Nitrate Reduction in a Hydrologically Restored Bottomland Hardwood Forest in the Mississippi River Watershed, Northern Louisiana
2016
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.
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