| The objective of this research is to determine the influence of liquid water content on the toxicity of petroleum hydrocarbons (PHC) to soil microorganisms in frozen soil. This research was conducted on soil collected from an aged diesel fuel spill site at Casey Station, East Antarctica, as well as on spiked diesel contaminated soil from Macquarie Island, a sub-Antarctic island.;Petroleum hydrocarbon toxicity in polar soil was assessed by sampling 32 locations at an aged diesel spill site at Casey Station, East Antarctica. Samples were taken nine times throughout an austral summer to encompass frozen, thaw and refreeze periods. Toxicity was assessed using potential activities of substrate induced respiration, total respiration, nitrification, denitrification, and metabolic quotient, as well as microbial community composition and bacterial biomass. The most sensitive indicator was community composition with an EC25 of 800 mg kg-1, followed by nitrification (2000 mg kg -1), microbial biomass (2400 mg kg-1) and soil respiration (3500 mg kg-1). Despite changes in potential microbial activities and composition over the frozen/thaw/refreeze period, the sensitivity of these endpoints to PHC did not change with liquid water or temperature.;The influence of liquid water (&thetas;liquid) on nutrient supply rate and gas diffusion, which are important factors in microbial degradation of PHC, was determined using contaminated soil from Casey Station. Freezing reduced nutrient supply rate of both NH4 + and NO3-. However, an increase in &thetas; liquid was linked to increases in nitrate and ammonia nutrient supply rates in frozen soil. Similarly for gas diffusion, decreases in D s due to freezing were much more pronounced in soils with low &thetas; liquid compared to soils with higher &thetas; liquid contents. Further research is needed to determine whether bioremediation in cold regions could be enhanced during the period of time where the soil temperature is below 0°C by controlling factors that increase the amount of liquid water.;The influence of liquid water content on the in situ toxicity of PHC to soil microorganisms was evaluated using stable isotope dilution technique to measure gross mineralization and nitrification, which was compared to the toxicity endpoints of potential microbial activities. Liquid water content did not have a significant effect on either gross mineralization or nitrification. Gross nitrification was sensitive to PHC contamination, with toxicity decreasing over time. The EC25 value for gross nitrification was 400 mg kg-1 for 1 month incubation period. In contrast, gross N mineralization was not sensitive to PHC contamination. Toxic response of gross nitrification to PHC contamination was comparable to PNA with similar EC25 values determined by both measurement endpoints (400 mg kg -1 for in situ nitrification compared to 200 mg kg -1 for PNA), indicating that potential microbial activity assays are good surrogates for in situ toxicity of PHC contamination in Polar Regions. Based on ecotoxicological data collected, the recommended soil quality guideline for on PHC contamination in polar soils would be 200 mg kg-1.;Suitable soil biogeochemical toxicity endpoints for PHC contamination were identified using sub-Antarctic soil from Macquarie Island spiked with diesel fuel. The sensitivity of nitrification, denitrification, carbohydrate utilization and total soil respiration to diesel fuel was assessed. Potential nitrification activity (PNA) was the most sensitive indicator of contamination assessed for nitrogen cycling, with a PHC concentration effecting microbial activity by 20% of the control response, EC20, of 190 mg PHC kg -1 soil. |
