Fate of Nitrate in Vegetated Brackish Coastal Marsh
- Christine M. VanZomerena,
- John R. White *a and
- Ronald D. DeLaunea
The Caernarvon Diversion meters Mississippi River water into coastal marshes of Breton Sound, Louisiana (29°51ʹ 40.15ʺ N, 89°54ʹ43.62ʺ W). Elevated levels of N in river water have sparked concerns that nutrient-loading may affect marsh resilience and belowground biomass, as evidence from several marsh fertilization studies suggests. These concerns have resulted from casual observations that fresh and brackish Breton Sound marshes, closest to the Mississippi River levee suffered extensive damage from Hurricane Katrina. The goal of this study was to determine the fate of nitrate (the dominant inorganic N form in the Mississippi River) in Breton Sound Estuary marshes. We hypothesized that the majority of the nitrate will be removed by denitrification and that nitrate-loading will not affect belowground biomass over several months of loading. To test this hypothesis, a mass balance study was designed using 15N-labeled nitrate. Twelve plant-sediment cores were collected from a brackish marsh and six cores received deionized water (control), while another six (treatment) received 2 mg L−1 of 15N-labeled potassium nitrate twice a week for 3 mo. A set of three control and treatment cores were destructively sampled after 3 mo and analyzed for 15N in the aboveground and belowground biomass and the soil. The N isotopic label allowed for a mass balance to distinguish N removal pathways, including denitrification, surface algae uptake, soil microbial uptake and incorporation into aboveground and belowground biomass of the macrophytes. Twelve hours after the addition of the 2 mg N L−1 water, nitrate levels were typically below detection. Approximately 64% of all added labeled nitrate was unaccounted for which suggests gaseous loss. The remaining 15N was incorporated in plant and soil compartments, the majority being the aboveground component. There were no significant differences in belowground biomass production between the nitrate loaded and the control cores after 3 mo.Please view the pdf by using the Full Text (PDF) link under 'View' to the left.
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