Nutrient dynamics of agricultural drainage ditches in the Hoagland Ditch watershed in northern Indiana

Abstract

Drainage ditches are used extensively in the Midwest to remove standing water from poorly drained agricultural fields. However, drainage ditches are thought to expedite downstream nutrient transport and prevent denitrification within soils. Little research has currently been performed on the nutrient dynamics within Midwestern ditches, however, studies point to agricultural and drainage in the Midwest as the source of the hypoxia in the Gulf of Mexico. This study focused on seasonal nutrient dynamics within three agricultural drainage ditches with different management practices (dredging, vegetated, and unvegetated) in the Hoagland Ditch watershed in northern Indiana. Short-term nutrient injections of nitrate, phosphate, and ammonium were performed in the ditches to determine nutrient uptake length (Sw) during spring and fall conditions. Injections of phosphate and ammonium into the vegetated ditched yielded shorter Sw than in the nonvegetated ditch, indicating greater retention and processing of nutrients in the vegetated ditch. Nitrate uptake was determined to be negligible due to high ambient nitrate concentrations. To determine sediment-nutrient interactions, sediment extractions including equilibrium phosphorus concentration (EPCo), phosphorous sorption index (PSI), and exchangeable phosphorous (Ex-P) were performed. Dredging disturbed the bottom sediments and affected Ex-P, PSI, and EPCo sediment extraction experiments at Robinson Ditch. This ditch experienced increased Ex-P and decreased PSI, possibly due to ditch dredging