Reduction nitrification-denitrification

The biogeochemical cycling of nitrogen is very much controlled by redox reactions. This perspective is presented in Figure 24.3 for the redox reactions that take place in the water column and sediments. The major pathways of reduction are nitrogen fixation, assimilatory nitrogen reduction, and denitrification. The major oxidation processes are nitrification and anaerobic ammonium oxidation (anammox). Each of these is described next in further detail. 

Nitrification-denitrification involves the conversion of NH " to NO , the oxidation of NOj" to N03 , and the reduction of NOj" to NO ". The gases N O and are used in the microbially mediated processes involved in the nitrification-denitrification phenomenon. 

Several comprehensive studies of N assimilation in the North Pacific trades biome have been conducted over the past several decades. Gundersen and his colleagues (1974, 1976) were the first to estabhsh N2 fixation as a source of new N to the open ocean ecosystem, and concluded that it was a more important source of fixed N than wet deposition from the atmosphere (see Case Studies section). They also made measurements of the rates of nitrification, denitrification and assimilatory nitrate-reduction. These latter experiments involved the addition of fairly high concentrations of exogenous N substrates (NH4 , N02, NOa ) and extended incubations (days to months), so the rates reported must be viewed as potential rates at best. 

Coupling of nitrification, denitrification, dissimilatory nitrate reduction to ammonium, and anammox... 

Figure 21.1 Microbial nitrogen cycling processes in sedimentary environments on a coral reef (A) nitrogen fixation (B) ammonification (C) nitrification (D) dissimilatory nitrate reduction and denitrification (E) assimilatory nitrite/nitrate reduction (F) ammonium immobilization and assimilation. Adapted from D Elia and Wiebe (1990). Anammox (the anaerobic oxidation of NH4" with NO2 yielding N2 ) is not represented, as it has not yet been shown to occur on coral reefs, but may be found to be important in reef sediments.

Figure 12 Major reduction-oxidation reactions involving nitrogen. The reactions are numbered as follows (1) mineralization, (2) ammonium assimilation, (3) nitrification, (4) assimilatory or dissimilatory nitrate reduction, (5) ammonium oxidation, (6) nitrite oxidation, (7) assimilatory or dissimilatory nitrate reduction, (8) assimilatory or dissimilatory nitrite reduction, (9) denitrification, (10) chemodenitrification, (11) anaerobic ammonium oxidation, and (12) dinitrogen fixation (after Capone, 1991) (reproduced by permission of ASM Press from Microbial Production and Consumption of Greenhouse Gases Methane, Nitrogen Oxides, and Halomethanes, 1991).

NOj m Intermediate in nitrification, denitrification, and NOf reduction Waters Toxic for fish... 

FIGURE 1. Schematic view of biogeochemical nitrogen cycle 1, nitrogen fixation 2, mineralization 3, immobilization 4, nitrification 5, nitrate assimilation 6, dissimilatory nitrogen reduction 7, denitrification (Rosswall, 1982). 

At last, the inhibition technique takes advantage of the property of acetylene to block the reduction of N O to N after it is injected into the sediment. The total amount of N O produced is then the measure for the denitrification rate as it is easy to determine by gas chromatography (Andersen et al. 1984) or by microsensors (Christensen et al. 1989). The advantage of this method is that analyses can be carried out rapidly and sensitively. Problems are (a) N O reduction is sometimes incomplete, (b) a homogenous distribution of in the pore water is difficult to maintain, (c) inhibits nitrification in the sediment meaning that the coupled system (nitrification / denitrification) might be seriously affected due to the applied method, and... 

The sediments are also important sites for ther removal of fixed nitrogen from coastal waters. Because there is relatively little NOs in the shallow waters overlying coastal and estuarine sediments, diffusion of NOs from bottom water is not a major source of N for denitrification. However, rapid organic matter oxidation results in the release of NH4+ to the pore waters. NH4+ can be converted to Ni by a nitrification/denitrification cycle or by NH4+ oxidation coupled to the reduction of NOs ... 

The nitrate flux from the aerobic zone (where the concentration is high) to anaerobic zone (where the concentration is low and the demand for electron acceptors is high) is regulated by many factors nitrate concentration gradient, nitrification rate, reduction or denitrification rate, and bioturbation and mixing. 

Soils inclnding wetland soils are important sonrces of atmospheric nitrous oxide. A wide range of processes may produce nitrous oxide, as well as minor amounts of NO, but not all of these seem to be fully understood. The main biological processes of nitrous oxide formation in soils are shown in Figure 16.5. They include nitrification, denitrification, the dissimilatory reduction of nitrate to ammonium, and the assimilatory reduction of nitrate wherein N is incorporated in the cell biomass. Additionally, some NO and nitrous oxide may be released due to chemo-denitrification and pyro-denitrification. Of these processes, nitrification and denitrification are the most important with respect to nitrous oxide production. 

There are strong nitrification and denitrification in Zhujiang River Estuary sediments and the average nitrification, denitrification, and nitrate reduction rates ranged from 0.32 to 2.43 imnol/(m h), 0.03 to 0.84 mmol/(m h), and 4.17 to 13.06 mmol/(m -h), respectively. The vertical profiles of the sediments showed that the nitrification and denitrification processes mainly took place in the depth from 0 to 4 cm and there were differences at different sampling sites. The rates of nitrification, denitrification, and nitrate reduction were dominated by Eh, nitrate, and ammonium concentrations in sediment and DO in overlying water (Xu et ah, 2005). 

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 discussed in Chapter 5, in submerged soils nitrification occurs in aerobic sites at the iloodwater-soil and root-soil interfaces. Denitrification occurs upon diffusion of the NO, to the anaerobic bulk soil. Denitrification is favoured over dissimilatory reduction to NH4+(NO, -> NO2 NH4+) because of the large ratio of available carbon to electron acceptors in submerged soils. Denitrification is likely to proceed completely to N2 with little accumulation of N2O because of the very large sink and therefore steep concentration gradient of O2, and because carbon is less likely to be limiting (Section 5.1). 

Anaerobic metabolism occnrs nnder conditions in which the diffusion rate is insufficient to meet the microbial demand, and alternative electron acceptors are needed. The type of anaerobic microbial reaction controls the redox potential (Eh), the denitrification process, reduction of Mu and SO , and the transformation of selenium and arsenate. Keeney (1983) emphasized that denitrification is the most significant anaerobic reaction occurring in the subsurface. Denitrification may be defined as the process in which N-oxides serve as terminal electron acceptors for respiratory electron transport (Firestone 1982), because nitrification and NOj" reduction to produce gaseous N-oxides. hi this case, a reduced electron-donating substrate enhances the formation of more N-oxides through numerous elechocarriers. Anaerobic conditions also lead to the transformation of organic toxic compounds (e.g., DDT) in many cases, these transformations are more rapid than under aerobic conditions. 

It is remarkable that the nitrogen cycle employs extreme oxidation-reduction states of nitrogen. Nitrification carries N from 3 to 5, whereas denitrification reduces N from 5+ to 0. A more direct cycle using only redox states of N between... 

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. 

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