Nitrous oxide how is it formed

In moist soils, the rate of gas diffusion and aeration are smaller, and greater amount of NO would react before being released to the atmosphere. In such condition, the more reduced oxide N 2 O would be the final dominant product released to the atmosphere.

Under supersaturation conditions anaerobic soil , most part of the N 2 O is reduced to N 2 , before being released to the atmosphere Davidson et al. In spite of these reports on production and emission of N 2 O, soils can sink N 2 O from the atmosphere. This phenomenon is still poorly understood, but it can be extremely important in agricultural systems. Factors influencing the consumption of N 2 O by soils are still unclear, but negative fluxes have been reported in a wide range of conditions frequently but not always associated to low availability of N and O 2 in soils, i.

The consumption of N 2 O also depends on soil properties, water content, soil temperature, pH and availability of labile organic C and N. Such variety of conditions suggests that a great number of processes should be related to this phenomenon. The longer the N 2 O remains in the soil, whether because it was produced in deeper layers or due to slow diffusion to the atmosphere, the greater amounts of N 2 O will be used as electron acceptor and greater will be the emissions of N 2 Chapuis-Lardy et al.

Despite the influence of climatic conditions on the N 2 O production, soil management practices affecting factors previously described and benefiting microbial activity can also influence the N 2 O production. Among them, soil tillage, recycling N from crop residues and the application of N-fertilizers mineral or organic are of great importance. However, there are considerable shortcomings in the current state of knowledge on how dynamic factors can control denitrification, and much of this is due to the high spatial and temporal variability of denitrification rates in the field Groffman et al.

The effects of soil tillage practices on N 2 O emissions result from changes in the soil structure, soil aeration, microbial activity, rate of residue decomposition and rate of N mineralization, as well as soil temperature and moisture. No-tillage NT have been reported as a soil management system that increases N 2 O emissions, when compared to conventional tillage CT Liu et al. The higher soil moisture, due to the crop residue in NT Baggs et al. This higher temperature could have stimulated the enzymatic activity of nitrifiers and denitrifiers, enhancing the microbial N 2 O production.

In the CT system, such mechanism would be dissipated by the tillage in the upper soil layer, which increases the O 2 concentration in soil and consequently decreases the N 2 O emission, in spite of the greater mineralization rate of crop residue and organic matter promoted by soil tillage Baggs et al. According to Baggs et al. Nevertheless, not all reports agree with greater emissions in NT.

Metay et al. This was also reported by Liu et al. Passianoto et al. Laboratory experiments conducted by Huang et al. Six et al. Van Kessen et al. Omonode et al. This occurs because, in areas with early NT system, the soil moisture is changed, favoring the denitrification process and thus causing N loss and inducing N deficiency in plants, reducing the crop production Six et al. Many of these conclusions were drawn for temperate climate regions, but we believe that similar responses would be observed in tropical regions.

Thus, in studies comparing different soil tillages, it is important to take into account the time since NT has been introduced. The biochemical composition of plant residues added to the soil is responsible for higher or lower N 2 O emissions Gomes et al.

Siqueira Neto et al. They explained this by the high amount of N applied to the corn field, in contrast to the biological N via microorganism fixation used as N source by the soybean crop.

Jantalia et al. In agreement with such finding, Siqueira Neto et al. In this case, in plots with straw, the high C and N contents did not limit the N 2 O biological production, while in plots without straw the C content limited the microbial activity and, thus, the N 2 O production was lower. Pastures with 4 and 10 years old had N 2 O emissions similar or slightly smaller than in forest areas, while in pastures older than 10 years emissions were smaller than in forests Neill et al.

Fernandes Cruvinel et al. As a result of the increase in food production and the consequent use of N-fertilizers, two thirds of the N 2 O emissions were related to crop production, in Mosier Once the N 2 O emissions by nitrification and denitrification depend on the N content in the soil Akiyama et al.

