12.4.1: Terrestrial
From the major processes involved in the nitrogen cycle in soil and sediments, four microbially mediated reaction pathways can contribute to \(N_2O\) emissions (Baggs, 2011; Quick et al., 2019): nitrification and nitrifier denitrification in oxic environments and denitrification and dissimilatory nitrate reduction to ammonium (DNRA) in suboxic and anoxic environments. The reactive nitrogen species are oxidized or reduced through a sequence of electron transfer steps, promoted by enzymatic reaction pathways. In all of these pathways, \(N_2O\) is produced as an intermediate reaction product (Fig. 1). In aquatic ecosystems, denitrification is regarded as the predominant source of \(N_2O\), and nitrifier-denitrification is likely more significant than nitrification (Quick et al., 2019). These processes can co-occur over a broad range of oxygen ( \(O_2\))/redox and moisture content (MC) conditions, within oxic/anoxic microsites in sediments (Jørgensen and Revsbech, 1985; Seitzinger et al., 2006).
Denitrification, a facultative anaerobic process, is the reduction of nitrate (\(NO^{-}_{3}\) ) or nitrite (\(NO^{-}_{2}\) ) to \(N_2O\) and di-nitrogen ( \(N_2\)) performed by heterotrophic bacteria (denitrifiers). Denitrifying microorganisms also include ammonia-oxidizing chemolithotrophic bacteria, which reduce \(NO^{-}_{2}\) to \(N_2O\) aerobically, archaea, fungi and other eukaryotes (Baggs, 2011). Part of the denitrifying bacteria and archaea are missing the genes encoding the enzymes involved in the reduction of nitric oxide (NO) and \(N_2O\) to \(N_2\), which can lead to incomplete pathways and \(N_2O\) release (Stein & Klotz, 2016). Denitrification enzymes are inhibited by \(O_2\), particularly \(N_2O\) reductase, which catalyzes the reduction of \(N_2O\) to \(N_2\). Thus, under suboxic conditions, \(N_2O\) may be the end product of denitrification (Knowles, 1982). Apart from \(O_2\) conditions, several other environmental factors control \(N_2O\) production from denitrification, specifically the \(N_2O\) yield (\(N_{2}O\)/(\(N_{2}O\)+ \(N_2\))), including water content, \(NO^{-}_{3}\) availability, C quality and availability and C:\(NO^{-}_{3}\) (Quick et al., 2019).
Nitrification and nitrifier denitrification occur under different environmental conditions and both oxidize ammonia. Nitrification is the oxidation of ammonia (\(NH_{3}\)) or ammonium (\(NH^{+}_{4}\)) to \(NO^{-}_{2}\) by ammonia oxidizers (cohort I; primary nitrifiers) and to \(NO^{-}_{3}\) by nitrite oxidizers (cohort II; secondary nitrifiers). Ammonia/um can be directly oxidized to nitrate by complete ammonia oxidizers (comammox, cohort III). Cohorts II and III only include chemolithotrophic microbes (Stein & Klotz, 2016). Under certain conditions, ammonia oxidizers can significantly contribute to \(N_2O\) emissions by two reactions along this pathway, hydroxylamine oxidation (biotic and abiotic) and chemodenitrification (abiotic). The main factors influencing hydroxylamine oxidation are aerobic conditions and \(NH_{3}\)availability, whilst chemodenitrification is limited by \(NO^{-}_{2}\) availability and may occur under fluctuating redox conditions (Quick et al., 2019). Nitrifier denitrification, strictly carried out by ammonia oxidizers, converts \(NH_{3}\)to \(N_2\) gas. It is supported by different \(O_2\) conditions, having both oxidation and reduction steps. The first steps are oxidative (ammonia is oxidized to (\(NO^{-}_{2}\) ) and the final steps are reductive (\(NO^{-}_{2}\) is sequentially reduced to NO, \(N_2O\) and \(N_2\)). Factors influencing \(N_2O\) production from nitrifier denitrification include \(O_2\) conditions, \(NH^{+}_{4}\)and C availability (Quick et al., 2019). It differs from nitrification and coupled nitrification-denitrification as there is no (\(NO^{-}_{3}\) involved.
DNRA is performed by both bacteria and fungi, using C as an electron donor. During nitrate ammonification, nitrate is reduced to \(NO^{-}_{2}\) and \(NH^{+}_{4}\), and \(N_2O\) is produced as a by-product during the \(NO^{-}_{2}\) reduction stage. Reducing conditions are an important factor controlling this process, which is mostly anaerobic but can also occur under relatively oxic conditions, being less sensitive to \(O_2\) than denitrifiers (Giles et al., 2012). The C:(\(NO^{-}_{3}\) ratio is also considered an important controlling factor in the process (Quick et al., 2019).
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