1 Introduction
Nitrous oxide (laughing gas; N 2 O) is the dominant nitrogenous greenhouse gas
(GHG) on Earth and contributes to ozone layer depletion [1, 2]. Its current atmospheric concentration amounts to about 332 ppbv (13 nM), and its average atmospheric lifetime is 114 years. The N 2 O concentration is approximately 1,250-fold
lower than that of carbon dioxide (CO 2 ) but, noteworthily, N 2 O has an almost
300-fold greater potential for global warming effects, based on radiative capacities.
Hence, N 2 O accounts for up to 10% of total emissions when the impact of individual GHGs on global warming is expressed in terms of CO 2 equivalents (CO 2 e), a
unit that has been approved by the Intergovernmental Panel on Climate Change
(IPCC) [1]. For comparison, the atmospheric N 2 O concentration in the year 1900
was about 277 ppbv (17% lower than today), and it has been concluded from ice
core studies that the N 2 O concentration was, most likely, never higher than 280
ppbv over the last 800.000 years [3, 4]. The main reason for the dramatically rising
N 2 O concentration is the worldwide intensification of agricultural fertilization
practices that is resulting in a severe anthropogenic imbalance of the global biogeochemical nitrogen cycle [3, 5–11]. This development started with the invention
of man-made nitrogen fixation by the Haber–Bosch process that allowed the
ever-expanding application of synthetic nitrogen-based fertilizers [12]. Hence,
agriculture and livestock farming are the main drivers of anthropogenic N 2 O
emissions with minor contributions from fossil fuel combustion, biomass burning
and wastewater treatment [1, 3, 10]. The IPCC Special Report Climate Change and
Land from 2019 (revised in 2020) quotes: ‘Agriculture, Forestry and Other Land
Use (AFOLU) activities accounted for around 13% of CO 2 , 44% of methane (CH 4 ),
and 81% of nitrous oxide (N 2 O) emissions from human activities globally during
2007–2016, representing 23% (12.0 ± 3.0 GtCO 2 e yr
−1 ) of total net anthropogenic
emissions of GHGs (medium confidence)’. [13]. Thus, humankind faces a severe
challenge in terms of N 2 O emissions since agricultural activities will have to
increase further to nourish the growing world population.
At room temperature, N 2 O is a rather inert, colourless and non-flammable gas
that can nevertheless act as a biocide, for example, by promoting vitamin B 12
deficiency and affecting vitamin B 12 -dependent enzymes. N 2 O is used as an
anaesthetic in surgery and dentistry but also served as a recreational drug, hence its
colloquial name ‘laughing gas’. In natural environments, N 2 O is generated by
various microbial metabolic processes including nitrate or nitrite reduction via nitric
oxide (NO) and N 2 O to produce dinitrogen gas (N 2 ), the so-called denitrification
pathway, as well as dissimilatory nitrate/nitrite reduction to ammonia (DNRA)
(Fig. 1) [14–20].
In addition, N 2 O is also produced by aerobic ammonia-oxidizing bacteria via the
so-called nitrifier denitrification pathway, by ammonia-oxidizing Thaumarchaeota
and by some methane-oxidizing bacteria that denitrify nitrate and/or nitrite under
hypoxic conditions (Fig. 1) [17, 20–25]. Furthermore, N 2 O effluxes from plants
have been reported [26]. Among the microbial processes, only organisms
186
J. Simon
Nitrous oxide (laughing gas; N 2 O) is the dominant nitrogenous greenhouse gas
(GHG) on Earth and contributes to ozone layer depletion [1, 2]. Its current atmospheric concentration amounts to about 332 ppbv (13 nM), and its average atmospheric lifetime is 114 years. The N 2 O concentration is approximately 1,250-fold
lower than that of carbon dioxide (CO 2 ) but, noteworthily, N 2 O has an almost
300-fold greater potential for global warming effects, based on radiative capacities.
Hence, N 2 O accounts for up to 10% of total emissions when the impact of individual GHGs on global warming is expressed in terms of CO 2 equivalents (CO 2 e), a
unit that has been approved by the Intergovernmental Panel on Climate Change
(IPCC) [1]. For comparison, the atmospheric N 2 O concentration in the year 1900
was about 277 ppbv (17% lower than today), and it has been concluded from ice
core studies that the N 2 O concentration was, most likely, never higher than 280
ppbv over the last 800.000 years [3, 4]. The main reason for the dramatically rising
N 2 O concentration is the worldwide intensification of agricultural fertilization
practices that is resulting in a severe anthropogenic imbalance of the global biogeochemical nitrogen cycle [3, 5–11]. This development started with the invention
of man-made nitrogen fixation by the Haber–Bosch process that allowed the
ever-expanding application of synthetic nitrogen-based fertilizers [12]. Hence,
agriculture and livestock farming are the main drivers of anthropogenic N 2 O
emissions with minor contributions from fossil fuel combustion, biomass burning
and wastewater treatment [1, 3, 10]. The IPCC Special Report Climate Change and
Land from 2019 (revised in 2020) quotes: ‘Agriculture, Forestry and Other Land
Use (AFOLU) activities accounted for around 13% of CO 2 , 44% of methane (CH 4 ),
and 81% of nitrous oxide (N 2 O) emissions from human activities globally during
2007–2016, representing 23% (12.0 ± 3.0 GtCO 2 e yr
−1 ) of total net anthropogenic
emissions of GHGs (medium confidence)’. [13]. Thus, humankind faces a severe
challenge in terms of N 2 O emissions since agricultural activities will have to
increase further to nourish the growing world population.
At room temperature, N 2 O is a rather inert, colourless and non-flammable gas
that can nevertheless act as a biocide, for example, by promoting vitamin B 12
deficiency and affecting vitamin B 12 -dependent enzymes. N 2 O is used as an
anaesthetic in surgery and dentistry but also served as a recreational drug, hence its
colloquial name ‘laughing gas’. In natural environments, N 2 O is generated by
various microbial metabolic processes including nitrate or nitrite reduction via nitric
oxide (NO) and N 2 O to produce dinitrogen gas (N 2 ), the so-called denitrification
pathway, as well as dissimilatory nitrate/nitrite reduction to ammonia (DNRA)
(Fig. 1) [14–20].
In addition, N 2 O is also produced by aerobic ammonia-oxidizing bacteria via the
so-called nitrifier denitrification pathway, by ammonia-oxidizing Thaumarchaeota
and by some methane-oxidizing bacteria that denitrify nitrate and/or nitrite under
hypoxic conditions (Fig. 1) [17, 20–25]. Furthermore, N 2 O effluxes from plants
have been reported [26]. Among the microbial processes, only organisms
186
J. Simon