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TECHNICAL REPORTS

Landscape and Watershed Processes

Dissolved Organic Nitrogen Regulation in Freshwaters

^a School of Agricultural and Forest Sciences, Deniol Road, University of Wales, Bangor, Gwynedd, LL57 2UW, UK
^b Centre for Ecology and Hydrology, Bangor, Gwynedd, LL57 2UW, UK
^c Cardiff School of Biosciences, Cardiff University, P.O. Box 915, Cardiff CF10 3TL, UK

^* Corresponding author (d.jones{at}bangor.ac.uk).

Received for publication November 13, 2002. Dissolved organic nitrogen (DON) has been hypothesized to play a major role in N cycling in a variety of ecosystems. Our aim was to assess the seasonal and concentration relationships between dissolved organic carbon (DOC), DON, and NO^–₃ within 102 streams and 16 lakes within catchments of differing complexity situated in Wales. Further, we aimed to assess whether patterns of land use, soil type, and vegetation gave consistent trends in DON and dissolved inorganic nitrogen (DIN) relationships over a diverse range of catchments. Our results reinforce that DON constitutes a significant component of the total dissolved N pool typically representing 40 to 50% of the total N in streams and lakes but sometimes representing greater than 85% of the total dissolved N. Generally, the levels of DON were inversely correlated with the concentration of DIN. In contrast to DIN concentrations, which showed distinct seasonality, DON showed no consistent seasonal trend. We hypothesize that this reflects differences in the bioavailability of these two N types. The amount of DON, DOC, and DIN was significantly related to soil type with higher DON export from Histosol-dominated catchments in comparison with Spodosol-dominated watersheds. Vegetation cover also had a significant effect on DON concentrations independent of soil type with a nearly twofold decrease in DON export from forested catchments in comparison with nonforested watersheds. Due to the diversity in catchment DON behavior, we speculate that this will limit the adoption of DON as a broad-scale indicator of catchment condition for use in monitoring and assessment programs.

Abbreviations: DIN, dissolved inorganic nitrogen • DOC, dissolved organic carbon • DON, dissolved organic nitrogen

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