Nitrate Reduction in a Hydrologically Restored Bottomland Hardwood Forest in the Mississippi River Watershed, Northern Louisiana

Abstract

Core Ideas River–floodplain connectivity is low in the Mississippi River basin. Floodplain restoration led to an increase in NO3 reduction from river water. Recovery of NO3 reduction was faster than other soil microbial measures. Nitrogen loading from the Mississippi River leads to formation of water column hypoxia in the northern Gulf of Mexico every summer. Bottomland hardwood (BLH) forests located within the Mississippi River watershed could play a crucial role in reducing NO3− loading to the Gulf of Mexico. However, much river–floodplain connectivity has been muted due to building of levees and land conversion for agriculture. Restoring floodplain–river connectivity can potentially reduce river NO3−. Mollicy Farms, a 6475‐ha BLH site in northern Louisiana, is the largest floodplain reconnection and BLH reforestation project in the Mississippi River Basin. Soil properties, including microbial measures (microbial biomass N, potentially mineralizable N, and β‐glucosidase activity) and NO3− reduction rates were compared with a control site. Nitrate reduction rates in the restored site were 28% lower than in the control site (11.8 ± 3.4 vs. 16.4 ± 8.1 mg N m−2 d−1), with the potential removal of ∼48.1 Mg of NO3–N from the Ouachita River annually. Other soil microbial measures, however, were >50% lower in the restored site compared with the control site, demonstrating that NO3− reduction has responded more quickly to hydrologic reconnection. Therefore, NO3− reduction in restored floodplain wetlands may have a relatively more rapid trajectory of recovery, allowing hydrologic reconnection to be an effective tool for enhancing NO3− reduction in the Lower Mississippi alluvial valley and reducing N flux to the coastal ocean.