Nitrogen Cycling In Vegetated and Nonvegetated Coastal Brackish Marshes Receiving Mississippi River Diverted Nitrate.

The Caernarvon Diversion meters Mississippi River water into the brackish and salt marshes of Breton Sound during spring flooding.  Elevated levels of nitrate delivered through the Caernarvon Diversion have recently sparked concerns that nitrate loading is affecting plant resilience, in particular, potentially affecting belowground biomass. This concern resulted from the observation that Breton Sound marshes suffered damage from Hurricane Katrina. Our hypothesis is that the nitrate from the Mississippi River is primarily removed by denitrification. To test this hypothesis, 12 plant-sediment cores were collected from a brackish marsh located proximal to Delacroix, Louisiana. Six received dionized water (control), while another six (treatment) received 2 mg/L 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 aboveground and belowground biomass as well as denitrifying enzyme activity.  The remaining three treatment cores received 20 mg/L of ¹⁵N-labeled potassium nitrate twice a week for one month and the fate of the ¹⁵N- nitrate was used to distinguish different removal pathways, including denitrification, surface algae uptake, incorporation into aboveground biomass, and incorporation into belowground biomass. Results show that just 24 hours after the addition of 2 mg/L nitrate, levels were below detection, which suggests denitrification is the major removal pathways. The largest amount of labeled N from the mass balance analysis was unaccounted for at 64%, which is gaseous loss.  The largest plant pool of ¹⁵N was found in the aboveground biomass at 24% with very little present in the belowground biomass at ~ 3%.  Results suggest that the addition of nitrate-N from the river is a very small portion of the plant total N budget at <2%.