As a consequence, the loss of N 2 O can also increase, because the NO 2 - formed during the nitrification process can be used as electron acceptor, if O 2 is limited, and also because the denitrification can occur after the nitrification, when soil conditions are favorable. When the NO 3 - availability decreases, N 2 O emissions will also decrease, because denitrification is reduced Hellebrand et al.

On the other hand, N-fertilization implies a higher plant biomass production, and then more crop residues and carbon sources would be available in the soil, what could increase N 2 O emissions for a long period, after the N-fertilizer application Hellebrand et al. A mathematical relationship between accu-mulated N 2 O emission and amount of N applied as fertilizer is not well defined.

Chen et al. Signor et al. In addition, the type of fertilizer also influences the behavior of N 2 O emissions. In general, ammoniacal fertilizers increase N 2 O emissions slower than nitric fertilizers, because nitric sources can be denitrified immediately, while ammonia sources still have to be nitrified before the denitrification. Carmo et al. Zanatta et al. Shimizu et al. Pelster et al. They compared emissions induced by calcium ammonium nitrate, poultry manure, liquid cattle manure and liquid swine manure and suggested that the manure application only increases N 2 O emissions in soils with low C content, i.

Nevertheless, there are some reports of no differences in N 2 O emissions induced by different N sources. Bergstrom et al. However, during 24 days of evaluation, these authors carried out just eight samplings. They also highlighted the limited number of measurements in each sampling day and the spatial variability on soil N 2 O emissions to explain why they did not detect differences between N sources. N 2 O emissions induced by N-fertilizers are concentrated in some weeks after the fertilizer application.

The largest N 2 O fluxes occur during the first two weeks after the fertilizer application Bergstrom et al. Therefore, researches should give priority to measurements of N 2 O emissions during the first two weeks after fertilization Schils et al. In conditions of soil saturation but not soaked , the split application of KNO 3 decreased the emission of N 2 O, as compared to a single application Ciarlo et al. However, if a single dose of N-fertilizer at seeding is followed by rain, it can produce elevated N 2 O emissions Tan et al.

Splitting the doses of N-fertilizers increases their efficiency and reduces losses by leaching and denitrification, implying in benefits for reducing GHG emissions and ensuring natural resources preservation Tan et al. Splitting N rates is also important to supply N during the crop cycle, mainly in periods in which it is more requested, ensuring a higher crop yield.

The fertilizer application depth also influences the N 2 O emission Liu et al. Emissions are smaller when the N-fertilizer is deposited at The application at In this case, slurry applications in inner soil layers are indicated to reduce NH 3 volatilization and increase the amount of mineral N entering the soil.

The interactions of N-fertilizers and other factors influencing N 2 O emissions should also be highlighted. Fertilizer applications during dry weather result in small emissions of N 2 O than the application under moist conditions Smith et al. However, with small N availability, even in conditions of high moisture content, emissions will be reduced Denmead et al. The use of slow releasing fertilizers is an important strategy to reduce N 2 O emissions induced by N-fertilizers, because they are involved in slower nutrient release, despite the fact that the rate, patterns and duration of the release are not well controlled Shaviv The use of controlled release N-fertilizer in southern Brazil promoted lower N 2 O emissions, in comparison to ammonium nitrate, calcium nitrate, ammonium sulphate and uran Zanatta et al.

Slow release fertilizers are classified in three groups: i organic-N low-solubility compounds, that include biologically and chemically decomposing products, as urea-formaldehyde and isobutyledene-diurea, respectively; ii fertilizers with a physical barrier, i.

Jiang et al. However, the application of urea-formaldehyde and urea with nitrification inhibitors hydrochinone and dicyandiamide reduced N 2 O emissions and this was not accompanied by a decrease in the biomass production.

Moreover, N-fertilizers stabilized or bio-amended by inhibitors are classified as slow-acting nitrogen Shaviv Nitrification inhibitors are also indicated as an important strategy to reduce N 2 O emissions induced by N-fertilizers Zanatta et al.

During a two years study, Pfab et al. The DMPP efficiency is dependent on temperature and soil water content. However, the use of DCD for seven years, in three pasture soils, did not change the soil microbial C and N, protease and deaminase activities, and had no negative impact on the abundance of archea and bacteria in the soils, indicating the DCD as an effective N 2 O mitigation technology Guo et al.

