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You are here: UMWiki>FR_3104_5104 Web>ClassWikiProjects>NitRification (15 Dec 2006, thom2333)

Nitrification

Nitrification is a biological process by which reduced nitrogen compounds from dead organic matter (usually ammonia) are oxidized to nitrite and nitrate. This process was discovered by the Russian microbiologist, Sergei Winogradsky (Wikipedia). The process of nitrification oxidizes ammonia into nitrates which can be utilized by plant root systems. The classical definition is the process whereby NH4+ is oxidized via NO2- to NO3-.

"Nitrification is generally carried out by autorophic bacteria which derive their energy solely from these oxidations of NH4+ and NO2- and not from oxidations of carbon containing compounds" (R.J. Haynes 1986).

Limited nitrogen within the soil inhibits the plants ability to utilize its immediate resources for optimun plant growth and development.

Bacteria Involved

"Several genera of autorophic bacteria are able to oxidize ammonium to nitrite, including Nitrosomonas, Nitrosolobus, and Nitrospira, while Nitrobacter appears to be the dominant or only nitrite oxidizer in terrestrial ecosystems" (R.J. Haynes 1986).

Reactions in the soil are generally caused by the activities of two small groups of chemoautorophic bacteria. One group, the NH4+ oxidizers, begins the process with the formation of NO2-, while a second group, the NO2- oxidizers, completes the process by converting NO2- to NO3- as promptly as it is formed.

Nitrosomonas exist in almost every ecosystem. Nitrosomonas are mostly lithotrophic which means they live in the soil and are generally aerobic. The nitrosomonas form colonies when enough ammonia and oxygen is present (R.J. Haynes).

Nitrobacter a key rod-shaped bacteria that can reside in both soil and water, is important in the process of breaking down nitrites into nitrate.

Ammonium oxidizers

In soils, Nitrosospira appears to be generally well represented, often accompianed by approximentely equal numbers of Nitrosomonas, while the Nitrosolobus is present generally in low numbers. Nitrosomonas is known to be the main genus associated with sewage or manured agricultural land.

There is much diversity among the genera Nitrosomonas, Nitrosospira and the Nitrosolobus. A team of experts (Williams and Haynes 1995) found examples in one soil where at least four classifications of Nitrosomonas, five of Nitrosospira and one of the Nitrosolobus were present. Several reasons include and defined in detail later are: substrate concentration, pH, moisture content, and other environmental parameters that allow a diverse popluation of NH4+ oxidizers to coexist in multiple niches within the soil (R.J. Haynes 1986).

Nitrite oxidizers

Nitrobacter appears to be the only genus of NO2- oxidizer in the soil though NO2- oxidation generally occurs rapidly as NO2- is formed. NO2- rarely accumulates within the soil profile or within nature for that matter. There is however diversity within the Nitrobacter species. There are two that are known and coexist within the same soil and are observed to have different growth rates. (R.J. Haynes)

Chemistry Processes

"A biological interchange of organic nitgroen with inorganic nitrogen occurs through the simultaneous processes of mineralization and immobilization. First part of mineralization is the breakdown of the organic N into amino acids and then ammonification to NH3 by energy yielding enzymatic reactions" (Williams and Haynes 1995).

R(organic nitrogen radical)-NH2 + H20 ---> NH3 + R--OH + energy

"The NH3 is quickly hydrolysed into NH4+ and the latter can then be nitrified in a two-step process to NO3-. Generally in soils the nitrification process is carried out by a rather small group of bacteria known as autorophic bacteria which derive thier energy from oxidation of N compounds. The first step NH4+ is oxidized to NO2- by Nitrosomonas and a small number of other organisms we know as Nitrosolubus and Nitrospira" (Williams and Haynes 1995) It follows as:

2 NH4+ + 3 02 ---> 2NO2- + 2H2O + 4H+

The NO2- is quickly converted to NO3- by Nitrobacter

2NO2- + O2 ---> 2NO3- + energy

We know that ammonification can occur in both aerobic and anaerobic conditions, and nitrification can only occur in an aerobic state. Thus, where oxygen is limited, such as a puddle in a field, NH4+ will accumulate. NO3- rarely accumulates in soil and we know this from prior knowledge that plants remove NO3- through root uptake and NO3- is also lost through denitrification, leaching, and erosion.

Factors Regulating Nitrification:

1. Substrates and Products: Briefly, autorophic nitrifiers are dependent on either NH4+ or NO2- as a specific energy source so that substrate concentration can be a very important influencing factor. There also can be too much NH4+ concentration in the soil and R.J. Haynes claims that the maximum tolerable NH4+ concentrations for nitrification to occur vary between 400 and 800 ug N gm-1. When the concentration is high, NH4+ attributes to toxic levels of NH3 at high pH.

2. Soil pH: The soil pH is a known limiting factor of nitrification. Nitrifiers prefer a range pH of 7 to 9. Nitrobacter can be slowed if there are toxic levels of NH3 and a pH greater than 7.5 (R.J. Haynes 1986).

