Subsurface nutrient export from a cropped field to an agricultural stream: Implications for targeting edge-of-field practices

Abstract

It is understood that the major transport pathways for the soluble nutrients nitrate (NO3-N) and orthophosphorus (OP) from cropped fields to streams in the U.S. Cornbelt are tile drainage and groundwater seepage. The relative contribution of each, however, has not been well quantified and can vary between fields and watersheds. In this study, we used intermittent grab sample water quality monitoring and tile flow measurements, and the groundwater model MODFLOW to source dissolved nutrients from a cropped field to a low-order stream in the intensively-drained and cropped Des Moines Lobe landform of north central Iowa. Based on monitoring of eight tile outlets, nine groundwater wells and the receiving stream over a two-year period, nutrient loads from tiles were found to contribute approximately 98 % of the nitrate load to Hardin Creek. The loading pattern for OP was also dominated by discharge from constructed drainage, with 99.7 % sourced back to field tiles. Results from the farm field fit within the scaling pattern observed within the Des Moines Lobe region of Iowa showing that water yields and NO3-N loads are dominated by tile drainage at the field and local watershed scale. Loads dominated by tile flows suggest edge-of-field interception and treatment of tile water for mitigation of stream nutrients. Study results are consistent with local and regional assessments showing tile drainage to be an important pathway for both water and nutrients in the stream network of the US Cornbelt.