Despite the fact that slow release fertilizers and nitrification inhibitors may reduce N 2 O emissions, they are more expensive than conventional fertilizers and have not been widely used Cameron et al.

The International Panel on Climate Change IPCC suggests a methodology to estimate soil N 2 O emissions, according to mineral and organic fertilizers, and also for the mineralization of N from crop residues added to soil.

This value, known as emission factor, was suggested to simplify the calculation in the life cycle study of agricultural crops Brentrup et al. Table 2 presents the emission factors reported in several papers, for many crops, where values vary from 0. Schils et al. The high emission factor value reported by Denmead et al. The emission factor seems to be directly influenced by the N dose.

However, the largest emission factor occurred for the smallest N dose applied to the soil. Although there are not enough long-term datasets to provide the information needed to design emission factors for different climate zones or soil types, the use of specific emission factors that reflect regional variability in climate, soil type and management is a requirement to improve greenhouse gases inventories Thomson et al.

Considering the previous comments and observations made by Thomson et al. So, it is interesting to use manure or organic fertilizers as N source when it is possible, because their N 2 O emissions are lower than for mineral N-fertilizers;. Moreover, it reduces the N available in the soil, especially when heavy rains are expected, and decrease the proportion of N lost as N 2 O.

So, splitting the rate of N-fertilizer recommended to the crop and applying the fertilizers to the inner soil layers can be used to reduce N 2 O emissions;. For example, when Cu concentration in the soil is low and pH values are below 7. Then, using liming to reduce soil acidity and adjusting micronutrients, specially Cu contents, can reduce N 2 O emissions;. The composition of plant-derived C flow or N uptake demand can be changed to avoid N losses to the atmosphere as N 2 O.

Moreover, the release of biological nitrification inhibitors could be promoted by plant breeding;. In order to increase sustainability, it is important to understand how agricultural inputs can positively or negatively affect the biological process and ecosystem services that support agriculture. Nitrous oxide is an important greenhouse gas, due to its high global warming potential times higher than CO 2. Agricultural soils are the main N 2 O source to the atmosphere, and the key processes affecting its production nitrification and denitrification are influenced by various factors that can be modified by agricultural management practices.

The available N in the soil is the most important of these factors and is directly related to the N-fertilizers application. Therefore, if, on one hand, the use of N-fertilizer is important to provide that plants reach a desirable yield, on the other hand, a portion of this added N can be lost to the atmosphere as N 2 O, enhancing the greenhouse effect. Despite the importance of N-fertilizers to nitrous oxide emissions, many questions about it remain unclear.

The behavior of N 2 O emissions from different N sources, different responses according to the crop the fertilizer is applied to, influence of native soil organic matter content and quality, interactions between N and other nutrients, as well as the real influence of micronutrients on N 2 O emissions should be better explored in future researches.

Another important point is to explore the N plant demands and N 2 O emissions, trying to find management practices to maximize plant yield and minimize N 2 O emissions, also considering splitting the N recommended dose.

Information on how differently N-fertilizers induce N 2 O emissions is useful in GHG inventories and contributes to reliable final values in these studies. However, nowadays, N 2 O emissions in agricultural soils are estimated by using a default emission factor that was not necessarily obtained in the place for which the estimative is being done, and do not taking into account the environmental specific characteristics where the fertilizer is applied soil type and characteristics, crop, type and quantity of N added to the area , what can generate unreliable GHG inventories.

In Brazil, there are few studies available about this theme. When elaborating Brazilian GHG inventories or life-cycle assessments, it is important to use information generated under specific climate and soil conditions, and for crops planted in the country.

So, it is necessary to develop long term studies, comprising different N sources and rates, evaluating N 2 O emissions during all the crop cycle and also taking into account the influence of crop residues on these emissions. It is also important to repeat these measurements in different regions, in order to obtain consistent values that will really contribute to the national GHG estimatives.

