Denitrification nitrate, nitrification

The tr ectories for denitrification, nitrate uptake, and sedimentary denitrification were each calculated using a closed-system Rayleigh model with an isotope discrimination factor ( ) in the middle of the range given in Rg. 29.2. The two nitrification tr ectories shown were calculated with a mass balance model for remineralization and nitrification of organic matter produced from average deep water N03, or diazotrophic organic matter with a of —2%o (Montoya etal,2m2). 

In addition to the oxidising of ammonia to nitrites and nitrates (nitrification), there also goes on a reverse process (denitrification) whereby nitrites and nitrates are destroyed by the action of certain organisms, e.g., a nitrate is converted directly into nitrogen, the final result being shown by some such end equation such as this —... 

Nitrification in wetlands is restricted to aerobic zones of soil and water column or under drained soils conditions, where ammonium is oxidized to nitrate. Nitrification reaction supports denitrification by supplying heterotrophs with nitrate as their electron acceptor. In a broader sense, nitrification is defined as the conversion of organic or inorganic compounds from reduced state to a more oxidized state. Three groups of microorganisms are capable of oxidizing ammonium under aerobic conditions ... 

In the process of ammonification, hydroxylamine (NH2OH) is an important intermediate in two directions, denitrification direct to ammonium as well as the oxidation of ammonium to nitrate (nitrification). As long as nitrogen remains in its reduced form (NH4), it remains in the local environment because of its affinity for soil absorption and its rapid uptake by biota. NH4 is in equilibrium with NH3, which can escape to the atmosphere, depending on pH, temperature, soil moisture, soil type and atmospheric NH3 partial pressure. The equilibrium between emission and deposition (gas uptake) is called the compensation point, similar factors control the emission/dry deposition of NO. 

Ammonia is also produced from burning of coal and bacterial decomposition of proteinaceous organic matter. Conversion of ammonia to ammonium salts will occur rapidly both in the atmosphere and in the soil but will result in less acidity of the soil. It is not known to what extent ammonia reduces excess greenhouse gases such as carbon dioxide in the atmosphere to form ammonium carbamate. Microbial assimilation of ammonium compounds consumes alkalinity of the soil as well as converting ammonium ions to nitrate. Nitrification is carried out by many aerobic bacteria. Nitrification is pH sensitive. Denitrification, which is an anaerobic process, is also pH sensitive. 

Denitrification is a process in which facultative organisms will reduce nitrate to nitrogen gas in the absence of molecular oxygen. This consequendy results in the removal of BOD. The denitrification process also generates one hydroxyl ion so that alkalinity requirements are reduced to half when both nitrification and denitrification are practiced. 

Figure 3. The general nitrogen model for illustrating the bio geochemical cycling in Forest ecosystems. Explanations for the fluxes 1, ammonia volatilization 2, forest fertilization 3, N2-fixation 4, denitrification 5, nitrate respiration 6, nitrification 7, immobilization 8, mineralization 9, assimilatory and dissimilatory nitrate reduction to ammonium 10, leaching 11, plant uptake 12, deposition N input 13, residue composition, exudation 14, soil erosion 15, ammonium fixation and release by clay minerals 16, biomass combustion 17, forest harvesting 18, litterfall (Bashkin, 2002).

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). 

As will be discussed further in this chapter, there is now much evidence to suggest that NO is an obligatory intermediate in the denitrification pathway. Furthermore, there is evidence that NH3 nitrifiers can synthesize the denitrification apparatus in addition to the nitrification apparatus and that the former system can produce NO and N2O (also N2 in at least one case) from nitrite under low partial pressures of O2. It is possible therefore that NO may be an intermediate in the denitrification activity of nitrifiers and so arise as a secondary consequence of NH3 oxidation. NO can also be ptoduced by nondenitrifying organisms under certain conditions. For example, NO can be slowly produced by the anaerobic reduction of nitrite, but only in absence of nitrate, by a variety of enteric bacteria. Some of the NO can be further reduced to N2O. 

Nitrogen transformations such as nitrate assimilation, denitrification, and decomposition (eqs 10-12) contribute to alkalinity by consuming H+. In contrast, both ammonium assimilation and nitrification (eqs 13 and 14) consume alkalinity via the production of H +. 

Ammonia is oxidized in nature to nitrate via several intermediates in the process of nitrification. Nitrate may be reduced to nitrite by either a dissimilatory or an assimilatory process. Nitrite may be assimilated into the cell via reduction to ammonia, or it may be reduced by microorganisms to N20 and N2 in denitrification. A major part of the total nitrogen in this pathway is lost to the atmosphere. However, in turn, atmospheric dinitrogen is converted to ammonia by various bacteria in nitrogen fixation. 

Denitrification takes place in anoxic environments where nitrate and nitrite act as electron acceptors (oxidizers) and nitrification reactions then reverse NOf => NO2 => NO. The processes of denitrification (H2) on land are important channels for nitrogen to get into the atmosphere. The intensity of these processes depends on temperature, humidity, pollution of soils with poisonous chemicals, and pH. The quantitative and functional characteristics of these dependences have been well studied. The global model need only take into account temperature and humidity ... 

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