Isotopic evidence for alteration of nitrous oxide emissions and producing pathways contribution under nitrifying conditions
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Commentaire : Nitrous oxide (N2O) is an important ozone-depleting substance and contributes to the greenhouse effect with high global warming potential. Nitrification, nitrifier-denitrification and denitrification are major producing processes of N2O either through the hydroxylamine oxidation or the nitrite reduction pathways. The N2O emissions from a nitrifying biomass of a submerged fixed-bed biofilm reactor were investigated with N2O isotopocules, molecules that only differ in either the number or position of isotopic substitutions. The isotope composition of substrates and fractionation mechanisms influence both nitrogen and oxygen isotope ratios of N2O (15N and 18O, respectively). In addition to this, each N2O producing pathway produces N2O with a specific range of nitrogen isotopomer site preference (15N-SP). Using both 15N, 18O of substrates and products in combination with 15N-SP allows distinction of the different N2O producing processes. The relative contribution of these processes was thus determined as a response to several environmental variables. The effect of oxygenation level (0 to 21 % O2 in the gas mix), temperature (13.5 to 22.3 °C), and ammonium concentrations (6.2 to 62.1 mg N L-1) were tested. Our results showed that whatever the imposed conditions, the nitrifying biomass produced N2O with nitrite reduction being the main N2O producing pathway. The N2O emissions were sensitive to oxygenation, temperature, and NH4+ concentrations. Among the environmental variables tested, temperature appears to be the main control on N2O producing pathways under nitrifying conditions, due to its dissimilar effects on ammonium and nitrite oxidizing activities. This finding allows possible mitigation of N2O emissions (e.g. in Waste Water Treatment Plants) by optimizing temperature allowing high nitrification rates and low N2O emissions.
Commentaire : Nitrous oxide (N2O) is an important ozone-depleting substance and contributes to the greenhouse effect with high global warming potential. Nitrification, nitrifier-denitrification and denitrification are major producing processes of N2O either through the hydroxylamine oxidation or the nitrite reduction pathways. The N2O emissions from a nitrifying biomass of a submerged fixed-bed biofilm reactor were investigated with N2O isotopocules, molecules that only differ in either the number or position of isotopic substitutions. The isotope composition of substrates and fractionation mechanisms influence both nitrogen and oxygen isotope ratios of N2O (15N and 18O, respectively). In addition to this, each N2O producing pathway produces N2O with a specific range of nitrogen isotopomer site preference (15N-SP). Using both 15N, 18O of substrates and products in combination with 15N-SP allows distinction of the different N2O producing processes. The relative contribution of these processes was thus determined as a response to several environmental variables. The effect of oxygenation level (0 to 21 % O2 in the gas mix), temperature (13.5 to 22.3 °C), and ammonium concentrations (6.2 to 62.1 mg N L-1) were tested. Our results showed that whatever the imposed conditions, the nitrifying biomass produced N2O with nitrite reduction being the main N2O producing pathway. The N2O emissions were sensitive to oxygenation, temperature, and NH4+ concentrations. Among the environmental variables tested, temperature appears to be the main control on N2O producing pathways under nitrifying conditions, due to its dissimilar effects on ammonium and nitrite oxidizing activities. This finding allows possible mitigation of N2O emissions (e.g. in Waste Water Treatment Plants) by optimizing temperature allowing high nitrification rates and low N2O emissions.