Nitrification is one of the crucial steps of the nitrogen cycle that occurs in soil. It is an aerobic process that involves two successive oxidation reactions, where the ammonia first oxidizes into nitrites, and then nitrites get oxidized into nitrates. It occurs in the soil, which includes members of autotrophic bacteria and archaea that facilitate the interconversion of soil ammonia into the inorganic nitrates. The microorganisms that participate in the process of nitrification refers as “Nitrifying bacteria”.
The energy yield in nitrification is quite low, and the process is relatively slower than the other processes occurring in the nitrogen cycle. Thus, nitrification is an oxidative process whereby ammonia oxidizes into nitrate via the production of nitrite as an intermediate.
Definition of Nitrification
Nitrification can define as the biological process, where ammonia (NH3) or ammonium (NH4+) first converts into nitrites (NO2–) and then into nitrates (NO3–). Its mechanism involves two distinct energy-producing reactions, which is further used to fix carbon dioxide.
Oxidation of ammonia to nitrite: It is the first reaction, where the nitrosifying or ammonia-oxidizing bacteria catalyzes the transformation of NH3 to NO2– . This reaction is metabolically inefficient, which requires 34 moles of NH3 to fix 1 mole of CO2.
Oxidation of nitrite to nitrate: It is the second reaction, where the nitrite-oxidizing organisms catalyzes the transformation of NO2– to NO3–. This reaction is even less efficient, which needs around 100 moles of NO2– to fix 1 mole of CO2.
Nitrification is a two-step process mediated by the specific groups of microorganisms present in the soil.
Nitritation: It is the initial step, which involves oxidation of ammonia into nitrogen dioxide or nitrites. Nitrosomonas are the special group of bacteria that participates in nitritation. This reaction uses an enzyme system, i.e. ammonium monooxygenase. The chemical equation for this process is:
NH3 + 1.5O2 –> NO2– + H+ + H20
Nitration: It is the second phase, which facilitates oxidation of nitrites (NO2–)into nitrates (NO3–). Nitration involves nitrite-oxidizing bacteria like Nitrobacter and Nitrospira species via an enzymatic action of nitrite oxidoreductase (NOR) to complete the process. The chemical equation for this reaction is:
NO2– + ½O2 –> NO3–
After these two reactions, the organic form of nitrogen (soil ammonia) has been converted into an inorganic form of nitrogen (nitrate), which plants can use. The chemical equation for the entire nitrification process summarizes into a chemical reaction as:
2NH3 + 3O2 –> 2NO2– + 2H+ + 2H20
- It is an aerobic chemoautotrophic process that requires oxygen for the ammonium and nitrite oxidation via ammonia-oxidizing and nitrite-oxidizing aerobes.
- Ammonia-oxidizing bacteria (e.g. Nitrosomonas) are the soil microorganisms that mediate the ammonia oxidation reaction.
- Nitrite-oxidizing bacteria (e.g. Nitrobacter) mediates the nitrite oxidation reaction.
- Both the nitrifying reactions are thermodynamically favourable, but occurs slowly.
- The efficiency or rate of nitrification is limited due to the nitrifying organisms that are less efficient metabolically.
- slow growth rates of nitrifying organisms.
- Most nitrifying bacteria are autotrophic, while few are heterotrophic.
Factors Affecting Nitrification
The efficiency of nitrification reaction depends upon the various environmental factors. The factors affecting the rate of nitrification include the following factors:
The nitrifying microorganisms are pH sensitive, and require pH between 6.6 to 8.0. The adequate supply of Calcium ions, dihydrogen phosphate and other essential elements in the soil environment provides favourable conditions for the microflora participating in the process of nitrification.
- At pH above 6.0: Nitrification rates get slowed down.
- At pH below 4.5: Nitrification is completely inhibited.
Although, few nitrifying heterotrophic fungi can directly fix the organic nitrogen to nitrate without oxidizing ammonia, as they are resistant to the acidic pH in the soil through slime production.
Nitrifying microorganisms function as the obligate aerobes, which need oxygen to produce inorganic nitrogen like nitrate or nitrate ions. Nitrification occurs at the optimum aeration porosity between 50–60% of the total soil porosity. Nitrification process in aggregated soil mainly constricts to the outer region, which is due to speedy diffusion of oxygen into the atmosphere.
Nitrification mainly occurs at an optimum soil moisture potential of − 0.3 to − 1.6 MPa. In waterlogged or saturated conditions, the nitrifying bacteria are not feasible to carry out nitrification. During water stress, the autotrophic bacteria are more likely affected in comparison to the fungal nitrifier.
The optimum range of temperature that favours the growth of nitrifying bacteria lies between 28°C and 36°C. In forest soil, the nitrification can occur at the temperature of 0°C is due to the predominance of low-temperature-adaptive nitrifying soil fungi. The temperature fluctuations in the soil also affect the rate of nitrification. Nitrification efficiently occurs during the time of spring and fall.
Soil Organic Matter
Nitrification includes the autotrophic nitrifying organisms that do not need soil organic matter as either a carbon or energy source.
Population of Nitrifying Organisms
Soils may differ in their ability to nitrify NH3 because of the diversified population density of nitrifying organisms. There are three categories of nitrifying organisms.
- First category includes ammonia-oxidizing organisms that function as obligate autotrophs and fix CO2 by using the Calvin cycle.
- Second category involves autotrophic ammonia-oxidizing archaea, which can fix CO2 using the 3-HP (hydroxypropionate pathway).
- Third category includes aquatic bacteria of the Planctomycetes phylum.
The use of pesticide reduces the efficiency of nitrification due to their toxic effect on soil microbiota. In contrast, tillage increases the rate of nitrification by improving soil aeration.
Plowed grasslands produce adequate inorganic nitrate. Brachiaria humidicola and Azadirachta indica produce root exudates containing brachialactone, which shows 60%–90% inhibitory effect on nitrification.
The nitrification process plays a significant role in agriculture (where it can facilitate nitrate leaching and determines the availability of fertilizer nitrogen), and in wastewater treatment systems (where it prevents groundwater contamination by removing excess of nitrogen).