Evaluation of Nitrate Nitrogen Fluxes from a Tile-Drained Watershed in Central Iowa

Abstract

Nitrate N fluxes from tile-drained watersheds have been implicated in water quality studies of the Mississippi River basin, but actual NO3–N loads from small watersheds during long periods are poorly documented. We evaluated discharge and NO3–N fluxes passing the outlet of an Iowa watershed (5134 ha) and two of its tile-drained subbasins (493 and 863 ha) from mid-1992 through 2000. The cumulative NO3–N load from the catchment was 168 kg ha−1, and 176 and 229 kg ha−1 from the subbasins. The outlet had greater total discharge (1831 mm) and smaller flow-weighted mean NO3–N concentration (9.2 mg L−1) than the subbasins, while the larger subbasin had greater discharge (1712 vs. 1559 mm) and mean NO3–N concentration (13.4 vs. 11.3 mg L−1) than the smaller subbasin. Concentrations exceeding 10 mg L−1 were common, but least frequent at the outlet. Nitrate N was generally not diluted by large flows, except during 1993 flooding. The outlet showed smaller NO3–N concentrations at low flows. Relationships between discharge and NO3–N flux showed log–log slopes near 1.0 for the subbasins, and 1.2 for the outlet, considering autocorrelation and measurement-error effects. We estimated denitrification of subbasin NO3–N fluxes in a hypothetical wetland using published data. Assuming that temperature and NO3–N supply could limit denitrification, then about 20% of the NO3–N would have been denitrified by a wetland constructed to meet USDA-approved criteria. The low efficiency results from the seasonal timing and NO3–N content of large flows. Therefore, agricultural and wetland best management practices (BMPs) are needed to achieve water quality goals in tile-drained watersheds.