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Sorry, a shareable link is not currently available for this article. Provided by the Springer Nature SharedIt content-sharing initiative. In this theory, plants are considered just a medium to transport N 2 O produced by soil microorganisms; however, laboratory-based studies have provided clear evidence that plants produce and emit N 2 O although the underlying mechanisms are unknown Goshima et al.

Therefore, N 2 O emitted by plants might originate from two sources, namely, the soil microorganisms and plants. Studies, which have hypothesised that plant-emitted N 2 O is produced by soil microorganisms, have only measured the fluxes from plants and concluded that plant-emitted N 2 O might be produced by soil microorganisms. N 2 O produced in plant cells might also use the same pathway, that is, transpiration, to release it to the atmosphere.

This raises the question whether measuring the fluxes alone provides substantial evidence to prove the hypothesis, because flux measurement methods can just estimate the emission of N 2 O and cannot distinguish the sources. More robust methods such as isotope studies would provide more insights to distinguish the sources of N 2 O. For examples, injecting 15 N-N 2 O into the root zone and measuring the subsequent fluxes would elucidate whether plants are a medium for N 2 O transport or not.

However, no study has injected 15 N-labeled N 2 O into the soil zone and measured the subsequent N 2 O emission from plants. Moreover, more powerful tools such as site preference SP measurement would provide insights to distinguish the sources of N 2 O emitted by plants under field conditions.

In the natural environment, if plant emitted N2O constitute significant amount of both sources soil micro-organisms and plant cells produced N2O , it will be highly challenging to distinguish the portion of the sources.

A recent field experiment reported considerably lower N 2 O concentrations in soil water than in tree stems Ward et al. Similarly, plants exposed to NH 4 did not emit N 2 O despite the high rate of N 2 O production in the rhizosphere Smart and Bloom, , indicating that N 2 O emitted by plants might not be produced by soil microorganisms and that N 2 O emitted through transpiration might be a less significant process than N 2 O production in plants.

Furthermore, the hypothesis that plants are just a conduit for soil microorganisms-produced N 2 O is not supported by a recent study of Lenhart et al.

They provided new evidence that dual isotopocule fingerprints of N 2 O emitted by plants differed from that produced by all known microbial or chemical processes, indicating that plant-emitted N 2 O is produced in plant cells.

Although plants are known to produce N 2 O and emit it to the atmosphere, the exact mechanisms of N 2 O production in plant cells are unknown Goshima et al. This might be the reason that most studies on N 2 O fluxes in plants Chang et al. Studies, which have claimed that plants could produce N 2 O, have not elucidated a possible production pathway. Therefore, the main objective of this study was to review the possible pathway of N 2 O formation in plant cells.

After absorption, NO 3 is directly reduced in the root, transported to the leaf for reduction Maathuis, ; Hachiya and Sakakibara, , or stored in the vacuoles and remobilised when the external supply is limited van der Leij, et al. NO 3 is a major source for N 2 O formation in both soil Thomson et al. Isotope labeling methods have demonstrated that plants as well as other eukaryotic organisms emit N 2 O, only when supplied with NO 3.

For example, when 15 N-labeled NO 3 was supplied as a source of N to various species of plants Goshima et al. This evidence clearly shows that NO 3 is the precursor of N 2 O in lichens, higher plants, and animals. Therefore, if plants were just a medium of transportation of soil-produced N 2 O as hypothesised by many studies Chang et al.

Similarly, if N 2 O emitted by plants is produced by microorganisms nitrifying and denitrifying bacteria , NH 4 supplementation should contribute to N 2 O emission from plants and aseptically grown plants should not emit N 2 O.

Moreover, the processes in the soil microbial communities and higher organisms may be different, or the denitrification process may be common between microorganism and plants, as NO 3 is the substrate for denitrification. However, in animals, NO 3 from food is reduced by bacteria in the digestive tracks Lundberg et al.

Moreover, germ-free mice are reported to possess NR activity, and the activity is catalysed by xanthine oxidoreductase, which is significantly high in the gastrointestinal tissues, compared with other tissues Jansson et al.

