Synthetic fuel oil effects on microbial activity and nitrogen transformations in a soil

The effects of a solvent refined coal oil (SRC-II) on microbial processes in a Captina silt loam soil was examined. The soil samples were maintained under environmental conditions favorable for most aerobic microbial activities. Soil was treated with four oil concentrations ranging from 0.2% to 8.6% (wt/wt). Oxygen uptake rates, total viable cell counts, numbers of nitrifying bacteria and inorganic nitrogen concentrations were monitored before oil addition and at regular intervals for three months afterwards. Organic carbon, total nitrogen and soil pH were also measured before and after application of the oil.

The SRC-II coal oil affected soil processes at all treatment levels. The lowest oil concentration (0.2%) decreased numbers of nitrifying bacteria while increasing total viable cell numbers and net nitrogen mineralization. The higher oil concentrations reduced oxygen uptake rates and total viable cells as well as nitrifier numbers. Soil treated with a 1.7% oil concentration showed significant increases in respiration rates and cell densities after two months, while no significant increases were observed at oil levels of 3.4% and 8.6%.

The application of the coal oil to soil samples raised the carbon to nitrogen ratio of the soil. The sum of nitrate and ammonium nitrogen in the oil-treated soils was never significantly lower than the control soil levels indicating that nitrogen was not limiting to decomposition. However, the toxicity of the oil towards the nitrifying bacteria resulted in an accumulation of ammonium in treated soils. This may affect plant establishment on soils contaminated with a synthetic fuel oil.