Comparison of nitrogen solute concentrations within alder (Alnus incana ssp. rugosa) and non-alder dominated wetlands

Abstract

This study examined differences in nitrogen solutes and groundwater flow patterns between a riparian wetland dominated by the N₂-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 NO₃⁻ was up to 16 µmol l⁻¹ greater in the alder reach, with peaks following maxima in groundwater dissolved inorganic nitrogen (DIN). NO₃⁻ 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 NH₄⁺ 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 NH₄⁺) and a January thaw (60 µmol l⁻¹ each of NH₄⁺ and NO₃⁻). 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 NO₃⁻ at 25 cm. These results indicate that nitrification is stimulated in the presence of alders and oxidized exchange flow, producing NO₃⁻ that may contribute to elevated channel water NO₃⁻ during periods of peak flow. Copyright © 2004 John Wiley & Sons, Ltd.