Nitrogen cycle assimilation

Figure 24-1 The nitrogen cycle. Conversion of N2 (oxidation state 0) to NH4+ by nitrogen-fixing bacteria, assimilation of NH4+ by other organisms, decay of organic matter, oxidation of NH4+ by the nitrifying bacteria Nitrosomas and Nitro-bacter, reduction of N03 and N02 back to NH4+, and release of nitrogen as N2 by denitrifying bacteria are all part of this complex cycle.1...

Figure 1. A simplified watershed nitrogen cycle, with major pathways (arrows) and their effects on the watershed hydrogen budget (numbers in circles) shown. Circled numbers represent the number of hydrogen ions transferred to the soil solution or surface water ( +1) or from the soil solution or surface water (-l)for every molecule of N03 or NH4 + that follows a given pathway. For example, nitrification follows the pathway for NH4 + assimilation into microbial biomass ( + l) and is leached out as N03 ( + 1), for a total hydrogen ion production of +2 for every molecule of N03 produced.

Nitrogen The next step in the nitrogen cycle is the assimilation of inorganic nitrogen, in... 

From a quantitative perspective, the most important process of the fixed nitrogen cycle in the ocean is the assimilation of NOJ and NH4 into organic nitrogen in the... 

Mitamura, O. (1986). Urea metaboHsm and its significance in the nitrogen cycle in the euphoric layer of Lake Biwa. II. Half-samrarion constant for nitrogen assimilation by fractionated phytoplankton in different trophic areas. Arch. Hydrobiol. 107, 167—182. 

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 6 Microbial transformations of the nitrogen cycle. Pathways depicted are 1—N2 fixation 2—DIN assimilation 3—ammonium regeneration 4—nitrification 5—nitrate/nitrite reduction and 6—denitrification.

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). 

Figure 28. Hypothetical anaerobic nitrogen cycle based on the following thermodynamically permissible reactions (1) ammonium oxidation to dinitrogen by carbon dioxide,. sulfate or ferric iron (no evidence at present, possibly kinetically limited) (2) dinitrogen fixation by various organic and inorganic reductants (known) (3) ammonium oxidation by nitrite or nitrate producing dinitrogen (known) (4) denitrification (known) (5) nitrite or nitrate respiration (known) (6) ferric iron oxidation of ammonium to nitrite or nitrate (no evidence at present) (7) nitrate assimilation (known) (8) ammonium assimilation and di.s,similation (known) (Fenchel etai, 1998).

Fig. 3.17 Summary of the nitrogen cycle (oxidation states of nitrogen shown in parentheses). Ammonium assimilation and ammonification can occur in oxic and anoxic environments, as can nitrogen fixation (although the most prolific bacteria are aerobes).

The stable isotope composition of sedimentary organic matter has widely been used to describe the state of the oceanic nitrogen cycle as it allows conclusions on nitrogen sources and transformation processes (i.e. assimilation or denitrification), which are subject to isotopic fractionation (i.e. Francois et al. 1992 Altabet and Francois 1994 Altabet et al. 1999 Thunell et al. 2004). We will briefly discuss this issue because early diagenesis... 

The cyclic transformation of nitrogenous compounds is of great importance in the total turnover of this element in the biosphere. The main features of the biological nitrogen cycle are illustrated schematically in Fig. 4.16. Plants and algae assimilate nitrogen as either nitrate or ammonia to form... 

The isotope fractionation factors associated with reactions of significance to the lacustrine nitrogen cycle are summarised in Table I. Detailed reviews of the physiology of N assimilation, mechanisms of fractionation, their measurement and the range of known fractionations are given in Owens (1987), Handley Raven (1992), Fogel Cifuentes (1993) and Goericke et al. (1994). 

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