3. Aeration and Moisture: "Generally, nitrification occurs at soil moisture in the range of -10 to -33 kPa, also depending on soil characteristics" (R.J. Haynes). When there is excess water, oxygen becomes limiting and nitrification is slowed drastically. Also, inadvertentely, according to R.J. Haynes, when the soil is dry, understood as the permanent wilting point, nitrification becomes inhibited. Something that is extremely interesting is that on dry soils, if any decent amount of moisture is added, mineralization of the organic nitrogen in the soil rapidly breaks down, creating an accumulation of NO3- in the soil.

4. Temperature: Ideal temperatures range between 25-35 degrees Celcius. However, the nitrifiers are able to adapt as their climate changes and still nitrify at much higher and lower temperatures, even in freezing soils.

5. Limiting Supply of Ammonium under Vegetation: If the supply of NH4+ is limiting, the nitrification rate is inhibited. When vegetation is present, competition exists for NH4+ between the plants and the organisms at work.

6. Nutrient Deficiencies: There are deficiencies other than nitrogen, for an exapmle, phosphourous, can limit the activity of the bacteria and thus stress the production of nitrite.

Other limiting factors: Pesticides, Trace Element Toxicities, Allelopathic Substances (R.J. Haynes 1986).

Assets of Nitrification

Nitrogen is taken up by plants in the forms of NO3- and NH4+. Every plant differs on its preference but most prefer to utilize NO3- and NH4+. It is necessary that nitrification take place, to readily make nitrogen available and easier for the plant to utilize. We know that most plants are only composed of 1-5% nitrogen as dry matter, so nitrogen is not a significant factor to plant weight. However, nitrogen is a main component in photosynthesis and the leaves contain about 1-6% nitrogen reinforcing the need for nitrogen to be made available to the plant.

Another interesting concept is the avoidance of ammonium toxicity. Nitrification decreases the levels of NH4+ in the soils and therefore reduces the chances of NH4+ toxicity to plants.

A decrease in ammonia volatilization occurs when nitrification limits the opportunity by breaking nitrogen down into nitrate rather than losing it as NH3. However, one downfall is when nitrification increases the chances of gaseous losses in the forms of N2 and N20.

Nitrification involves the removal of exchangeable NH4+ from the soil system and tends to bring about the release of fixed NH4+. When soils are found with large amounts of fixed nitrogen (unavailable), the nitrifiers are important in making allowing the fixed nitrogen to become available to plants (R.J. Haynes 1986).

Drawbacks of Nitrification

Nitrification uses a large amount of energy when NO3 concentration levels increase in the soil and decreases that of NH4+.

There is also a loss of nitrogen, and it can be a significant amount lost within ecosystems. NH4+ is generally retained by negatively charged particles and NO3- is not attracted to negative particles and becomes very mobile within the soil and is easily leached. Nitrification also is lost in a gaseous form of N20.

Environmental and health hazards are also a concern. Leaching of NO3- into groundwater, can lead to troubles in drinking water, lakes and rivers. Also, some researches have expressed concern that the emission of N20 into the atmosphere contributes to the degredation of the stratospheric ozone layer (R.J. Haynes 1986).

Soil acidification can be a problem in soils resulting from nitrification. It is demonstrated with the following formula: NH4+ + 2O2 <----> NO3- + H20 + 2H+

Conclusions

Nitrfication is a process within the ecosystem in which NH4+ is oxidized to NO3- via NO2- and can be seen occuring in all soils where NH4+ is present. The most important pathway of nitrification is by the actions and contributions of the nitrifying bacteria. The four genera of autotrophs as mentioned in the beginning, that are able to oxidize NH4+ to NO2: Nitrosomonas, Nitrosococcus, Nitrosospira, Nitrosolobus, and the mighty genus Nitrobacter is known to oxidize NO2- and NO2-. The bacteria prefer certain soil temperatures, pH levels, soil moisture content and other general qualities. Nitrification, as awesome as it is, also has drawbacks. One being the process results in acidification of the soil. During the nitrification process N can be, and is lost as N20 and NO3- and can undergo denitrification in anoxic soil sites with the release of N2 and N20 gases. Many see nitrification to have fewer assests than drawbacks, however, as small of a process as nitrification is, nitrification is still important.

References

Haynes, R.J., Williams, P.H. 1995. Nitrogen Nutrition in Higher Plants. Nitrogen in the Plant Environment: 1-18.

Haynes, R.J., K.C. Cameron, K.M. Goh, R.R. Sherlock. 1986. Mineral nitrogen in the plant-soil system. Nitrification: 127-156.

Malzer, Gary. Plant Nurtients in the Environment. Soils 3416: Nitrification.

Prosser, Jim. http://www.abdn.ac.uk/~mbi010/nitrification.htm

Wikipedia. http://en.wikipedia.org/wiki/Nitrification

Topic revision: r12 - 15 Dec 2006 - 16:28:20 - thom2333
 
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