Repeated freeze–thaw cycles and their effects on mineralization of hexadecane and phenanthrene in cold climate soils

Arctic soil samples were experimentally subjected to freeze–thaw cycles (FTC) between + 5 °C and − 5 °C and to stable temperature regimes of + 5 °C and − 5 °C to investigate how the microorganismsʹ ability to mineralize oil contamination is affected by different temperature regimes. Hydrocarbon mineralization was monitored in oil contaminated soil microcosms spiked with radiolabeled hexadecane and phenanthrene. Both fertilized and unfertilized soils were used. Mineralization of radiolabeled compounds was demonstrated in all treatments. In soils kept stable at − 5 °C hydrocarbon mineralization was low, but detectable for hexadecane and phenanthrene, both in fertilized and unfertilized soil. Greatest mineralization rates were determined in fertilized soil samples kept stable at + 5 °C where up to 54 mg hexadecane/kg/day and 11 mg phenanthrene/kg/day were measured. Mineralization did not come to a complete stop when the temperature was changed to − 5 °C, but was considerably reduced compared to the + 5 °C mineralization rates. FTC stimulated hexadecane mineralization compared to constant temperature. Phenanthrene mineralization, on the other hand, was constrained by FTC. FTC did not seem to affect the microbial populations in the soil in a negative way, and the total number of cultivable microorganisms increased during the experimental period. The freeze–thaw experiments demonstrate that mineralization of both hexadecane and phenanthrene is occurring in soils undergoing seasonal temperature fluctuations, both above and below freezing temperatures.