Nitrate nitrogen cycle

Thus, from the viewpoint of environmental risk assessment (critical loads) the most important exposure pathways are nitrate leaching and denitrification, which are both very sensitive to anthropogenic pollution. These links of biogeochemical nitrogen cycle should be firstly quantitatively parameterized to assessing environmental risk. 

In denitrification, part of the biological nitrogen cycle, nitrate in the soil is converted via four enzymatic reactions stepwise to nitrite, nitric oxide and nitrous oxide to finally yield gaseous nitrogen. 

Nitrite reductases (NiRs)—enzymes found in several strains of denitrifying bacteria— catalyze the one-electron reduction of nitrite anion to nitric oxide (Equation 1). - In addition to the importance of this process in the global nitrogen cycle (Figure 1), further incentive for the study of the denitrification process is provided by its environmental impact, ranging from the production of NO as a pollutant and NjO as a potent greenhouse gas, to lake eutrophication due to farm runoff that contains high concentrations of nitrates and nitrites. 

The involvement of HA during bacterial conversion of nitrate to NH3 (known also as the nitrate ammonification phase of the nitrogen cycle) has been studied at the molecular level as part of an effort to delineate the mechanism of conversion of nitrite to NH3 by a group of multiheme cytochromes of bacterial origin. The overall reduction reaction is depicted in equation 3 for cytochrome c-nitrite reductase " ... 

The study of the biosynthesis of inorganic nitrogen oxides has a long and distinguished history, principally in the field of microbial and plant nitrogen metabolism (Zumft, 1993). Relatively recently, it has been found that NO, in addition to other species such as nitrite, nitrate, and NjO, is a true intermediate in the biological nitrogen cycle, as described in detail in Chapter 9. 

The nitrogen cycle entails formation of ammonia by bacterial fixation of N2, nitrification of ammonia to nitrate by soil organisms, conversion of nitrate to ammonia by higher plants, synthesis of amino acids from ammonia by all organisms, and conversion of nitrate to N2 by denitrifying soil bacteria. 

Manufacture of ammonium nitrate The nitrogen cycle Problems with fertilisers... 

In times past, before it was possible to prepare chemically bound nitrogen from atmospheric N2, nitrates were mined as Chile saltpeter in arid regions of Chile and used as fertilizer. Where would such nitrogen fall in the nitrogen cycle shown in Figure 2.2 ... 

Koike, I., and Sprensen, J. (1988) Nitrate reduction and denitrification in marine sediments. In Nitrogen Cycling in Coastal Marine Environments. SCOPE 33 (Blackburn, T.H., and Sprensen, J., eds.), pp. 251-274, John Wiley, New York. 

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