Mechanisms of nutrient attenuation in a subsurface flow riparian wetland

Riparian wetlands are transition zones between terrestrial and aquatic environments that have the potential to serve as nutrient filters for surface and ground water due to their topographic location. We investigated a riparian wetland that had been receiving intermittent inputs of NO3- and PO43- during storm runoff events to determine the mechanisms of nutrient attenuation in the wetland soils. Few studies have shown whether infrequent pulses of NO3- are sufficient to maintain substantial denitrifying communities. Denitrification rates were highest at the upstream side of the wetland where nutrient-rich runoff first enters the wetland (17-58 mug N2O-N kg soil(-1) h(-1)) and decreased further into the wetland. Carbon limitation for denitrification was minor in the wetland soils. Samples not amended with dextrose had 75% of the denitrification rate of samples with excess dextrose C. Phosphate sorption isotherms suggested that the wetland soils had a high capacity for P retention. The calculated soil PO43- concentration that would yield an equilibrium aqueous PO43- concentration of 0.05 mg P L-1 was found to be 100 times greater than the soil PO43- concentration at the time of sampling. This indicated that the wetland could retain a large additional mass of PO43- without increasing the dissolved PO43- concentrations above USEPA recommended levels for lentic waters. These results demonstrated that denitrification can be substantial in systems receiving pulsed NO; inputs and that sorption could account for extensive PO43- attenuation observed at this site.