Evaluating the effects of gross nitrogen mineralization, immobilization, and nitrification on nitrogen fertilizer availability in soil experimentally contaminated with diesel

Sandy clay loam soil was contaminated with 5000 mg kg^-1 diesel, and amended with nitrogen (15.98 atom% ¹⁵N) at 0, 250, 500, and 1000 mg kg^-1 to determine gross rates of nitrogen transformations during diesel biodegradation at varying soil water potentials. The observed water potential values were -0.20, -0.47, -0.85, and -1.50 MPa in the 0, 250, 500, and 1000 mg kg^-1 nitrogen treatments respectively. Highest microbial respiration occurred in the lowest nitrogen treatment suggesting an inhibitory osmotic effect from higher rates of nitrogen application. Microbial respiration rates of 185, 169, 131, and 116 mg O₂ kg^-1 soil day ^-1 were observed in the 250, 500, control and 1000 mg kg^-1 nitrogen treatments, respectively. Gross nitrification was inversely related to water potential with rates of 0.2, 0.04, and 0.004 mg N kg^-1 soil day^-1 in the 250, 500, and 1000 mg kg^-1 nitrogen treatments, respectively. Reduction in water potential did not inhibit gross nitrogen immobilization or mineralization, with respective immobilization rates of 2.2, 1.8, and 1.8 mg N kg^-1 soil day^-1, and mineralization rates of 0.5, 0.3, and 0.3 mg N kg^-1 soil day ^-1 in the 1000, 500, and 250 mg kg^-1 nitrogen treatments, respectively. Based on nitrogen transformation rates, the duration of fertilizer contribution to the inorganic nitrogen pool was estimated at 0.9, 1.9, and 3.2 years in the 250, 500, and 1000 mg kg^-1 nitrogen treatments, respectively. The estimation was conservative as ammonium fixation, gross nitrogen immobilization, and nitrification were considered losses of fertilizer with only gross mineralization of organic nitrogen contributing to the most active portion of the nitrogen pool.