Production of Nitrite from the Photodegradation of Dissolved Organic Matter in Natural Waters

Abstract

Significant concentrations of nitrite NO 2 -were produced from the photodegradation of humic substances (HS) isolated from a variety of natural waters in coastal North Carolina. Nitrite concentrations were 40-118% higher after light exposure relative to initial levels, while no statistical differences were observed in dark controls before and after irradiation. The amount of nitrite produced upon irradiation was positively correlated to the concentration of HS added (r ) 0.97, p < 0.001) and to the length of irradiation. The average production rate, normalized to both humic substance concentration and time of light exposure, for all HS studied ranged from 9.7 to 17 [nM (W-h) -1 m -2 mg -1 ] × 10 5 . This translates into an average yearly nitrite production of 2 × 10 6 mol yr -1 in the top one meter of Onslow Bay. When natural waters were irradiated, changes in nitrite were influenced by both initial nitrite and humic substance concentrations. The rate of nitrite photochemical formation in typical coastal waters (4 nM h -1 ) was smaller than its direct photolysis rate (23 nM h -1 ) and smaller than reported rates of ammonium generation via DOM photodegradation (50 nM h -1 ) and of the same order of magnitude as NH 4 + incorporation during humification (1 nM h -1 ). The photochemical release of biologically available nitrite from biologically refractory humic substances has significant implications with respect to nitrite biogeochemistry and N cycling in natural waters and suggests HS bound N is more biogeochemically labile than previously thought.

Photochemical production of dissolved inorganic nitrogen and primary amines from dissolved organic nitrogen in waters of two estuaries and adjacent surficial groundwaters

“…This release, in addition to the release of NH 4 + , may increase the productivity of Nlimited bacteria as they degrade humic substances. Nitrite is also produced by the photolysis of humic substances (Kieber et al 1999) and from NO 3 -by a number of reactions energized by UVB light (Zafiriou & True 1979). However, NO 2 -production was observed in only 1 of the 8 irradiated samples in the present study in which NO 2 -concentrations were above the limit of detection.…”

“…One process that may contribute to the bioavailability of riverine DON is photochemical decomposition. Photochemical processes have been shown to result in the release of ammonium (NH 4 + ), dissolved primary amines (DPA), nitrite (NO 2 -), and unidentified labile N compounds when more refractory DON compounds are exposed to sunlight (Bushaw et al 1996, Bushaw-Newton & Moran 1999, Kieber et al 1999, reviewed in Moran & Zepp 1997. Of these, the observed rates of photoproduction of NH 4 + are generally the greatest, between 0.04 and 0.4 µM h -1 (Bushaw et al 1996, Gao & Zepp 1998, Gardner et al 1998).…”

Origin, causes and effects of increased nitrite concentrations in aquatic environments

“…A totally different source of nitrite accumulation influenced by light is the production of significant concentrations of nitrite (40-118% relative to initial levels) from the photodegradation of humic substances (Kieber et al 1999).…”

Photochemical production of ammonium in the oligotrophic Cyprus Gyre (Eastern Mediterranean)

“…However, the role of photochemistry in open ocean nutrient budgets has received little attention, even though NH estuarine and coastal seawater (Bushaw et al, 1996;Gardner et al, 1998;Bushaw-Newton and Moran, 1999;Morell and Corredor, 2001;Koopmans and Bronk, 2002;Buffam and McGlathery, 2003), while rates of up to 4700 nmol L −1 h −1 (corrected for dilution during ultrafiltration), were observed in freshwaters (Bushaw et al, 1996;Gao and Zepp, 1998;Wang et al, 2000;Koopmans and Bronk, 2002;Buffam and McGlathery, 2003;Vähätalo et al, 2003). In addition to ammonium, photoproduction of nitrite (NO − 2 ) and low molecular weight organic nitrogen compounds such as dissolved primary amines and dissolved free amino acids were also observed (Spokes and Liss, 1996;Bushaw-Newton and Moran 1999;Kieber et al, 1999;Tarr et al, 2001;Wiegner and Seitzinger, 2001;Koopmans and Bronk, 2002;Buffam and McGlathery, 2003). However, the associated photoproduction rates of these N compounds are generally one order of magnitude lower than corresponding rates of photoammonification.…”