Nitrogen retention in the riparian zone of catchments underlain by discontinuous permafrost JONATHAN A. O’DONNELL AND JEREMY B. JONES JR Institute of Arctic Biology, University of Alaska Fairbanks, Fairbanks, AK, U.S.A. SUMMARY 1. Riparian zones function as important ecotones that reduce nitrate concentration in groundwater and inputs into streams. In the boreal forest of interior Alaska, permafrost confines subsurface flow through the riparian zone to shallow organic horizons, where plant uptake of nitrate and denitrification are typically high. 2. In this study, riparian zone nitrogen retention was examined in a high permafrost catchment (approximately 53% of land area underlain by permafrost) and a low permafrost catchment (approximately 3%). To estimate the contribution of the riparian zone to catchment nitrogen retention, we analysed groundwater chemistry using an end- member mixing model. 3. Stream nitrate concentration was over twofold greater in the low permafrost catchment than the high permafrost catchment. Riparian groundwater was not significantly different between catchments, averaging 13 lM overall. Nitrogen retention, measured using the end-member mixing model, averaged 0.75 and 0.22 mmol N m )2 day )1 in low and high permafrost catchments, respectively, over the summer. The retention rate of nitrogen in the riparian zone was 10–15% of the export in stream flow. 4. Our results indicate that the riparian zone functions as an important sink for groundwater nitrate and dissolved organic carbon (DOC). However, differences in stream nitrate and DOC concentrations between catchments cannot be explained by solute inputs from riparian groundwater to the stream and differences between streams are probably attributable to deeper groundwater inputs or flows from springs that bypass the riparian zone. Keywords: denitrification, discontinuous permafrost, groundwater, mixing model, nitrate Introduction Riparian zones can be important sites for reducing nitrogen concentration in groundwaters and the input of nitrogen into streams (Lowrance, Todd & Asmus- sen, 1984; Peterjohn & Correll, 1984; Cooper, 1990). Assimilation by plants and microbes, and consump- tion via denitrification are the primary mechanisms accounting for the loss of nitrate as ground water flows through riparian zones. In the boreal forest of interior Alaska, terrestrial primary production is commonly nitrogen limited (Van Cleve et al., 1983) and thus plant assimilation in the riparian zone is probably an important sink. Further, nitrate loss via denitrification is generally enhanced in riparian zones where anoxic conditions and soil rich in organic matter provide optimal conditions (Hedin et al., 1998; Devito et al., 2000; Hill et al., 2000; Sobczak, Findlay & Dye, 2002). In the boreal forest of interior Alaska, catchments are underlain by discontinuous permafrost, which is an important feature controlling catchment hydrology and the flow of water through the riparian zone (Slaughter & Kane, 1979). Permafrost affects catch- ment hydrology by confining the majority of ground- water flow to shallow organic soil horizons (Woo, Correspondence: Jonathan A. O’Donnell, Institute of Arctic Biology, 311 Irving I, University of Alaska Fairbanks, Fairbanks, AK 99775. U.S.A. E-mail: ftjao1@uaf.edu Freshwater Biology (2006) 51, 854–864 doi:10.1111/j.1365-2427.2006.01535.x 854 Ó 2006 The Authors, Journal compilation Ó 2006 Blackwell Publishing Ltd