However, under hypoxic and anoxic conditions, root NO 3 uptake increases with the activation level of NR Botrel and Kaiser, ; Rockel et al. A 15 N isotope labeling study has showed that 15 N-NO 2 assimilation into amino acids is sharply reduced under hypoxic conditions Oliveira and Sodek, The accumulated NO 2 in the cytoplasm enters the mitochondria with the help of proteins in the chloroplast Sugiura et al.

Moreover, mitochondrial inner membrane anion channels may import NO 2 to the mitochondria Gupta and Igamberdiev, Using 15 N isotope labeling method, it has been demonstrated that NO 2 is another precursor of N 2 O formation in plants and algal cells.

For example, when 15 N-labeled NO 2 was supplied to aseptically grown tobacco plants Goshima et al. The enzyme NR has been proved to play a role in N 2 O production in plants. For example, when tobacco plants were supplied with NO 3 and tungstate NR inhibitor , N 2 O production was inhibited in the plants Goshima et al. A similar role of NR has been observed in algae Plouviez et al. This indicates that NO 2 has to be transported to other cell organelles rather than plastid.

Overall, the available evidence indicates that exogenous NO 2 along with endogenous NO 2 derived through NO 3 reduction in the cytosol by NR plays a role in N 2 O formation in plant cells. We previously discussed NO 3 reduction to NO 2 in the cytosol. NO has several essential roles in plant and animal cells Wendehenne et al. The oxidative pathway is dependent on L-arginine, polyamine, or hydroxylamine, whereas the reductive pathway is dependent on NO 3 and NO 2 Benamar et al.

The oxidative pathway of NO formation is dominant when the oxygen supply to cells is sufficient, whereas the reductive path is dominant under hypoxic conditions. By shifting from the oxidative to reductive pathway, the cells maintain the level of NO along with the physiological and pathological oxygen and proton gradients Lundberg et al.

It may be essential to shift processes, as plants may experience hypoxia due to soil environmental conditions. Although the mechanisms of NO 2 transport to the mitochondria are not precise, it is evident that the mitochondria are a site of reduction of NO 2 to NO.

For example, the mitochondria have been reported to reduce NO 2 to NO under hypoxic and anoxic conditions in fungi Kobayashi et al. However, the enzymes involved in the mitochondrial reduction of NO 2 to NO are not clear. Nitric oxide synthases NOS have been reported to be present in plant Guo and Crawford, and animal mitochondria Giulivi et al. However, the NOS activity in the mitochondria of plants is questioned Moreau et al.

Tischner et al. The mitochondrial respiratory chain is responsible for NO production using NO 2 as the substrate under low pH, hypoxic, or anoxic conditions Castello et al. Mitochondrial and bacterial electron transport chains ETCs are involved in NO production from NO 2 under hypoxic conditions than under normoxic conditions Horchani et al.

Ascenzi et al. The mitochondrial molybdopterin enzymes in the reduced form catalyse the reduction of NO 2 to NO, and the rate was increased when the pH was decreased from 7.

Although at the molecular level, the reductive pathway for NO formation is well documented, at the field scale, the emission of NO is less documented. For instance, when plants were supplied with NO 3 , NO was emitted under anoxic conditions Klepper, ; Rockel et al.

The leaf NO 2 level and NO emission under anoxic conditions were significantly higher than those under normoxic conditions Rockel et al.

These findings suggest that NO 2 can be reduced to NO in the mitochondria; however, the involvement of various enzymes within the mitochondria raises the question whether these enzymes catalyse the reduction process simultaneously or they function differently under varied cell environment. NO is a signaling molecule in cells, and several studies have focused on its formation in the mitochondria. However, studies on the reduction of NO to N 2 O in the mitochondria are limited, although there is a strong indication that this process exists Gupta and Igamberdiev, The inner membrane of the mitochondria has an enzyme called cytochrome c oxidase CcO.

The similar properties of O 2 and NO facilitate the binding of NO to CcO, and this activity is pronounced under oxygen-limited conditions Ghafourifar and Cadenas, Therefore, mitochondria can be a potent site of N 2 O formation under oxygen-limited conditions, and it should be a focus of future research.

