Phototransformations of Dissolved Organic Nitrogen

Within the last decade it has become obvious that photochemical reactions play an important role in the transformations of organic matter. Most studies in this respect have focused on organic carbon. However, photochemical reactions are not restricted to carbon, but also affect other elements including nitrogen (see below), sulfur (e.g. Brugger et ah, 1998 Dierdre et ah, 2003 Hatton, 2002), phosphorus (observations limited to humic-rich lake water—see Mopper and Kieber, 2002), and metals (see Zepp, 2003). 

The interest in environmental photochemistry is additionally stimulated by the awareness of an increasing flux of harmful UVB radiation due to stratospheric ozone depletion (e.g. Crutzen, 1992 Smith et ah, 1992). As a consequence, there has been a sizable increase in the number of publications concerning interactions between sunlight and both biotic and abiotic components of aquatic environments (Fig. 10.1). 

The data that have accumulated so far are summarized and discussed in several excellent reviews. Of those, information on the impact of sunlight on aquatic nitrogen compounds can be found in Bronk (2002), Kieber (2000), Mopper and Kieber (2002), Moran and Covert (2003), and Zepp (2003). A reader interested in mechanisms behind the reported phenomena is advised to turn to more chemistry-oriented works by Boule et ah (1999) and Mopper and Kieber (2000). The present chapter is a supplement and update of the above-mentioned studies, specifically focusing on phototransformations of dissolved organic nitrogen (DON). 

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