Denitrification potential in a Louisiana wetland receiving diverted Mississippi River water

Abstract

Excess nitrate in Mississippi River water entering offshore areas is reported to contribute to low oxygen (hypoxia) conditions in the Gulf of Mexico. Excessive algal growth driven by the excess nitrogen results in a decrease in dissolved oxygen in bottom water. Reintroduction of Mississippi River waters into a Louisiana coastal wetland has the potential to reduce the amount of nitrate reaching offshore waters. In this study, reduction in the concentration of added NO₃ ⁻ was determined in sediment–water-columns collected from a wetland site in Breton Sound estuary receiving nutrient inputs from the Mississippi River. The capacity of a wetland to process nitrate in floodwater was determined in the laboratory. The rates of NO₃ ⁻ removal (determined from change in nitrate concentration in the floodwater) averaged 97 mg N m⁻² d⁻¹ over 16 d for a 1750-mg NO₃-N m⁻² addition, and 170 mg N m⁻² d⁻¹ over 16 d for a 3500-mg NO₃-N m⁻² addition. The total N₂O-N emissions from the 1750- and 3500-mg NO₃-N m⁻² additions were 19 and 54 mg N m⁻² accounting for 1.1% and 1.5% of the applied NO₃-N, respectively. Using the acetylene-inhibition technique, the average denitrification rate was determined to be 57 and 87 mg N m⁻² d⁻¹ (21 and 32 g N m⁻² yr⁻¹) during the most active denitrification period of 5 d after incubation for 1750 and 3500 mg NO₃ ⁻-N m⁻² of added nitrate in floodwater, respectively. The total N evolved over 11 d as N₂O + N₂ was equivalent to 436 and 921 mg N m⁻² (24.9% and 26.3%, respectively, of added N). Increasing the amount of NO₃ ⁻ applied to the overlying water increased the rate of NO₃ ⁻ loss and N₂O emission significantly. The thickness of the oxidized surface sediment layer was also influenced by the NO₃ ⁻ application to the floodwater with a significant linear correlation between nitrate addition and thickness of the oxidized layer (r = 0.9998, p = 0.01). This study indicates that wetlands receiving diverted Mississippi River water have the potential to process and remove NO₃ ⁻ in the river water, reducing the amount of NO₃ ⁻ reaching to offshore areas.

Keywords:

• NO3− loading
• Denitrification
• Acetylene blockage technique
• NO3− reduction
• N2O emission, Mississippi River
• Wetland

Acknowledgments

Funding for this study was provided in part by the Louisiana Water Resource Research Institute and the Louisiana Department of Natural Resources, Coastal Restoration Division.