| Summary: | The Caernarvon Diversion meters Mississippi River water into coastal marshes of Breton Sound. Elevated levels of nitrogen in river water have sparked concerns that nutrient loading may affect marsh resilience and belowground biomass, as evidence from several marsh fertilization studies. These concerns resulted from observation that fresh and brackish Breton Sound marshes suffered extensive damage from Hurricane Katrina. The goal of this study is to determine the fate of nitrate (the dominant inorganic nitrogen form in the Mississippi River) in Breton Sound Estuary marshes. We hypothesized that most nitrate would be removed by denitrification and that nitrate loading would not affect belowground biomass. To test this hypothesis, a mass balance study was conducted using 15N-labeled nitrate. Twelve plant-sediment cores were collected from a brackish marsh located proximal to Delacroix, Louisiana. Six cores received dionized water (control), while another six (treatment) received 2 mg L-1 of 15N-labeled potassium nitrate twice a week for three months. A set of three control and treatment cores were destructively sampled after three months and analyzed for 15N in the above and below ground biomass, as well as the soil. The remaining three treatment cores received 20 mg L-1 of 15N-labeled potassium nitrate twice a week for one month, and a similar mass balance was determined to distinguish N removal, including denitrification, surface algae and microbial uptake and incorporation into aboveground and belowground biomass. Twelve hrs after the addition of 2 mg N L-1 water for each flooding event, nitrate levels were below detection (0.014 mg NO3- L-1. In comparison, after 24 hrs, 20 N mg L-1 water column nitrate levels were approximately zero. The 15N analyses determined 68, 65, and 74% of added labeled nitrate as unaccounted for, which represents gaseous losses. The remaining 15N was incorporated in plant and soil compartments. Labeled N data from the 2 mg N L-1 treatment and 20 mg N L-1 treatment suggests denitrification as the major removal pathway for nitrate in Caernarvon Diversion. Comparison of nitrate loss in bayou sediment and marsh soil suggests nitrate removal would be enhanced if diverted Mississippi River water flooded the marshes.
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