Nitrogen nitrification

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In nature, nitrogen [1] is found in several different forms: as gases in the atmosphere, in soil as organic and inorganic components and in plant- and animal cells. The handling of nitrogen in nature is often referred to as the Nitrogen cycle and consists of several different microbial processes where each step results in different nitrogen forms. Hence, the availability of plant nitrogen is highly dependent on microbial activity. Perhaps most important in this respect is the ability of bacteria to oxidize NH4+ to NO3-, as different plants prefer uptake of either the positive or negative N species.

Nitrification is a process conducted by a specialized group of bacteria [2]. The production occurs in two steps; first ammonia (NH3) is oxidized to nitrite (NO2-) by ammonia-oxidizing bacteria, and second nitrite (NO2-) is oxidized to nitrate (NO3-) by nitrite-oxidizing bacteria (Myrold, 1998; Kowalchuk et al., 2001; Prosser et al., 2002).

In nitrification, soil microbes combine ammonia with oxygen to form nitrate. Nitrifying bacteria are both chemolithotrophic and autotrophic meaning that they derive energy from the oxidation of the nitrogen compounds that can be used for assimilation of carbon dioxide. The capacity to nitrify is limited to only a few bacterial genera.

Ammonia oxidation is mediated by the enzyme ammonia monooxygenase resulting in the product hydroxylamine (NH2OH). This reaction requires a small amount of energy. In the second step, NH2OH is converted to NO2- by hydroxylamine oxidoreductase enzymes, an energy-yielding reaction. In the third step, nitrite oxidoreductase oxidizes NO2- to NO3-. The nitrite oxidoreductase is membrane-bound and transfers oxygen from water to NO2- while conveying a pair of electrons to the electron-transport chain for production of ATP through oxidative phosphorylation.

For a very long time it was assumed that specific genera of bacteria were the only organisms responsible for the oxidation of NH3 in soil. But recently it has been discovered that also archaea may play an important role in the ammonia oxidation.

One common way to characterize nitrification is to measure the so-called potential ammonium oxidation rate (PAO).

Factors affecting nitrification in arable soils

In arable soils, the nitrogen transformations that take place after the application of nitrogen fertilizers depend on many factors like weather conditions (temperature and precipitation), soil properties (organic matter and soil texture), and type of fertilizer (Mulvaney, 2005). Specific factors affecting nitrification besides density and composition of nitrifyer population are aeration, substrate (ammonia) availability and pH (Myrold, 1998; Nobuhiko et al., 2006).

Berg et al (1988) showed in short term field experiments that the nitrification potential and number of ammonium oxidizers had increased in response to increased soil moisture. Clay mineral content can also affect nitrification activity. Macura et al (1980) observed a higher nitrification rate after the addition of montmorillonite. Both moisture and clay content affect substrate availability and oxygen status.

Leggett and Iskandar (1980) showed that the optimal pH for Nitrosomonas and Nitrobacter spp. was around 8. This is in concordance with observations that nitrification in arable land is favored by a high pH with an optimum around 8.5 (Norman et al., 1987). The effect of the type of fertilizer on nitrification was studied in an experiment where three sandy soils were treated with either urea or ammonium sulphate (NH4)2SO4, respectively (Eno et al., 1957). It was concluded that nitrification was more rapid with urea than (NH4)2SO4, due to the increase in pH associated with hydrolysis of urea by soil ureases.

References

Myrold, D.D. 1998. Microbial nitrogen transformations. p. 259-294. In Principles and Applications of Soil Microbiology (D.M. Sylvia, J.J. Fuhrmann, P.G. Hartel, and D.A. Zuberer, eds.). Prentice Hall, Upper Saddle River, NJ.

Kowalchuk, G. A. & Stephen, J. R. 2001. Ammonia-oxidizing bacteria: a model for molecular microbial ecology. Annu. Rev. Microbiol. 55, 485–529.

Prosser, J. I. & Embley, T. M. 2002. Cultivation-based and molecular approaches to characterisation of terrestrial and aquatic nitrifiers. Antonie Van Leeuwenhoek 81, 165–179.

Mulvaney, R. L. 2005. Factors affecting the efficient use of nitrogen fertilizers. Illinois Fertilizer Conference Proceedings. [Online] Available from: http://frec.cropsci.uiuc.edu/1992/report13/ . [2007-11-18].

Nobuhiko, F., Takuji, S., Shuji, H., & Satosh, N. 2006. Factors affecting nitrification in arable soils in Hokkaido, Japan: Influence of applied nitrogen concentration, form of nitrogen source, soil pH and soil organic matter. Pedologist. ISSN: 0031-4064. Vol.50; No.2; Page. 81-90.

Berg, P., & Rosswall, T. 1988. Abiotic factors regulating nitrification in a Sweden arable soil. SperingerLink. P.247,254. [Online] available from: http://www.springerlink.com/content/p154344lj081645g/ [26-12-2007].

Macura, J., & Stotzky, G. 1980. Effect of montmorillonite and kaolinite on nitrification in soil. Folia Microbiol 25:90-105.

Leggett, D. C., & Iskandar, I. K. 1980. Improved enzyme kinetic model for nitrification in soils amended with ammonium. CRREL, US Army Corps of Engineers. Hannover, New Hamsphire, USA.

Norman, R. J., Kurtz, L. T, and Stevenson, F. J. 1987. Distribution and recovery of nitrogen-15-labeled liquid anhydrous ammonia among various soil fractions. Soil Science Society of America Journal, 51:235-241.

Eno, C. F., & Blue, W. G. 1957. The comparative rate of nitrification of anhydrous ammonia, urea, and ammonium sulfate in sandy soils. Soil Science Society of America Proceedings, 21:392-396.

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