Similar to the observations in plants, macrofauna and earthworms are also found to emit N 2 O when supplied with NO 3 and under O 2 -limited conditions Horn et al. Moreover, in other studies, listed in Table 1 , when 15 N-labeled NH 4 was used as a substrate, there was no N 2 O emission. This shows that NO 3 metabolism at the cellular level produces N 2 O in both plants and animals. The gut of insects has a hypoxic environment Johnson and Barbehenn, , which may explain the higher level of N 2 O production in the gut Stief et al.

Moreover, axenic algae supplied with NO 2 produced significantly higher levels of N 2 O under dark conditions than under light conditions Guieysse et al. The low emission of N 2 O under light conditions may be due to the supply of photosynthetic O 2 to the cells. Table 1 Compilation of the substrates, mediums and products that used labeled N sources and their subsequent measurements of N 2 O emissions. Figure 1 Potential pathway of N 2 O formation in plant cells.

NR represents nitrate reductase and Pgb represents phytoglobin Brudvig et al. The pathway is active in presence of NO 3 and NO 2 , and when plants experience hypoxia and anoxia. Based on experimental evidence gathered from various studies, we propose that the reductive pathway of NO formation in the mitochondria and further reduction of NO by the mitochondrial ETCs contributes to the formation of N 2 O in eukaryotic cells, as presented in Figure 1.

The process is catalysed by various enzymes, and it might be pronounced under hypoxic and anoxic conditions but not under normoxic conditions. The proposed pathway is further supported by the existence of a denitrifying pathway, and the associated enzymes and genes in Globobulimina species and the localisation of enzymes in the mitochondria Woehle et al.

As higher animals possess well developed respiratory and circulatory systems that transport O 2 , they may not experience hypoxia. However, plants lack such sophisticated systems to transport O 2 Voesenek et al. Field studies have reported high N 2 O emission from plants under flooded conditions Rusch and Rennenberg, ; Machacova et al.

NO 3 is not only a major nutrient in plant cells but also a signaling molecule Zhao et al. Several studies have reported that NO 3 plays a role in hypoxia tolerance. For example, NO 3 maintains the growth of plants under oxygen-limited conditions, and its absence disturbs plant growth Horchani et al. NO 3 nutrition in plants decreases the total respiration rate and reactive oxygen species levels Wany et al. Under oxygen-limited conditions, NO 3 protects the ultrastructure of mitochondria Vartapetian et al.

The addition of NO 3 to the root zone of plants released significantly less amount of ethanol compared with roots supplied with NH 4 under hypoxic conditions Oliveira et al. This suggests NO 3 plays an important role to decrease alcoholic fermentative metabolism in plants during hypoxia Oliveira et al.

Overall, these findings suggest that NO 3 and NR play an important role to maintain the integrity of plant cells under oxygen-limited conditions. NO 2 is also reported to play important roles under oxygen-limited conditions in plants. Benamar et al. The supply of NO 2 decreased lipid peroxidation and reactive oxygen species formation Gupta et al.

NO 2 supplemented roots released significantly less amount of fermentative ethanol during hypoxia than NH 4 -supplemented roots Oliveira et al.

NO helps plants to cope under several environmental stresses. For example, NO is essential for the homeostasis of O 2 level in plants under oxygen-limited conditions Gupta and Igamberdiev, NO production in the mitochondria has several implications in plants as illustrated in Figure 2. For example, NO can break seed dormancy and stimulate seed germination in plants Beligni and Lamattina, ; Bethke et al. Similarly, under hypoxic stress, NO is vital for the formation of aerenchyma in the roots Wany et al.

NO production in the mitochondria under low-oxygen conditions can help in ATP synthesis, preventing excessive depletion of energy Stoimenova et al. NO 3 -NO 2 -dependent NO production in plant roots decreases fermentative ethanol production during hypoxia Oliveira et al.

