Evaluating Nitrate Reduction in a Hydrologically Restored Bottomland Hardwood Forest: Is Restoration Working?.

N fertilizers are often heavily applied to cropland to increase plant production since nitrogen tends to be a limiting factor of primary productivity in terrestrial ecosystems. Concomitant with the gain of feeding a growing world population, N pollution of our aquatic systems has become pervasive. The Mississippi River has seen a dramatic increase in the nitrate load over time, helping trigger a hypoxic zone in the Northern Gulf of Mexico (GOM) every summer. Bottomland Hardwood (BLH) forests located along the Mississippi River and its tributaries can play a crucial role in reducing nitrate loads prior to waters reaching the GOM. However, almost 80% of the BLHs in the Lower Mississippi Alluvial Valley (LMAV) have been modified and the construction of 2,700 km of levees along both sides of the lower Mississippi River has limited the area of floodplain interacting with river floodwaters by 90%. Restoring floodplain wetlands and increasing their connection along rivers can reduce water nitrate by increasing opportunities for nitrate reduction. Mollicy Farms, a 6,475 hectare BLH site in northern Louisiana, is the largest floodplain reconnection and bottomland hardwood reforestation project in the U.S. and borders the Ouachita River, a tributary of the Mississippi River. To better understand the role of Mollicy Farms in reducing river nitrate, denitrification rates and soils characteristics of the site were compared to those of a natural BLH. Denitrification rates in Mollicy (10.956 ± 3.0 mg/m²/day) were, on average, lower than the natural BLH rates (12.4 ± 5.9 mg/m²/day); however, there was no significant difference between the sites. There was also no difference in OM content determined by Loss on Ignition. Mean bulk density was higher in Mollicy (0.94 ± 10 g/cm³) relative to the natural BLH (0.65  ± 0.17 g/cm³⁾, while moisture content was lower (38.8 ± 2.7 % vs 48.7±10.3%). These results suggest that restoring floodplain wetlands can be a useful tool for enhancing nitrate reduction.