Denitrification in a South Louisiana wetland forest receiving treated sewage effluent
- Cite this article as:
- Boustany, R.G., Crozier, C.R., Rybczyk, J.M. et al. Wetlands Ecol Manage (1996) 4: 273. doi:10.1007/BF02150540
Although denitrification has the potential to reduce nitrate (NO3−) pollution of surface waters, the quantification of denitrification rates is complex because it requires differentiation from other mechanisms and is highly variable in both space and time. This study first measured potential denitrification rates at a wetland forest site in south Louisiana before receipt of secondary wastewater effluent, and then, following 30 months of effluent application, landscape gradients of dissolved nitrate (NO3−) and nitrous oxide (N2O) were measured. A computer model was developed to quantify N transformations. Floodwater NO3− and N2O concentrations were higher in the forest receiving effluent than in the adjacent control forest. Denitrification rates of NO3−-amended soil cores ranged from 0.03–0.45 g N m−2 d−1 with an overall mean of 0.10 g N m−2 d−1. Effluent N is being applied at a rate of approximately 0.034 g N m−2 d−1, with approximately 95% disappearing along a 1 km transect. In the treatment forest, floodwater NO3− concentrations decreased from 1000 μM at the inflow point to 50 μM along the 1 km transect. Nitrous oxide concentrations increased from 0.25 μM to 1.2 μM within the first 100 m, but decreased to 0.1 μM over the next 900 m. The initial increase in N2O was presumably a result ofin situ denitrification. Model analyses indicated that denitrification was directly associated with nitrification and was limited by the availability of NO3− produced by nitrification. Due to different redox potential optima, coupling of nitrification and denitrification was a function of a balance of environmental conditions that was moderately favorable to both processes. N removal efficiency was largely dependent on the proportion of effluent NH4+ to NO3−. When NH4+/NO3− was ≤1, average N removal efficiency ranged from 95–100%, but ratios that were >1 reduced average efficiencies to as low as 57%. Actual effluent NH4+/NO3− loading ratios at this site are approximately 0.2 and are consistently <1.