Therefore, it is critical to regulate its concentration in cells, as a higher amount of NO is formed under hypoxic and anoxic conditions. As both products of NO metabolism in the mitochondria, that is, NO 2 and N 2 O, are non-toxic, their formation might play a protective role in the mitochondria.

Therefore, scavenging of NO is essential to protect cells from high NO toxicity. Phytoglobins are reported to scavenge NO in the cytosol Igamberdiev et al. Additionally, purified mitochondria have been reported to scavenge exogenous NO Gupta et al. NADH might act as an electron donor to reduce cytochrome c oxidase, leading to an increase in NO scavenging in purified mitochondria.

As discussed in our proposed pathway of N 2 O formation in the mitochondria, the conversion of NO to N 2 O by the reduced form of CcO might be the potential pathway regulating excessive NO formed under oxygen-limited conditions in the mitochondria.

Mitochondria are not only a source of NO, but also an important sink and target of NO Igamberdiev et al. Therefore, N 2 O formation in the mitochondria via the NO 3 -NO 2 -NO pathway might be a strategy to protect cells and mitochondrial components from excessive NO formed under oxygen-limited conditions.

Many catalytic properties of CcO from denitrifying bacteria Paracoccus denitrificans and eukaryotic organisms are similar Ludwig, ; Kadenbach et al. As eukaryotic mitochondrion is considered to be evolved from P. Although during the evolution most of genes of the bacterium transferred to the nucleus, few remained in the mitochondrial DNA including genes of CcO Kadenbach et al. Therefore, it may be possible that CcO of higher organisms might also possess similar properties like that of its ancestor, P.

The significant negative relationship between N 2 O consumption and CO 2 respiration rates in plants and lichens Machacova et al. This N 2 O consumption observed in these eukaryotes might be at the site of CcO, as this enzyme is formed from the last two enzymes of denitrification.

There are also reports of emission of 15 N-labeled N 2 from wheat crops supplied with 15 N-labeled NO 2 Vanecko and Varner, , suggesting that under certain cell conditions, mitochondria may also metabolise N 2 O to N 2. However, to date, N 2 emission from plants is less reported. It may be due to the advanced systems of O 2 regulation in plants, and this might inhibit the complete process of denitrification. A recent study, which measured N 2 O and N 2 emission from soil—plant systems, showed that N 2 O and N 2 emitted by NO 3 -rich soil—plant systems was three times higher than that by NH 4 -supplemented soil—plant systems and bare soil Senbayram et al.

Further experiments at the molecular level mitochondria are needed to explore the reason for the significant negative relation between N 2 O emission and respiration rate in plants and lichen, as reported by Machacova et al.

To cope with the problems of global warming and ozone layer depletion, a good understanding of N 2 O formation processes in various source is critical. Therefore, the N 2 O formation process in plants is a matter of concern. Considering available evidence, we conclude that there is strong possibility that plant cells produce N 2 O in the mitochondria under hypoxic and anoxic conditions. The theory that plants are only a conduit for N 2 O produced by soil-inhabiting microorganisms might be an ambiguous explanation.

The root zone may sense hypoxia and anoxia due to the soil environmental conditions, which may favour N 2 O formation in the root mitochondria. As some studies have shown that N 2 O emission from tree stems is higher than that from soils in natural habitats Welch et al. Furthermore, we have highlighted the reduction of NO to N 2 O in the mitochondria, and therefore, it would be valuable to reassess the role of mitochondrial ETC under both hypoxic and anoxic conditions.

Although, N 2 O is a potent greenhouse gas ,its formation in the mitochondria might help to protect the integrity of the mitochondria and protect cells from the toxicity of NO accumulation during hypoxia. AT wrote the manuscript. CH supervised the whole work. All authors contributed to the article and approved the submitted version.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

We are thankful to Prof. Moreover, we are grateful to Prof. Oene Oenema and Prof. Jiafa Luo for their valuable comments. Alber, N. The occurrence and control of nitric oxide generation by the plant mitochondrial electron transport chain. Plant Cell Environ. Alegria, A. Quinone-enhanced ascorbate reduction of nitric oxide: role of quinone redox potential.