Contaminant and plant-derived changes in soil chemical and microbiological indicators during fuel oil rhizoremediation with Galega orientalis

Abstract The aim of this work was to evaluate the effects of vegetation and hydrocarbon (HC) contamination on the development of soil chemical and biological status during rhizoremediation of fuel oil contamination with the legume Galega orientalis . Uncontaminated and unvegetated references monitored in parallel with the rhizoremediation treatment enabled the identification of the partial effects. A 21-week greenhouse experiment simulated one growing season with a single 3000 ppm contamination event in the beginning. For a comprehensive view of the restoration process, the following soil parameters were assessed by ten destructive samplings at increasing intervals: plant growth, HC content, pH, C/N ratio, DNA content, culturable oil-degrader numbers and six enzymatic activities. After 21 weeks, 90% and 87% of the initial HC load was depleted in the rhizoremediation treatment and the unvegetated reference, respectively. Contamination retarded the growth of Galega ; a majority of the hydrocarbons were degraded by the indigenous soil microbial community by week 6, when the legume seedlings were still very small. In the end of the experiment, when HC contamination had decreased to the clean soil threshold level, Galega biomass in the rhizoremediation treatment reached that of the uncontaminated reference. In contrast, fuel oil stimulated the growth and activity of the soil microbial community and may have masked vegetation-associated changes. The amounts of total and oil-degrader micro-organisms mirrored the HC degradation curve and seem an efficient and ecologically relevant tool to monitor the biodegradation process of light HC contamination. No negative HC effects were seen in soil enzymatic activities either, but aminopeptidases were induced by contamination. Vegetation-associated upward trend was observed for aminopeptidases and phosphomonoesterase. Biological HC degradation raised soil pH. Part of the fuel oil carbon remained in the soil, perhaps assimilated by the degrader micro-organisms. The results of this study support the theory of high soil resilience to moderate contamination with light hydrocarbons, demonstrate the power of microbiological methods in monitoring bioremediation, and back up the current legislative clean soil threshold level for hydrocarbon contamination.