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 (NO 3 — ) concentrations. The western or "control"site had an undeveloped upland. Our objectives were (1) to evaluate if groundwater NO 3 — 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 and N saturated. However, input—output analysis indicated that the enriched site was still a sink for upland derived NO 3 — . High rates of denitrification and storage of N in soil organic matter appear to moderate N saturation on the enriched site.