kg and 0 mg/Kg (control). Hydrocarbon-degrading organisms were also monitored in the rhizosphere soil using a most probable number (MPN) method for enumeration. Hydrocarbon concentrations were measured using Gas Chromatography. Lipase enzyme assays were also used to monitor hydrocarbon degradation rates. Seedling emergence was not adversely affected by the presence of hydrocarbon contamination (p>0.05). The grass was assessed for relative growth in contaminated and uncontaminated soils. The species survived for 120 days in the contaminated soil, and produced significantly more root biomass in the presence of contamination (10,000 and 5,000 mg/Kg) compared with the control (p<0.0001). There was a significantly increased the number of hydrocarbon-degrading organisms in the planted contaminated soil (p<0.01) compared with the unplanted contaminated soil and unplanted clean soil. This effect was first demonstrable at 17 days (approximately 10 days after germination) and continued throughout the experiment. Hydrocarbon concentrations in unplanted soil were reduced from 10,000 to approximately 5,000 mg/Kg during the course of the experiment (50%). In planted soil hydrocarbon concentrations were reduced from 10,000 mg/Kg to approximately 1,000 mg/Kg (90%). Hydrocarbon concentrations in planted soil were significantly lower than those in unplanted soil (p<0.01). Lipase activity in planted and control soils were significantly different from the point of germination onwards (p<0.05). Lipase activity was positively correlated with MPN for hydrocarbon degrading microorganisms (p= 0.03), and negatively correlated with hydrocarbon concentrations (p= 0.02). This investigation identified an Australian native grass species that is a candidate for further investigation for in situ rhizoremediation potential. Significantly, it was not necessary to add N and P to achieve a 90% reduction in hydrocarbon in the soil. Increased microbial numbers and improved plant growth indicate a symbiotic relationship between the plants and microorganisms.