Abstract
The Davis Pond freshwater diversion discharges nutrient-rich Mississippi River water to a 3,760 ha receiving marsh in upper Barataria Basin, LA. Excess nitrate in the Mississippi River has been linked to algal blooms and hypoxia in the Gulf of Mexico with potential to negatively impact Barataria Basin. We hypothesized that 1) soil denitrification enzyme activity (DEA) will increase with higher surface water nitrate concentrations, and 2) the spatial distribution of DEA in Davis Pond marsh will provide information about the extent nitrate loading at a specific discharge rate. Intact soil cores collected from the marsh received a continuous flow of nitrate solution (0.0, 0.5, 1.0, or 2.0 mg NO3-N l-1) for a period of 7, 20, or 45 days. Overall, DEA for the 1.0 mg NO3-N l-1 was significantly higher than the control treatment (P < 0.05). A strong positive correlation between DEA and surface water nitrate in the 0-5 cm (P < 0.05) and 5-10 cm (P < 0.001) soil horizons was observed on day 20. However, the correlation between DEA and nitrate was not significant on days 7 and 45. Measureable DEA was observed in the 0.0 mg NO3-N l-1 on all days, indicating the contribution of internal biochemical N cycling to DEA in organic wetland soils. Approximately 92% of all DEA was observed in the top 5 cm of soil, 7% occurred at 5-10 cm, and <1% below 10 cm. DEA was also quantified for 88 randomly distributed soil cores in Davis Pond marsh collected May - July, 2007. At a mean discharge rate of 39 m3 s-1, high rates of DEA (0.41 to 2.10 mg N2O-N kg-1 h-1) occurred in a 715 ha area proximal to the diversion inflow, while background rates (0 to 0.30 mg N2O-N kg-1 h-1) were observed outside this area. The 715 ha area contained > 80% of all the DEA observed in Davis Pond marsh, yet encompassed only 19% of the total marsh area. The area of elevated DEA included the highest observed surface water nitrate concentrations, suggesting DEA is a potential indicator of nitrate loading.
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