Nitrogen Dynamics in Two Created Riparian Wetlands over Space and Time

Abstract

Temporal and spatial variations of nitrogen (N) soil storages and fluxes were examined at two 1-ha created riverine wetlands in the U.S. Midwest. Soil N content (total N, organic-N, NO3-N, and NH4-N), N accumulation rates, and soil C:N ratios were compared between the two wetlands constructed 15 years earlier (one was planted and the other left to naturally colonize). Differences in wetland soil N content and accumulation were also examined in relation to proximity of river input and relative to a range of topographic features related to wetland water depth. The planted and naturally colonized wetlands showed similar rates of N accumulation. However, differences were detected related to the content of mineralized forms of N (NO3-N and NH4-N) that may relate to the history of vegetation communities at these wetlands. Significant spatial variation of N accumulation was detected within the wetlands, with the highest rates found in the deeper open water communities compared to shallow emergent marsh/edge vegetation communities (23.5±2.0 versus 17.3±1.3/18.2±1.4 g N m−2 y−1). Nitrogen budgets comparing two ages of the wetlands illustrate higher N accumulation rates (by 19%), higher N reduction in the surface water (47 to 52% reduction), and increased denitrification rates (by 13%) from year 10 to year 15. We also found out that nitrogen accumulation in the soil was 7.1 to 7.5% higher than were denitrification rates in these young wetlands. We discuss the importance of efforts such as this to support better understanding of N pathways in both created and natural wetlands, while providing critical data needed to improve modelling efforts and assess the long-term effectiveness of wetlands for improving water quality.