12.2.1: Chemistry of the nitrogen cycle

The nitrogen cycle is presented in Figure \(\PageIndex{1}\). Atmospheric N₂ is fixed into ammonia (NH₃) by free-living and symbiotic bacteria and archaea (diazotrophs), using the nitrogenase enzyme, the universal catalyst, to break the N₂ triple bond. In soil, NH₃ can be converted into ammonium ion (NH₄⁺), which can be oxidized to nitrate ion (NO₃^-), in a three-step process called nitrification. Nitrite (NO₂^-) and NO₃^- ions are generated during nitrification and they may be reduced during the denitrification process, that is the stepwise reduction of NO₃^- to N₂ by four enzymes, generating intermediate products: NO₂^-, NO and N₂O. During the NO₃^- ammonification to NH₄⁺, via NO₂^-, it can also occur NO₃^- reduction, thus producing N₂O (Thomson et al. 2012).

Figure \(\PageIndex{1}\): Nitrogen cycle. The oxidation states of N are given in parentheses and the names of the processes are indicated

Several concurrent processes are responsible for nitrogen gases emissions in agricultural soils (Bockman & Olfs 1998, Stevens & Laughlin 1998). Most of the N₂O 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 NH₄⁺ and NO₂^- into N₂, under anaerobic conditions, while some fungi can produce N₂ and N₂O 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 N₂O is produced in non-biological processes: the chemical decomposition of nitrite (chemidenitrification) and hydroxylamine oxidation (NH₂OH) (Bremner 1997). Chemidenitrification is the decomposition of NO₂^- that occurs in neutral and acidic soils, causing volatilization and fixation of NO₂^- in the soil organic matter (Bremner et al. 1980, Bremner 1997). The amount of N₂O produced this way is almost negligible (Bremner et al. 1980, Bremner 1997). Hydroxylamine is an intermediate compound in the oxidation of NH₄⁺ to NO₃^- that can produce much more N₂O than the chemidenitrification process (Bremner 1997). In neutral and acidic soils, N₂O is the main product of the NH₂OH oxidation, due to its reaction with Mn and Fe, while in calcareous soils (pH from 7.8 to 8.2) the NH₂OH reacts with CaCO₃ and the main product is N₂ (Bremner et al. 1980).

Nitrification and denitrification processes are further discussed, since they are the most important ones to the N₂O formation in soils. Nitrification is the aerobic oxidation of NH₄⁺ to NO₃^- caused by chemoautotrophic bacteria in two stages: nitritation, in which the NH₄⁺ is oxidized to NO₂^- by Nitrosomonas sp., Nitrosospira sp. and Nitrosococcus sp.; and nitratation, in which the NO₂^- is oxidized to NO₃^- by Nitrobacter sp., Nitrosospira sp. and Nitrococcus sp. (Moreira & Siqueira 2006). In general, nitrification can be summarized by the following reactions: nitritation: 2NH₄⁺ + 3O₂ → 2NO₂^- + H₂O + 4H⁺ + energy; nitratation: 2NO₂^- + O₂ → 2NO₃^- + energy. During this process, the NO₂^- concentration increases as NH₄⁺ is being oxidized and then decreases as NO₃^- is being formed.

Denitrification is the reduction process of NO₃^- to N₂, 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 N₂, or can be incomplete, and a variable fraction of N can be emitted as NO and N₂O (Figure 1).

Despite denitrification is responsible for the most part of the N₂O produced in the soil, nitrification can also produce N₂O, when O₂ is limited. Bremner (1997) presented results from many researches which showed that nitrifying microorganisms can significantly contribute to N₂O emissions from soils. According to this author, the N₂O 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. N₂O produced by nitrification can be decreased when nitrification inhibitors are used.

Under anaerobic condition, the concentration of NO₂^-, 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 N₂O and NO during nitrification (Snyder et al. 2009), as it follows:

References

BOCKMAN, O. C.; OLFS, H. W. Fertilizers, agronomy, and N2 O. Nutrient Cycling in Agroecosystems, Dordrecht, v. 52, n. 2/3, p. 165-170, 1998.

BREMNER, J. M. et al. Formation of nitrous oxide and dinitrogen by chemical decomposition of hydroxylamine in soils. Soil Biology and Biochemistry, Oxford, v. 12, n. 3, p. 263-269, 1980.

BREMNER, J. M. Sources of nitrous oxide in soils. Nutrient Cycling in Agroecosystems, Dordrecht, v. 49, n. 1-3, p. 7-16, 1997.

HAYATSU, M.; TAGO, K.; SAITO, M. Various players in the nitrogen cycle: diversity and functions of the microorganisms involved in nitrification and denitrification. Soil Science and Plant Nutrition, Tokyo, v. 54, n. 1, p. 33-45, 2008.

MOREIRA, F. M. S.; SIQUEIRA, J. O. Microbiologia e bioquímica do solo. 2. ed. Lavras: UFLa, 2006.

SNYDER, C. S. et al. Review of greenhouse gas emissions from crop production systems and fertilizer management effects. Agriculture, Ecosystems & Environment, Amsterdam, v. 133, n. 3-4, p. 247-266, 2009.

STEVENS, R. J.; LAUGHLIN, R. J. Measurement of nitrous oxide and di-nitrogen emissions from agricultural soils. Nutrient Cycling in Agroecosystems, Dordrecht, v. 52, n. 2-3, p. 131-139, 1998.

THOMSON, A. J. et al. Biological sources and sinks of nitrous oxide and strategies to mitigate emissions. Philosophical Transactions of The Royal Society Biological Sciences, London, v. 367, n. 1593, p. 1157- 1168, 2012.

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