Nutrient Content at the Sediment-Water Interface of Tile-Fed Agricultural Drainage Ditches

Abstract: Extensive network of tile drains present in the Midwest USA accelerate losses of nutrients to receiving ditches, rivers and eventually to the Gulf of Mexico. Nutrient inputs from agricultural watersheds and their role in affecting water quality have received increased attention recently; however, benthic sediment-nutrient interactions in tile-fed drainage ditches is still a matter of active research in consideration to nutrient discharge from tile drains. In this study, phosphorus (P) and nitrogen (N) contents and variability of nutrient retention ability of benthic sediments upstream and downstream from tile drain outlets were evaluated in managed agricultural drainage ditches in Indiana. Sediment samples were collected every three months upstream and downstream from selected tile drains in three ditches in northwest Indiana. Sediment equilibrium P concentrations (EPC0) were measured to examine P adsorption-desorption and equilibrium characteristics of benthic sediments in the ditches. P sorption index (PSI), exchangeable P (ExP), and exchangeable NH₄⁺-N (ExN) were measured to evaluate nutrient retention ability and readily available nutrient content of benthic sediments. Results indicated a dynamic interaction between benthic sediment and overlying water column where sediments were acting as a sink or a source of P. There were no differences in nutrient retention ability between sediments collected upstream and sediments collected downstream from the selected tile drains. While the data, except for ExN, was comparable to reported values by previous studies in Indiana’s drainage ditches, there was no particular seasonal pattern in the content of exchangeable nutrient content in sediments at all three sites. This study also suggested that nutrient uptake by benthic sediments in these drainage ditches is not always efficient; therefore watershed management should focus on minimizing the delivery of nutrients into ditches while maintaining their drainage functionality.