Isotopic evidence for alteration of nitrous oxide emissions and producing pathways contribution under nitrifying conditions

Abstract

Nitrous oxide (N2O) emissions by a nitrifying biofilm reactor were investigated with N2O isotopocules. The site preference of N2O ( 15 N-SP) indicated the contribution of producing and consuming pathways in response to changes in oxygenation level (from 0 to 21 % O2 in the gas mix), temperature (from 13.5 to 22.3 °C), and ammonium concentrations 15 (from 6.2 to 62.1 mg N L -1 ). Nitrite reduction, either nitrifier-denitrification or heterotrophic denitrification, was the main N2O producing pathway under the tested conditions. Nitrite oxidation rates decreased as compared to ammonium oxidation rates at temperatures above 20 °C and sub-optimal oxygen levels, increasing N2O production by the nitrite reduction pathway. Below 20 °C, a difference in temperature sensitivity between hydroxylamine and ammonium oxidation rates is most likely responsible for an increase in the N2O production via the hydroxylamine oxidation pathway (nitrification). A negative correlation between 20 the reaction kinetics and the apparent isotope fractionation was additionally shown from the variations of  15 N and  18 O values of N2O produced from ammonium.

Stimulation of N2O emission via bacterial denitrification driven by acidification in estuarine sediments

“…Similar to SP values, the fractional contributions of the three processes to δ 18 O of N 2 O production were expressed aswhere δ 18 O p denotes the δ 18 O values of N 2 O before reduction (not the measured δ 18 O values). δ 18 O B , δ 18 O F , and δ 18 O C represent the δ 18 O value for bacterial denitrification (17.4–26.5‰, 19 ± 3.6‰), fungal denitrification (30.2–51.9‰, 45 ± 10.8‰), and chemo‐denitrification (24.3–36.5‰, 10 ± 6.3‰), respectively (Baggs, 2008; Frame & Casciotti, 2010; Hu et al, 2015; Humbert et al, 2020; Jones et al, 2015; Maeda et al, 2017; Sutka et al, 2006, 2008; Yu et al, 2020; Table S3). The mean values of SP and δ 18 O estimated by the Monte Carlo method were the most appropriate for this study and used in the mass balance model (Wankel et al, 2017).…”

“…SP B , SP F , and SP C represent the SP values for bacterial denitrification (−7.5–3.7‰, 0 ± 2.3‰), fungal denitrification (30.2·39.3‰, 37 ± 3.2‰), and chemo‐denitrification (10–22‰, 16 ± 4.1‰), respectively (Table S3). The ranges of SP values for the three denitrification processes are summarized according to previous studies (Baggs, 2008; Decock & Six, 2013; Hu et al, 2015; Humbert et al, 2020; Jones et al, 2015; Sutka et al, 2008; Wankel et al, 2017; Yu et al, 2020; Zou et al, 2014). The mean values and standard errors for SP were obtained by estimating error propagation using the Monte Carlo sampling method in MATLAB and then were adjusted based on previous studies (Wankel et al, 2017; Zou et al, 2014).…”

Dissolved Nitrous Oxide and Hydroxylamine in the Northern South China Sea in Summer

“…In general, dissolved N 2 O concentrations in natural seawater increase with decreasing oxygen concentrations (Löscher et al, 2012). In addition, N 2 O is also affected by other environmental factors including temperature, salinity, pH, nutrient availability and physical disturbance such as typhoon as well as water mass exchange (Humbert et al, 2019;Li et al, 2022;Lin et al, 2016;Schulz et al, 2023;Su et al, 2019;Zhou et al, 2023).…”