Comparison of nitrogen solute concentrations within alder (Alnus incana ssp. rugosa) and non-alder dominated wetlands : Interrelationships between atmospheric deposition and landscape features of forest catchments in regulating surface water biogeochemistry

This study examined differences in nitrogen solutes and groundwater flow patterns between a riparian wetland dominated by the N 2 -fixing shrub, Alnus incana ssp. rugosa, and an upstream coniferous forested riparian wetland along a stream of the Adirondack Mountains, where some surface waters are susceptible to nitrogen excess. Channel water NO 3 - was up to 16 μmol l -1 greater in the alder reach, with peaks following maxima in groundwater dissolved inorganic nitrogen (DIN). NO 3 - at 25 cm depth was 30 μmol greater in the alder than in the conifer reach in April, and 24 μmol l -1 greater than channel water and 30 μmol l -1 greater than that of 125 cm groundwater in June. Dissolved organic nitrogen and NH 4 + concentrations increased between 25 and 75 cm depths in both wetlands during the growing season. Inorganic nitrogen increased between the hillslope and stream in both wetlands, with the greatest increases in the alder reach during the dormant season. Greatest subsurface DIN (120 μmol l -1 ) occurred at 75 cm in the alder reach, within 1 m of the stream, between November (120 μmol l -1 NH 4 + ) and a January thaw (60 μmol l -1 each of NH 4 + and NO 3 - ). Concentrations of deeper groundwater at 125 cm during this period were lower (10-30 μmol l -1 ). Lateral flow from the stream channel occurred in the alder reach during the dormant season, and channel water contribution to groundwater was correlated strongly to NO 3 - at 25 cm. These results indicate that nitrification is stimulated in the presence of alders and oxidized exchange flow, producing NO 3 - that may contribute to elevated channel water NO 3 - during periods of peak flow.