Nitrogen, among the macronutrients, is the most required by plants. It is a component of amino acids, proteins and genetic material (DNA and RNA). It is also associated with the growth and development of plants, as it is involved in the process of chlorophyll synthesis, which is responsible for the photosynthetic process.
In the atmosphere, nitrogen is present in the form of nitrogen gas, represented by the molecular formula N2 (N≡N), and constitutes 78 % of atmospheric air. Despite being found in large quantities in the atmosphere, nitrogen gas is not used by most living organisms, which are unable to fix and incorporate atmospheric nitrogen into living matter. Among the organisms capable of using this nitrogen are some types of bacteria (SANTOS, 2020).
In the lithosphere, the layer of the planet represented by the Earth's crust, there is about 98 % of the existing nitrogen, part of which is in the oceans, part composing rocks and soil. Specifically in the soil, nitrogen is found almost entirely (about 98 %) in organic form, and the other part is found in the mineral forms of ammonium (NH4+), nitrite (NO2–) and nitrate (NO3–).
The nitrogen cycle allows the cycling of this element in the environment, making it available to living beings and then to the environment again. This cycle comprises five specific stages: fixation (1), ammonification (2), nitrification (3), denitrification (4) and assimilation (5) which will be described below.
1 – Nitrogen Fixation
The nitrogen fixation process can occur in three ways: biological fixation, physical fixation and industrial fixation, described below.
a. Biological Fixation
The first stage of the nitrogen cycle, biological fixation, occurs through an enzymatic process, where atmospheric nitrogen (N2) is reduced to ammonia (NH3), through the action of free-living microorganisms, symbiotic or associated with plants, as shown in the chemical equation below:
N2 + 8e– +8H++16ATP → Nitrogenase → 2NH3 +H2 +16ADP+16Pi
According to Vieira (2017), the problem involving the fixation of atmospheric nitrogen is related to the presence of the triple covalent bond between the molecules, which makes this gas highly stable at room temperature. The breaking of this triple bond by microorganisms requires the presence of the enzyme nitrogenase.
N-fixing microorganisms2 can exist as free-living organisms and in associations with varying degrees of complexity with plants. These microorganisms can be divided into: 1. Non-symbiotic or free-living fixers; 2. Associative fixers, which form a casual and poorly structured relationship with roots or aerial portions of plants; and 3. Symbiotic fixers that fix N2 in organized associations with higher plants (VIEIRA, 2017).
Among the symbiotic fixatives, there are some genera of bacteria that stand out for their efficiency and specificity with leguminous plants, the so-called rhizobia, such as: Rhizobium, Sinorhizobium (Ensifer), Mesorhizobium, Azorhizobium and Bradyrhizobium. For symbiosis to occur ideally, some factors are decisive, such as nutrient availability (phosphorus, cobalt, molybdenum, nickel), soil pH (if low, it can inhibit the action of bacteria), temperature and adequate water regime.
b. Physical Fixation
It is worth noting that there is also physical fixation of nitrogen, where atmospheric N (N≡N or N2) is transformed into ammonium through physical phenomena, such as lightning, whose high energy separates nitrogen molecules and allows their atoms to bind with oxygen molecules in the air, forming nitrogen oxides, especially nitrate (NO3–). These are later dissolved in rainwater and deposited in the soil.
c. Industrial Fixation
Under high temperature and pressure, the bond between nitrogen and hydrogen occurs, forming ammonia (NH3). This process is known as Haber-Bosch, the starting point for the manufacture of various products for agriculture and industry.
2 – Ammonification
After fixation, the ammonification stage occurs, where organic nitrogen, in the form of ammonia (NH3) is mineralized into ammonium (NH4+) through enzymatic action and in the presence of water. The ammonia form is insoluble in water and highly toxic to plant tissues. Both aerobic and anaerobic microorganisms, exclusively heterotrophic, act in this process, using organic matter as a source of energy. Chemical equation of the ammonification process:
NH3 + H2THE → NH4+ + OH–
Nitrogen fertilization is a complement to the nitrogen supply capacity of soils, from the mineralization of their organic matter stocks, which are generally high in relation to the plants' needs (MESSIAS et al., 2008). According to these authors, it is common to classify nitrogen fertilizers as organic and chemical, depending on their form of action and general conditions of use.
Messias et al. (2008) suggests that mineral nitrogen fertilizers are divided into four groups: ammoniacal, nitric, nitric-ammoniacal and amide. Organic nitrogen fertilizers come from the mineralization of plant and animal residues, through the effective action of soil microbiota (MESSIAS et al., 2008).
Urea, widely used in agriculture, is a chemical fertilizer of the amide type and, when applied, is rapidly hydrolyzed through a bacterial process that uses the enzyme urease. This results in immediate nitrogen availability, which, from an edaphic and physiological point of view, is not desirable, since immediate release increases losses due to leaching and even volatilization.
Therefore, the use of organic fertilizers with amino acid matrices tends to be better utilized, since the release occurs gradually, minimizing losses and making nitrogen available throughout the entire production cycle.
In general, the ammonification process can be influenced by soil and climate conditions, such as humidity, aeration, temperature and pH. Furthermore, since it is a biological process, it can also be directly affected by soil microbial activities.
