USING NATURAL N ISOTOPES TO IDENTIFY NITRATE REMOVAL MECHANISMS IN CONSTRUCTED WETLANDS RECEIVING AGRICULTURAL TILE DRAINAGE

Constructed wetlands can be effective in removing nitrate from agricultural tile drainage. However, there is little research that identifies or quantifies removal mechanisms (primarily denitrification and uptake). We are conducting an investigation on the Gully wetland at the Franklin Demonstration Farm in Lexington, IL that is operated by The Nature Conservancy. The Gully wetlands consists of 3 sequential cells, with the area of each cell representing 3% of the tile drainage area. The natural N isotopic composition of nitrate, macrophytes, and sediments was analyzed in order to calculate nitrification, denitrification, and uptake rates using a N isotopic mass balance equation. The N isotopic composition was shown to be higher in the plants (duckweed and algae: 2.4-9.5‰, cattails: 4.6-7.9‰, arrowheads: 7.1-8.1‰) than the sediments (2.3-7.2‰). The N isotopic composition of N found in nitrate is ranged between 2.0-14.8‰ and progressively increase through each cell. Data collected from four rounds over a 1 year period indicate that denitrification is the dominant mechanism for removing nitrate, because the N isotopic ratios increase as nitrate concentrations decrease through the three cells according a Rayleigh fractionation model. The wetlands remain at least partially effective in removing nitrate by denitrification even in winter when ice covers the surface of the wetlands as indicated decreasing nitrate concentrations and increase N isotopic composition in samples collected in February. The isotopic data also show evidence of dilution by groundwater input and nitrification within the wetlands. Dilution can be seen in the data collected from cell three where groundwater with low nitrate causes a decrease in nitrate concentration without a change in the N isotopic composition. Nitrification can be seen in samples that fall to the right of a Rayleigh curve indicating an increase in the nitrate concentration with little change in the N isotopic composition. The nitrate added by nitrification is not expected to change the overall N isotopic composition because plant matter and sediment are similar in isotopic composition to dissolved nitrate.