Biostimulation of natural microbial assemblages in oil-amended vegetated and desert sub-antarctic soils

A field study was initiated in December 2000 in two selected soils of The Grande Terre (Kerguelen Archipelago) with the objective of determining the long-term effects of fertilizer addition on the biodegradation rate and the toxicity of oil residues under severe sub-Antarctic conditions. Two soils were selected. The first site supports an abundant vegetal cover; the second one was desert soil, devoid of plant material. These two soils were located in the vicinity of the permanent station of Port-aux-Francais (69degrees 42'E; 49degrees 19'S). A series of five experimental plots (0.75 x 0.75 in) were settled firmly into each of the studied soils. Each plot received 500 mL of diesel or Arabian light crude oil, and some of them were treated with a bioremediation agent: slow-release fertilizer Inipol EAP-22 (Elf Atochem). All the plots were sampled on a regular basis over a 1 year period. Heterotrophic and hydrocarbon-degrading microorganisms increased by two orders of magnitude during the first month of the experimentation in all treated enclosures, but differences appeared between the different plots. The microbial response was improved by bioremediation treatments. However, fertilizer addition had a greater impact on the desert soil when compared to the vegetated one. All chemical indices show a reduction of alkanes and light aromatics. Toxicity results show a high variability between treatments and environmental conditions. As a conclusion, it is clear that the microbial response was rapid and efficient in spite of the severe weather conditions, and the rate of degradation was improved by bioremediation treatments. However, after 1 year of treatment, the signal of a relatively high toxicity of oiled residues remained present in the two studied soils.