Main Nitrogen Species

In terrestrial ecosystems the symbiotic bacteria, particularly strains from the genus Rhizobium, play the most important role in N fixation. These bacteria are found on the roots of many leguminous plants (soybeans, clever, chickpeas, etc.). There are other symbiotic diazotrophs (nitrogen-fixing microbes), but till now the Rhizobium has been the most extensively studied. 

The cyanobacteria are considered the main N-fixing organisms in both fresh water and marine ecosystems. Some estimates have shown that these species are 

Two different pathways can be monitored for nitrates in terrestrial and aquatic ecosystems. The first is related to assimilatory nitrate reduction and the second to denitrification. 

The denitrification is still insufficiently quantitatively understood in aquatic ecosystems, especially by comparison to terrestrial ecosystems. There are different views on whether the oceans are a source or sink of nitrous oxide. Various data indicate that the ocean is, on average, supersaturated with respect to N2O, and that N2O supersaturations are positively correlated with N03 and negatively correlated with O2. A number of studies have suggested that denitrification is a major sink for fixed nitrogen in the oceans. Recent studies suggests that denitrification losses in the oceans are on the order of 0.13 x 10 tons/yr (9.2 x 10 mol N per year) that exceed known oceanic N inputs. More than half of this denitrification (0.067 x 10 tons/yr (4.8 x lO mol N per year) takes place in sediments, with the remainder in pelagic oxygen minimum zones (see Box 3 for more details). 

Estimating denitrification in North Atlantic continental shelf sediments (after Seitzinger Giblin, 1996) 

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The main nitrogen species in solution was nitrite, with a minor amount of nitrate. 

When reacting with HNO3, the metals Cu, Ag, Hg and Bi differ from others in that the main nitrogen species is NO ... 

Research on plasma-deposited a-C(N) H films has been frequently included in the general discussion of carbon nitride solids [2, 3]. However, the presence of hydrogen in its composition, and the complexity of the deposition process, which introduces the nitrogen species in the already intricate hydrocarbon plasma-deposition mechanism, make a-C(N) H films deserve special consideration. This is the aim of the present work to review and to discuss the main results on the growth, structure, and properties of plasma-deposited a-C(N) H films. As this subject is closely related to a-C H films, a summary of the main aspects relative to the plasma deposition of a-C H films, their structure, and the relationship between the main process parameters governing film structure and properties is presented... 

Vitamin C (ascorbate) (Fig. 9.5) has the ability to act as a reducing agent, i.e. it will tend to reduce more reactive species. This ability to reduce Fe3+ to Fe2+may be important in promoting iron uptake in the gut. Oxidation of ascorbate by reaction with reactive oxygen species or reactive nitrogen species seems to lead to its depletion. In vitro, vitamin C can also exert pro-oxidant properties. Fe3+ can react with ascorbate to form Fe2+ and the semi-dehydroascorbate or ascorbyl radical. The latter can react with hydrogen peroxide to form Fe3+, the hydroxyl radical and a hydroxide anion. A key question with regard to the pro- or anti- oxidant effects of ascorbate may therefore be the availability of transition metal ions. Neurons main-... 

Another "OH generating reaction is peroxynitrite decomposition to hydroxyl radical and nitrogen dioxide (see sect. 1.12). Also, actions of ionizing radiation and ultrasounds yielding water molecule decomposition are additional sources of "OH. The hydroxyl radical is probably the main reactive species in the so-called metal-catalyzed oxidation systems (or mixed-function oxidation systems) used in experiments where ascorbate and metal ions are employed (though formation of other products such as high-valency iron species ferryl Fe+4 or perferryl Fe+5 forms cannot be excluded) (N2, S50). 

In addition to ammonia, organic compounds present may also react with HCIO but at a much slower rate. Therefore, in the following discussion we will consider that any chlorine added to the water under discussion reacts mainly with these nitrogenated species in the form of HCIO (recall eq. 10.1a). This acid may ionize as follows ... 

The turnover time of NOj is not well known, but has been estimated to be about 3—7 days (Lipschultz et al, 1996). This makes NO the shortest lived among the nitrogen species considered here. The main reason for this fast turnover is that NO2, even more so than NH, represents an intermediary species (Figure 1.2). It is produced during NO3 assimilation, nitrification, and denitrification, and then immediately consumed again. 

The kinetic model for the gas phase reactions in the reburn and bournout zone enables the description of the influence of the reburn temperature and stoichiometry on the nitrogen species and hence is a suitable tool for the qualitative study of the influence of the main parameters. The simulation predicts a higher NO, reduction potential under ideal conditions than measured. 

Lary et al. (1997) were the first to suggest that soot may also affect northern hemispheric mid-latitude ozone, mainly through possible reactions involving reactive nitrogen species (see Rogaski et al., 1997). 

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