Nitrous oxide production from biological and chemodenitrification by Fe(II) in estuarine and coastal sediments

Abstract

Although biological processes are assumed the main sources of nitrous oxide (N2O), it has been increasingly highlighted that abiotic mechanisms are also at play. Here we investigated abiotic and biological N2O production from nitrate and nitrite reduction by Fe(II) in estuarine sediments through a series of anoxic incubations. N2O concentrations increased and then decreased with prolonged incubation time in biological treatment, which was attributed to biological reduction. N2O from chemodenitrification was not mediated by nitrate reduction, but instead was generated from nitrite reduction. Electrons transfer of Fe(II) oxidation decreased by 44.5% from abiotic to biological processes, reflecting that abiotic nitrite reduction was mainly responsible for Fe(II) oxidation. Pseudo-first-order rate constants of Fe(II) were higher in abiotic than biotic processes, and the constants of nitrite were higher in biotic than abiotic processes. 18ɛ:15ɛ ratios of nitrate and nitrite reduction were higher in abiotic than biological processes, suggesting that abiotic process represented a stronger oxygen isotope effect relative to nitrogen isotope effect than biological process. Site preference (SP) of N2O was higher in biological than abiotic processes and Fe(II) addition increased N2O production with accompanying high SP. Based on the reaction stoichiometry and electron transfer balance, chemodenitrification contributed to 20.2–38.4% of N2O production, and 61.6–79.8% of N2O was produced from biological denitrification. These result suggest that biological and chemodenitrification coupled with Fe(II) oxidation can produce N2O, which further mediate the kinetics of N2O production and have wide implications for coupled biogeochemical reactions between nitrogen and iron in global aquatic ecosystems.