Application of 15N tracing and bioinformatics for estimating microbial-mediated nitrogen cycle processes in oil-contaminated soils

Crude oil pollution can profoundly alter the nitrogen (N) cycle in the soil. Here, a 30-day incubation with 15N tracer approach was performed to assess the impacts of crude oil concentrations (medium: 10,000 mg kg-1; heavy: 50,000 mg kg-1) on soil N cycling based on a numerical model. Results showed that crude oil pollution significantly increased the gross N-transformation rates, but the rates of oxidation of recalcitrant organic N, the immbolization of NO3- and the adsorption of NH4+ changed differently as a function of hydrocarbon concentrations. There was no significant difference of the oxidation rate of recalcitrant organic N between the medium and heavy oil-contaminated soils (medium: 0.1149 mmol N kg-1 d-1; heavy: 0.1299 mmol N kg-1 d-1), but the rates of NO3- immobilization (0.1135 mmol N kg-1 d-1) and NH4+ adsorption were the highest (0.1148 mmol N kg-1 d-1) in the moderately oil-contaminated soils than those in the heavy polluted soil (0.0849 mmol N kg-1 d-1 and 0.0034 mmol N kg-1 d-1, respectively). The NO3- immobilization rate was 2.5-fold higher than its reduction rate, indicating that NO3- immobilization played a more important role during the process of NO3- transformation. Microbial community structure analysis indicated that phyla of Actinobacteria and Ascomycota respectively promoted the immobilization of NO3- to recalcitrant organic N and the reduction of NO3- to NH4+. The genus of Aspergillus was related to net NH4+ production, and the genera of Penicillium and Acremonium were responsible for oxidation of recalcitrant organic N to NO3-.