Long-term effect of temperature on N2O emission from the denitrifying activated sludge

The long-term effect of various temperature (4°C, 12°C, 20°C, 25°C and 34°C) on nitrous oxide (N2O) emission from lab-scale denitrifying activated sludge was studied in terms of activation energy, abundance of functional gene nosZ and its transcription. Results showed that temperature had a positive effect on N2O emissions as well as the maximum biomass-specific reduction rates of N2O and NO3−, ranging from 0.006% to 0.681% of (N2O + N2), 17.3–116.2 and 5.2–66.2 mg N g−1 VSS h−1, respectively. The activation energies (Ea) for N2O and NO3− reduction of 44.1 kJ mol−1 and 54.9 kJ mol−1, shed light on differences in denitrifying rate variation. The maximum NO3− reduction rates were more sensitive to temperature variation than the corresponding N2O reduction rates under long-term acclimation. As a result, the ratio between N2O and NO3− reduction rates declined to 1.87 at 34°C from 3.31 at 4°C, suggesting great potential capacity for N2O losses at high temperature. The copy numbers of denitrifiers as nosZ gene (×108 copies mL−1) and total bacteria as 16S rRNA gene (×1010 copies mL−1) did not show obvious relationship with temperature, having relative abundance of 0.42% on average. The transcriptional regulation of nosZ gene, in the range of 108–105 copies mL−1, was affected by reductase activity, substrate concentration as well as its duration. The active nosZ gene expression was accompanied with low reductase capacity, high dissolved N2O and the duration of N2O accumulation. These results provide insights into activation energy and gene expression responsible for N2O emission.