Nitrification and denitrification

Also known as denitrification. Denitrifying bacteria reduce nitrate and release molecular nitrogen (N2) or nitrous oxide (N2O) under anoxic conditions. Microorganisms and plants absorb and utilize nitrate for two completely different purposes. One is called assimilation nitrate reduction with nitrogen as nitrogen source: NO3-→NH4+→ organic nitrogen. Many bacteria, actinomycetes and molds can use nitrate as nitrogen nutrition. Another purpose is to reduce nitric acid to nitrogen (N2) by using NO2- and NO3- as the final electron acceptors of respiration, which is called denitrification or denitrification: NO3-→NO2-→N2↑. Only a few bacteria can carry out denitrification, and this physiological group is called denitrifying bacteria. Most denitrifying bacteria are heterotrophic bacteria, such as denitrifying micrococcus and denitrifying Pseudomonas. They use organic matter as nitrogen source and energy source for anaerobic respiration, and their biochemical process can be expressed as follows:

Energy of c6h12o6+12no3-→ 6h2o+6co2+12no2-+

Ch3cooh+8no3-→ 6H2O+10co2+4N2+8OH-+energy

A few denitrifying bacteria are autotrophic bacteria, such as Thiobacillus denitrificans, sulfur oxide or nitrate to obtain energy, assimilate carbon dioxide, and nitrate as the final electron acceptor of respiration. The following reactions can be carried out:

5S+6KNO3+2H2O→3N2+K2SO4+4KHSO4

Denitrification reduces nitrate to nitrogen, thus reducing the content of nitrogen nutrition in soil, which is unfavorable to agricultural production. In agriculture, intertillage is usually used to loosen soil to prevent denitrification. Denitrification is an indispensable link in nitrogen cycle, which can reduce NO3- flowing into rivers and oceans due to soil leaching, and eliminate the toxic effect of nitric acid accumulation on organisms.

Nitrification:

The process of ammonia oxidation to nitrite is completed by two groups of microorganisms: ammonia-oxidizing bacteria (AOB) and ammonia-oxidizing archaea (AOA)[ 1]. Ammonia-oxidizing bacteria can be found in β -proteobacteria and γ -proteobacteria of Proteobacteria [2]. At present, only one kind of ammonia-oxidizing archaea-Nitromonas [3] [4] has been isolated and found. The most studied ammonia-oxidizing bacteria in soil belong to the genus nitrosomonas and nitrosococcus. Although ammonia oxidation occurs in both bacteria and archaea in soil, the ammonia oxidation of archaea occupies the primary position in both soil and marine environment [5][6], which means that archaea may be the biggest contributor to ammonia oxidation in these environments. The second step (the step of oxidizing nitrite into nitrate) is mainly completed by nitrifying bacteria in bacteria. All the above steps will generate energy and combine it into adenosine triphosphate. Nitrifying organisms are chemoautotrophic bacteria, which use carbon dioxide as the carbon source for their growth. Some ammonia-oxidizing bacteria have an enzyme called urease, which catalyzes the decomposition of urea into two molecules of ammonia and one molecule of carbon dioxide. It was found that nitrosomonas europaea, like native ammonia-oxidizing bacteria, can assimilate carbon dioxide produced by urease reaction in calvin cycle to produce biomass energy, and harvest energy by oxidizing ammonia (another product of urease) into nitrite. This characteristic can explain why the existence of urea in acidic environment will promote the growth of ammonia-oxidizing bacteria [7].

Nitrification also plays an important role in the denitrification of municipal wastewater. Conventional denitrification is first nitrification and then denitrification. The consumption of this process is mainly spent on aeration (the process of bringing oxygen into the reactor) and providing additional carbon source (such as methanol) for denitrification.

Nitrification can also occur in drinking water. In the water distribution system, chloramine is often used as a secondary disinfectant, and free ammonia can be used as a substrate for ammonia-oxidizing microorganisms. This correlation reaction can reduce the residual disinfectant in the system [8].

These organisms can exist in most environments at the same time, and the final product they produce is nitrate. However, a system that only produces nitrite can be designed (see Sharon process).

Nitrification and ammoniation together constitute an inorganic process, which refers to the process of complete decomposition of organic matter and release of available nitrogen-containing compounds. This process completes the nitrogen cycle.