3 – Nitrification
Nitrification consists of the oxidation of ammonium (NH4+) to nitrate (NO3–), where nitrifying bacteria use ammonia as an energy source and carbon dioxide (CO2) as a carbon source (NANES, 2017). This process only occurs in the presence of oxygen, as it is carried out by aerobic bacteria. This stage is divided into two parts, as demonstrated by the following chemical equations:
2NH3 + 3O2 → 2H+ + 2NO2– + 2H2The + energy (eq. a)
2NO2– + The2 → 2NO3– + energy (eq. b)
Equation “a” corresponds to the nitrosation process, which occurs through the action of bacteria of the genus Nitrosomonas. At this point, nitrite is formed, which will be oxidized to nitrate in equation “b”, called nitration, carried out by species of the genus Nitrobacter. According to Vieira (2017), the bacteria that participate in nitrification use nitrogen compounds as a source of energy and CO2 as a carbon source, which require O2 in their metabolic processes and have very slow growth.
Among the factors that positively or negatively affect the nitrification process, we have soil aeration, since the bacteria responsible for this stage are aerobic; temperature (ideally between 26 ºC and 32 ºC); soil moisture; liming and the use of fertilizers.
C/N ratios also directly influence the nitrification process. According to Victoria et al. (1992), when high, they cause immobilization of mineral N, at least temporarily, ceasing nitrification due to lack of substrate and potentially causing nitrogen deficiency in plants. According to Moreira and Siqueira (2006), in a soil that offers favorable conditions for nitrification, the nitrate level is reasonably high, while the soil C/N ratio is low. Also according to these authors, when a large quantity of organic waste with a high C/N ratio is added, the microorganisms that act in the decomposition of organic matter become highly active, producing large quantities of CO2, which causes nitrate and ammonium to practically disappear from the soil.
4 – Denitrification
According to Victoria et al. (1992), biological denitrification and ammonia volatilization correspond to the most important gaseous losses of nitrogen from the soil.
The denitrification process involves the removal of oxygenated compounds that are bound to nitrogen, returning it in gaseous form to the atmosphere and completing the biogeochemical cycle of this element. This stage is only possible through the action of anaerobic soil microorganisms that, when respiring, use nitrate and carbon compounds as a source of energy, thus releasing gaseous N into the atmosphere. The chemical reaction is described below:
5C2H6The + 12NO3– + 12 H+ → 10CO2 + 21H2The + 6N2(g)
In the process of N formation2 some by-products such as nitrous oxide (N) may be formed.2O) and nitric oxide (NO) (NANES, 2017). These gases are highly toxic and contribute to global warming and ozone layer depletion. According to Bortoli et al. (2012), the emission of N2It has a greenhouse effect potential 300 times greater compared to CO2, currently being the third most important greenhouse gas present in the atmosphere, behind only CO2 (carbon dioxide) and CH4 (methane).
The main variables that affect the denitrification process in nature are: the concentration of nitrate and nitrite, the organic matter content, the presence of dissolved oxygen, the pH value range and the temperature. Of these factors, the concentration of endogenous nitrate present in the water is one of the main limiting factors of the process (CHAVES et al., 2003).
5 – Assimilation by the plant
The elements present in the soil solution are absorbed by the plants through the roots. The assimilation of nitrogen by the plant occurs through the mineral forms of NH4+ (ammonium) and NO3– (nitrate).
Nitrogen fertilization, what to apply?
Among ILSA Brasil's main products is Azoslow, a pelletized organomineral fertilizer that is produced using Italian technology and has as its main characteristic a formulation that promotes gradual release of nitrogen mediated by the action of microorganisms, originating from Azogel (an exclusive organic matrix from ILSA) in association with urea. Azoslow has 29% of total nitrogen in its composition, in addition to a high content of organic carbon and CTC. This high concentration of organic carbon effectively contributes to increasing microbial activity in the soil near the area where the fertilizer is applied.
The use of this organomineral reduces the number of nitrogen applications, due to its gradual release, thus ensuring the availability of this mineral for longer in the soil, which allows for greater savings and less use of machinery, equipment and labor, as has already been proven by scientific research in Brazil. In addition, it stimulates root development, allowing plants to explore a larger area of soil in search of water and nutrients, increasing productivity and profitability per hectare.
Authors
- Agricultural Eng. MSc. Aline Tramontini dos Santos
- Agricultural Engineer Ana Elisa Velho
- Agricultural Eng. MSc. Thiago Stella de Freitas
Bibliographic references
BORTOLI, M. et al. Nitrous oxide emission in biological nitrogen removal processes from effluents. Sanitary and Environmental Engineering, Rio de Janeiro, v. 17, n. 1, p. 01-06, 2012.
CHAVES, RA, ABE, DS, TUNDISI, JG, MATSUMURA-TUNDISI, T. Controlling factors of denitrification in the water column at the old Cepta/Ibama dam, Pirassununga, São Paulo. Technical and Scientific Bulletin – CEPNOR, Belem, v.3, n.1, p. 197-213, 2003.
MESSIAS, AS et al. Fertilization recommendations – Fertilizers, PE, p. 83-103, 2008.
MOREIRA, FMS; SIQUEIRA, JO Soil microbiology and biochemistry, UFLA Publishing House, Lavras, 2006. 729 p.
NANES, MB Influence of free ammonia on partial nitrification in anaerobic and aerobic series reactors. Federal University of Pernambuco (UFPE), Recife. 2017, 83 p. (Dissertation)
SANTOS, VS dos. “Nitrogen Cycle”; Brasil Escola. Available at: https://brasilescola.uol.com.br/biologia/ciclo-nitrogenio.htm. Accessed on September 19, 2020.
VICTORIA, RL, et al. Soil Microbiology (chap. 08), Brazilian Society of Soil Science, Campinas, 1992, 360 p.
VIEIRA, RF Nitrogen cycle in agricultural systems. Embrapa, Brasilia, 2017. 163p.
