Determination of organic nitrogen and urea

Organic nitrogen in the aquatic environments exists in a continuous size distribution, from truly dissolved organic compounds to macroheterotrophs. In the range of utilization of bottle samplers and sediment traps, the pool of organic nitrogen in the natura} waters is an assemblage of dissolved/particulate and living/nonliving forms, whose composition is stili not completely known. This organic fraction coexists with the dissolved inorganic nitrogen (DIN) (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 biologica} 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 their communities. The biogeochemistry of organic nitrogen in the aquatic ecosystems is not treated here and readers are addressed to the specific literature [1-8].