Ammonia ammonification

Autotrophic activity. Because of the low C N ratio and its declining value as carbonaceous residues are degraded there is substantial ammonification. With all mean treatment times greater than the doubling time of Nitrobacter sp. nitrification will occur provided that oxygen is not limiting. Smith and Evans (19) found that with DO levels above 15% of saturation, nitrification continued until the culture was limited by a fall in pH level. Up to 40% of the slurry ammonia was oxidised. The autotrophic activity never achieved steady state and cycled between periods of activity when the pH value was above about 5.5 and periods of inactivity when the pH value fell below 5.5. Complete nitrification of all ammonia only occurred if the pH value was controlled at about 7 by the addition of alkali. When the DO level was held within the range of 1 to 15% of saturation a system of simultaneous nitrification and denitrification was established. The reduction of nitrate allowed the pH value to remain above 6 and nitrification to continue. Thus more than 70% of the ammonia was oxidised. If the DO level was held below 0.1% of saturation, nitrification was inhibited (unpublished). 

Figure 10.5 Major processes involved in the biogeochemical cycling of N in estuaries and the coastal ocean (1) biological N2 fixation (2) ammonia assimilation (3) nitrification (4) assimilatory NC>3 reduction (5) ammonification or N remineralization (6) ammonium oxidation (speculative at this time) (7) denitrification and dissimilatory NO3 reduction to NH4+ and (8) assimilation of dissolved organic nitrogen (DON). (Modified from Libes, 1992.)...

Oxidation of ammonia to nitrite, N02, and nitrate, N03, is called nitrification the reverse reaction is ammonification. Reduction from nitrite to nitrogen is called denitrification. All these reactions, and more, occur in enzyme systems, many of which include transition metals. A molybdenum enzyme, nitrate reductase, reduces nitrate to nitrite. Further reduction to ammonia seems to proceed by 2-electron steps, through an uncertain intermediate with a -fl oxidation state (possibly hyponitrite, N202 ) and hydroxylamine ... 

Ammonification is the process by which the organically bound nitrogen of microbial, plant, and animal biomass is recycled after their death. Ammonification is carried out by a diverse array of microorganisms that perform ecological decay services, and its product is ammonia or ammonium ion. Ammonium is a suitable source of nutrition for many species of plants, especially those living in acidic soils. However, most plants cannot utilize ammonium effectively, and they require nitrate as their essential source of nitrogen nutrition. 

Many organisms, especially bacteria, decompose organic material with the release of ammonia, a process referred to as ammonification. 

Ammonia is the primary basic gas in the atmosphere and, after N2 and N20, is the most abundant nitrogen-containing compound in the atmosphere. The significant sources of NH3 are animal waste, ammonification of humus followed by emission from soils, losses of NH3-based fertilizers from soils, and industrial emissions (Table 2.8). The ammonium (NH ) ion is an important component of the continental tropospheric aerosol. Because NH3 is readily absorbed by surfaces such as water and soil, its residence time in the lower atmosphere is estimated to be quite short, about 10 days. Wet and dry deposition of NH3 are the main atmospheric removal mechanisms for NH3. In fact, deposition of atmospheric NH3 and NH4" may represent an important nutrient to the biosphere in some areas. Atmospheric concentrations of NH3 are quite variable, depending on proximity to a source-rich region. NH3 mixing ratios over continents range typically between 0.1 and lOppb. 

FIGURE 8.5 Oxidation and reduction reactions of nitrogen in wetlands. Numbers 1-7 refer to pathways of nitrogen reactions. 1 = ammonification 2 = immobilization 3 = nitrification 4 = denitrification 5 = dissimilatory nitrate reduction to ammonia 6 = dinitrogen fixation and 7 = ammonia volatilization. 

Mineralization is basically a sequence of enzymatic reactions. Ammonia is stable under anaerobic conditions therefore, it is the primary form of nitrogen in wetland soils. Globally, ammonia represents only about 15% of the nitrogen released into the atmosphere. Although ammonification can occur in both aerobic and anaerobic environments, a higher concentration of ammonia is seen in anaerobic conditions due to the lower demand for nitrogen by anaerobic organisms. 

The fate of ammonium nitrogen in wetland soils is summarized in Figure 8.25. Ammonification of organic nitrogen results in the release of ammonium into the soil solution, which is readily partitioned into dissolved phase and an adsorbed phase, maintaining a certain level of equilibrium between these two pools. Depending on the conditions found in wetlands, the fate of ammonium in soil solution includes (1) loss via volatilization as ammonia, (2) oxidation of ammonia by nitrifiers,... 

Such biochemical process of removing nitrogen from the bio-geo-chemical cycle is called ammonification. In the presence of ammonium is repidly oxidized with the participation of microorganisms to nitrate NO. This biochemical process of oxidizing ammonia is called nitrification. Autotrophic nitrification has 2 stages. First, bacteria of genera Nitrosomonas, Nitrosospira, Nitrosococcus, Nitrosolobus form nitrite ... 

Nitrite Ammonification Reductive Transformation of Nitrite to Ammonia... 

The reduction of nitrite to ammonia was described for bacteria with a fermentative rather than a respiratory metabolism [125]. However, growth of various bacteria by oxidation of non-fermentable substrates such as formate linked to the reduction of nitrite to ammonia, demonstrated that nitrite ammonification may also function as respiratory energy conserving process [126]. The enzymol-ogy and bioenergetics of respiratory nitrite ammonification have been recently reviewed [17]. In respiratory nitrite ammonification, nitrite is reduced to ammonia without the release of intermediate products, such as NO or N2O, in a six-electron step by a cytochrome c nitrite reductase, the so-caUed NrfA protein [Eq. (10)] [127-131]. 

In summary, ammonia oxidation is negligible (4%) compared with the main fate of NH3, deposition (40 %) and particle formation (55 %) the numbers in parenthesis provide the percentage of emitted NH3 (Moller 2003). Thus, ammonia remains in its oxidation state -3, is mainly seen as ammonium (NH4) in air and returns to soils and waters as ammonium, where it moves between the amino group (—NH2) in the biomass and nitrate through nitrification and ammonification. 

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