Nitrogen Treatment Revisited: A Study of Nitrate Removal by Denitrification in a 19 Year Old Constructed Wetland.

The intensively cultivated region of the Upper Mississippi River Basin, especially areas containing tile drainage, has been identified as the dominant source of riverine nitrate to the Gulf of Mexico, where it is a major contributor to summer hypoxia. Constructed wetlands designed to intercept tile drainage water have been shown to be effective at removing nitrate via denitrification. Three central Illinois wetlands were studied for nine wetland water years and were found to remove approximately 45% of the tile water nitrate. For this study, we re-evaluated these three wetlands 19 years after their initial creation and establishment, which was determined to be a major gap in current literature.  We determined an input/output budget for total N and investigated greenhouse gas fluxes from the wetlands at various times throughout the drainage season. Greenhouse gas fluxes were measured using floating and static chambers. These wetlands leaked a portion of their water under the berm when full, but the seepage water passed through a 15 m riparian buffer where more nitrate was removed. Therefore, we determined nitrate removal and nitrous oxide flux from seepage water throughout the drainage season as well. Currently, new patterned tile drainage systems are being rapidly installed throughout central Illinois. Since the creation of the three wetlands in 1993, the fields that drain into the wetlands have received new patterned tile drainage systems. This allowed us to compare and contrast N removal rates under different drainage area to wetland area ratios. Overall, we hope to show that these wetlands have maintained or increased their ability to process and remove N from tile drainage water. We also will better understand their overall greenhouse gas fluxes, from both flooded and dry wetland sediments.