Assessment of C:N ratios and water potential for nitrogen optimization in diesel bioremediation

Sandy clay loam soil contaminated with 5000, 10,000 or 20,000 mg/kg of diesel fuel no. 2 was amended with 0 (ambient nitrogen only), 250, 500, or 1000 mg/kg nitrogen (NH ₄Cl) to evaluate the role of C:N ratios and soil water potential on diesel biodegradation efficacy. The soil was incubated at 25°C for 41 days and microbial O ₂ consumption measured respirometrically. Highest microbial respiration was observed in the 250 mg N/kg soil treatments regardless of diesel concentration. Higher levels of nitrogen fertilization decreased soil water potential and resulted in an extended lag phase and reduced respiration. Application of 1000 mg/kg nitrogen reduced maximum respiration by 20% to 52% depending on contaminant levels. Optimal C:N ratios among those tested were 17:1, 34:1, and 68:1 for the three diesel concentrations, respectively, and were dependent on contaminant concentration. Nitrogen fertilization on the basis of soil pore water nitrogen (mg N/kg soil H ₂O) is independent of hydrocarbon concentration but takes into account soil moisture content. This method accounts for both the nutritional and osmotic aspects of nitrogen fertilization. In the soil studied the best nitrogen augmentation corresponded to a soil pore water nitrogen level of 1950 mg N/kg H ₂O at all diesel concentrations.