12.2.1: Chemistry of the nitrogen cycle
The nitrogen cycle is presented in Figure \(\PageIndex{1}\). Atmospheric N 2 is fixed into ammonia (NH 3 ) by free-living and symbiotic bacteria and archaea (diazotrophs), using the nitrogenase enzyme, the universal catalyst, to break the N 2 triple bond. In soil, NH 3 can be converted into ammonium ion (NH 4 + ), which can be oxidized to nitrate ion (NO 3 - ), in a three-step process called nitrification. Nitrite (NO 2 - ) and NO 3 - ions are generated during nitrification and they may be reduced during the denitrification process, that is the stepwise reduction of NO 3 - to N 2 by four enzymes, generating intermediate products: NO 2 - , NO and N 2 O. During the NO 3 - ammonification to NH 4 + , via NO 2 - , it can also occur NO 3 - reduction, thus producing N 2 O (Thomson et al. 2012).
Several concurrent processes are responsible for nitrogen gases emissions in agricultural soils (Bockman & Olfs 1998, Stevens & Laughlin 1998). Most of the N 2 O is produced through the biological processes of nitrification and denitrification. Autotrophic aerobic nitrification (by ammonia-oxidizing bacteria and nitrite-oxidizing bacteria) and anaerobic denitrification, mediated by denitrifying bacteria, are the main microbial processes in the nitrogen cycle in the soil. Other microorganisms are involved in nitrification and denitrification processes: anammox bacteria can convert NH 4 + and NO 2 - into N 2 , under anaerobic conditions, while some fungi can produce N 2 and N 2 O by denitrification and codenitrification, and archeae mediate nitrification in marine ecosystems and are capable of promoting denitrification in soils (Hayatsu et al. 2008)
A small fraction of N 2 O is produced in non-biological processes: the chemical decomposition of nitrite (chemidenitrification) and hydroxylamine oxidation (NH 2 OH) (Bremner 1997). Chemidenitrification is the decomposition of NO 2 - that occurs in neutral and acidic soils, causing volatilization and fixation of NO 2 - in the soil organic matter (Bremner et al. 1980, Bremner 1997). The amount of N 2 O produced this way is almost negligible (Bremner et al. 1980, Bremner 1997). Hydroxylamine is an intermediate compound in the oxidation of NH 4 + to NO 3 - that can produce much more N 2 O than the chemidenitrification process (Bremner 1997). In neutral and acidic soils, N 2 O is the main product of the NH 2 OH oxidation, due to its reaction with Mn and Fe, while in calcareous soils (pH from 7.8 to 8.2) the NH 2 OH reacts with CaCO 3 and the main product is N 2 (Bremner et al. 1980).
Nitrification and denitrification processes are further discussed, since they are the most important ones to the N 2 O formation in soils. Nitrification is the aerobic oxidation of NH 4 + to NO 3 - caused by chemoautotrophic bacteria in two stages: nitritation, in which the NH 4 + is oxidized to NO 2 - by Nitrosomonas sp., Nitrosospira sp. and Nitrosococcus sp.; and nitratation, in which the NO 2 - is oxidized to NO 3 - by Nitrobacter sp., Nitrosospira sp. and Nitrococcus sp. (Moreira & Siqueira 2006). In general, nitrification can be summarized by the following reactions: nitritation: 2NH 4 + + 3O 2 → 2NO 2 - + H 2 O + 4H + + energy; nitratation: 2NO 2 - + O 2 → 2NO 3 - + energy. During this process, the NO 2 - concentration increases as NH 4 + is being oxidized and then decreases as NO 3 - is being formed.
Denitrification is the reduction process of NO 3 - to N 2 , mediated by facultative anaerobic bacteria, which correspond to 0.1-5.0% of the total bacteria population in the soil (Moreira & Siqueira 2006). This process can be complete, resulting in N 2 , or can be incomplete, and a variable fraction of N can be emitted as NO and N 2 O ( Figure 1 ).
Despite denitrification is responsible for the most part of the N 2 O produced in the soil, nitrification can also produce N 2 O, when O 2 is limited. Bremner (1997) presented results from many researches which showed that nitrifying microorganisms can significantly contribute to N 2 O emissions from soils. According to this author, the N 2 O production during nitrification is increased when the soil pH and organic matter content increase and with increase in soil moisture (from air dried to field capacity) and in soil temperature (5-40ºC), by addition of nitrifiable N forms, animal manures and plant residues. N 2 O produced by nitrification can be decreased when nitrification inhibitors are used.
Under anaerobic condition, the concentration of NO 2 - , which is a toxic compost, increases in the soil (Khalil et al. 2004), and it may be alternatively used by the nitrifying microorganisms as an electron final acceptor, resulting in N 2 O and NO during nitrification (Snyder et al. 2009), as it follows:
References
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Excerpted from:
Signor, D., & Cerri, C. E. P. (2013). Nitrous oxide emissions in agricultural soils: a review. Pesquisa Agropecuária Tropical , 43 , 322-338. Accessed December 2023 at https://www.scielo.br/j/pat/a/yyfWQ9zyWJVdknyBfYFP3wD/ , CC-BY-4.0