12.4.1: Terrestrial
- Page ID
- 19357
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)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).
References
Baggs EM. 2011. Soil microbial sources of nitrous oxide: recent advances in knowledge, emerging challenges and future direction. Current Opinion in Environmental Sustainability 3(5):321–327 DOI 10.1016/j.cosust.2011.08.011
Giles M, Morley N, Baggs E, Daniell T. 2012. Soil nitrate reducing processes—drivers, mechanisms for spatial variation, and significance for nitrous oxide production. Frontiers in Microbiology 3:407 DOI 10.3389/fmicb.2012.00407.
Jørgensen BB, Revsbech NP. 1985. Diffusive boundary-layers and the oxygenuptake of sediments and detritus. Limnology and Oceanography 30:111–122 DOI 10.4319/lo.1985.30.1.0111.
Knowles R. 1982. Denitrification. Microbiological Reviews 46(1):43–70.
Quick AM, Reeder WJ, Farrell TB, Tonina D, Feris KP, Benner SG. 2019. Nitrous oxide from streams and rivers: A review of primary biogeochemical pathways and environmental variables. Earth-Science Reviews 191:224–262 DOI 10.1016/j.earscirev.2019.02.021.
Seitzinger S, Harrison JA, Bohlke JK, Bouwman AF, Lowrance R, Peterson B, Tobias C, Van Drecht G. 2006. Denitrification across landscapes and waterscapes: a synthesis. Ecological Applications 16:2064–2090 DOI 10.1890/1051-0761(2006)016%5b2064:DALAWA.
Stein LY, Klotz MG. 2016. Primer: the nitrogen cycle. Current Biology 26:R94–R98 DOI 10.1016/j.cub.2015.12.021.
Excerpted from
Pinto R, Weigelhofer G, Brito AóG, Hein T. 2021. Effects of dry-wet cycles on nitrous oxide emissions in freshwater sediments: a synthesis. PeerJ 9:e10767 http://doi.org/10.7717/peerj.10767 CC-BY-4.0