Effect of nitrogen amendment on respiration and respiratory quotient (RQ) in three hydrocarbon contaminated soils of different type

Three soil types (sandy gravel, silty clay and sandy loam) from sites historically contaminated with total petroleum hydrocarbon (TPH) were amended with NH4NO3 at concentrations ranging from 16 to 2133 mg/kg soildry weight. Microbial activity was measured as O2 consumption and CO2 production in order to assess nitrogen limitation. Although activity was stimulated in all three soils under NH4NO3 amendment (after 72 h), the level of nitrogen required was soil specific. For the sandy gravel and silty clay soils, O2 consumption and CO2 production both showed enhanced microbial activity when amended with 16 mg/kg soildry weight NH4NO3, whereas, these two parameters gave differing results for the sandy loam soil. Specifically, CO2 production and O2 consumption were stimulated with 66 mg/kg and 133 mg/kg soildry weight of NH4NO3 respectively. In addition, respiratory quotient kinetic analysis suggested different decomposition processes occurring in this soil under different NH4NO3 amendment concentrations.