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

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 16µmol 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 C 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 C ) and a January thaw (60 µmol l � 1 each of NH4 C 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.