The Effect of Restoration on Nitrate Reduction and Biogeochemical Functioning in Louisiana Wetlands: Bottomland Hardwood Forests and Deltaic Sediments
Nitrogen loading in the Mississippi River from increased N fertilization of agricultural land helps to trigger an area of hypoxic water in the northern Gulf of Mexico (GOM) every summer. Louisiana wetlands can play a vital role in removing nitrate from river waters prior to discharge in the GOM. However, Louisiana’s wetlands have experienced significant losses in recent years. Efforts to restore wetlands include reconnecting floodplain wetlands to rivers and utilizing river diversions to re-introduce sediment to coastal wetlands. Increasing wetland connection to rivers can reduce water nitrate by expanding opportunities for nitrate reduction. I examined soil physicochemical properties, microbial characteristics, and nitrate reduction rates in a hydrologically restored bottomland hardwood forest adjacent to the Ouachita River in Louisiana. Nitrate reduction rates in the restored site were only 28% lower than those in a natural site (11.8 ± 3.4 vs 16.4 ± 8.1 mg N m-2 day-1) (P<0.1), removing approximately 48.1 metric tons of nitrate from the Ouachita River annually. Results suggest that restoring floodplain wetlands can be useful for enhancing nitrate reduction in river floodwaters, improving water quality while reducing the areal extent of hypoxia in the northern GOM. I also investigated nitrate reduction in turbulent surface water conditions resulting from sediment diversions. There is a paucity of data on nitrate loss in areas of diversions where turbulent conditions impart significant shear stress on the sediment surface, suspending fine grained sediments. Sediment cores were collected from Wax Lake Delta in Louisiana and subjected to shear stresses using a flow-through erosional microcosm system for 24 hrs. Nitrate reduction rates were determined under high, medium, and zero shear stress conditions of 0.45, 0.2, and 0 Pa, respectively. Nitrate reduction rates under high, low, and zero shear stresses were 303 ± 65.6, 186 ± 55.1, and 18.7 ± 20.2 mg N m-2 day-1, respectively (P<0.001). Rates of nitrate reduction increased significantly with an increase in shear stress, indicating that turbulent flow conditions from river diversions can significantly increase nitrate reduction rates. Results from this research can help inform modelers in predicting potential nutrient impacts of river diversions on coastal receiving basins.