Istituto di Scienze Marine     
Cozzi S., Giani M. Determination of organic nitrogen and urea. Leo M.L. Nollet; Leen S. P. De Gelder (ed.). Boca Raton, London, New York: CRC Press, 2007.
Organic nitrogen in the aquatic environments exists in a continuous size distribution, from truly dissolved organic compounds to macro heterotrophs. In the range of utilization of bottle samplers and sediment traps, the pool of organic nitrogen in the natural waters is an assemblage of dissolved/particulate and living/non-living forms, whose composition is still not completely known. This organic fraction coexists with the dissolved inorganic nitrogen (nitrate, nitrite and ammonium) and the gaseous forms (N2 and NxOy). The transformations among the different pools of nitrogen in the aquatic ecosystems are mainly mediated by the biological processes. However, abiotic processes and ambient conditions concur to regulate the nitrogen cycling, because of their influence on the activity and abundance of living organisms and on the structure of communities. The biogeochemistry of organic nitrogen in the aquatic ecosystems is not treated here and readers are addressed to the specific literature (1; 2; 3; 4; 5; 6; 7; 8). Several compounds have been identified as constituents of the natural pools of organic nitrogen. Fulvic and humic substances, urea, dissolved combined amino acids, dissolved free amino acids, polypeptides, purines, pyrimidines, pteridines, methylamines, amino sugars and creatine contribute to the pool of dissolved organic nitrogen. Nucleic acids, chlorophylls, ATP, proteins, chitins, peptidoglycans, muramic acid and ornithine contribute mostly to the nitrogen associated to the particulate matter (1; 3; 8; 9; 10; 11). Inland waters and coastal marine areas also receive the contributions of a large variety of organic nitrogen compounds of anthropogenic origin, included their degraded residues, because of the discharges of urban and industrial waste waters (5), fertilizers and pesticides (12; 13), detergents and surfactants (14), drugs and antibiotics (15). Atmospheric deposition supplies a significant amount of urea and other organic nitrogen compounds to the aquatic environment (16; 17; 18). Nitrogen limitation of phytoplankton production can occur both in freshwaters, estuaries and oceans (19; 20). As nitrogen can be biologically available both in inorganic and organic forms, total (dissolved) nitrogen and particulate nitrogen are used as water quality indicators in monitoring programs of freshwaters, coastal waters (21; 22; 23), and wastewaters. The concentration of organic nitrogen in the aquatic ecosystems varies in dependence of their trophic state and of the external inputs. The lowest concentrations of dissolved organic nitrogen are found in deep ocean (2-5 mol N l-1), whereas higher and more variable values are typical of the surface ocean (1-13 mol N l-1) and coastal areas (3-19 mol N l-1). Estuarine and river waters have contents of dissolved organic nitrogen usually in the range 5-60 mol N l-1, although concentrations as high as 150 mol N l-1 have been reported (3; 4; 8; 11). Concentrations of urea generally vary from <1 mol N l-1, in oligotrophic and oceanic areas, up to 13 mol N l-1 in coastal, estuarine and river waters (4; 8; 24). Dissolved nitrogen in the aquatic environments is often present mostly as organic form (60-69% of TDN), being the most important exception the deep ocean, where the low contribution of the organic fraction to the total dissolved nitrogen (≈10%) is due to the high concentration of nitrate (3; 4; 8; 25). Concentration of particulate nitrogen in the deep oceans is typically <0.07 mol N l-1, whereas it ranges from 0.1 to 0.5 mol N l-1 in the surface ocean. Higher concentrations are found in the coastal marine waters (0.1-30 mol N l-1), estuarine areas (5-100 mol N l-1) and river waters (1-210 mol N l-1; 2; 26; 27). The nitrogen associated to the particulate is mostly considered to be organic. However, inorganic nitrogen may be found in suspended sediment particles, in particular as ammonium present in the interior of the particles, adsorbed on their surface, or contained in the pore waters (2; 7; 28; 29). Combined nitrogen present in the natural waters can be operationally considered as sum of dissolved inorganic nitrogen (DIN), dissolved organic nitrogen (DON) and particulate nitrogen (PN). Nitrate, ammonia, nitrite and nitrous oxide constitute the dissolved inorganic nitrogen, whereas the sum of inorganic and organic dissolved nitrogen constitutes the total dissolved nitrogen (TDN). Separation between PN and DON is commonly defined on the basis of the size fractionation at 0.45 Ám, which is generally achieved by filtration. However, this definition is arbitrary as the organic nitrogen pool in <0.45 Ám size fraction includes dissolved organic compounds at variable molecular weight (8; 30) as well as colloids, submicron organic/inorganic particles, variable fractions of pico- and femto-plankton (1; 2; 31). The possible presence of living and non-living submicron particles in this size fraction should be taken into consideration, even though it is usually negligible compared to 'truly' DON (8; 32; 33).
URL: http://www.crcpress.com/product/isbn/9780849370335
Subject azoto organico, urea
water analysis

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