Nitrification in soil - Terminology and methodology (review)

Nitrification is defined as a biological oxidation of ammonium to nitrite a nd nitrate, or more generally as the biological transformation of reduced f orms of nitrogen to oxidized forms. In many soils nitrification is a key pr ocess in nitrogen transformations as it converts the exchangeable cation, N H4+, to the mobile anions NO2- and NO3-, which rapidly undergo other transf ormations (denitrification to N-2 and N2O, assimilatory and disimilatory ni trate reduction to ammonia, nitrate respiration; Fig. 1) or are easily leac hed from the soil. High rates of nitrification thus usually lead to high lo sses of nitrogen from soil. Moreover, as the oxidation of nitrogen compound s during nitrification produces hydrogen ions (H+), nitrification results i n acidification of the soil. Most nitrification is carried out by chemolith oautotrophic bacteria belonging to the family Nitrobacteraceae. The family consists of two main groups, the ammonia-oxidizing or nitrosobacteria and t he nitrite-oxidizing or nitrobacteria (Tab. I). Nitrifying bacteria are obl igate aerobes and gain energy from the oxidation of reduced nitrogen compou nds to fix CO2 to organic carbon. Although nitrite and nitrate are main nit rification products, there is an increasing evidence that also some gaseous nitrogen species, namely NO and N2O, are produced as by products during th e autotrophic nitrification. In addition to chemolithoautotrophic nitrifyin g bacteria, many other (heterotrophic) soil bacteria and fungi have an abil ity to oxidize reduced nitrogen compounds, both mineral (NH4+) and organic in the process called heterotrophic nitrification - heterotrophy is related to use of organic compounds as a source of carbon for biomass synthesis. H eterotrophic nitrifiers apparently do not obtain energy from the process an d thus its physiological importance is not clear. Nevertheless, heterotroph ic nitrification can prevail in (micro)sites with unfavorable conditions fo r autotrophic nitrifiers in, for example, acidic forest soils. As with auto trophic nitrification, nitrogen gases may also be produced during heterotro phic nitrification, although the significance of both autotrophic and heter otrophic sources of NO and N2O has not been clearly demonstrated. It has be en recently found that substantial amounts of NO and N2O can be produced by autotrophic nitrifiers under conditions with lowered pO(2). At high pO(2) the organisms use molecular oxygen to oxidise NH; during nitrification, whi ch depletes the O-2 in their microenvironment. In order to maintain catabol ism, nitrifying bacteria may switch to using either nitrate or nitrite as a n electron acceptor in respiration reactions, saving any remaining O-2 for activation of NH4+ by ammonium monooxygenase enzyme. This in fact means tha t they carry out denitrification reactions which were found to be an import ant source of nitrogen gases. The process is called nitrifier denitrificati on and is believed to contribute substantially to NO and N2O production in many soils. Nitrification is controlled by many environmental variables (Fi g. 2), but the principal regulatory factors are ammonium as the substrate a nd partial pressure of molecular oxygen. The latter is a result of balance between soil air and soil moisture content, which is controlled both direct ly and indirectly by many environmental conditions. The only practical way for controlling the rate of nitrification in the fie ld is thus the use of specific chemical compounds known as nitrification in hibitors (Tab. II), or to manage soil N to prevent substatial NH4+ accumula tion at times when plant demand is small. Several techniques for estimation of nitrification rates in soil were developed, of which short-term nitrify ing enzyme assay is a promising tool for the indication of recent capacity of the soil to nitrify.