SYMPTOMS OF NITROGEN SATURATION IN A RIPARIAN WETLAND

Riparian forests are in a unique position in the landscape since they form a transition between uplands and aquatic systems. These ecosystems may be highly susceptible to nitrogen (N) saturation since they may be subject to high inputs of N from upland areas. We measured potential net N mineralization and nitrification, soil inorganic N levels, microbial biomass carbon (C) and N content, and the N content of litter as indicators of N saturation in two riparian zones on the eastern and western sides of a stream. The sites had similar soils, vegetation, and hydrology, but differing upland land use. The eastern or ''enriched'' site was downgradient of a dense residential housing development (built in the 1950s) that produced high groundwater nitrate (NO3-) concentrations. The western or ''control'' site had an undeveloped upland. Our objectives were (1) to evaluate if groundwater NO3- loading had induced changes in surface soil N-cycle processes that are symptoms of N saturation in the enriched site and (2) to evaluate these changes in relation to inputs and outputs of N to the site. Soil inorganic-N levels, litter N content, and potential net N mineralization and nitrification were significantly higher on the enriched site relative to the control site, suggesting that the enriched site was N saturated. However, input-output analysis indicated that the enriched site was still a sink for upland-derived NO3-. High rates of denitrification and storage of N in soil organic matter appear to moderate N saturation on the enriched site.