Continual Evaluation of Ditch Sediment and Phosphorus Dynamics in tile fed Drainage Ditches in the St. Joseph Watershed

Shalamar Armstrong and D. R. Smith

USDA-ARS, National Soil Erosion Research Laboratory, 275 South Russell St., West Lafayette, IN 47907. Shalamar Armstrong corresponding author. 
Email: Sarmstro@purdue.edu

 Abstract

Persistent phosphorus (P) amendments to agricultural land continue to be the leading cause of eutrophication and surface water degradation. Therefore, understanding P transport and interaction with agricultural drainage ditch sediments is vital for the development of effective predictive models and best management practices to reduce further P contamination. Our objective was to evaluate the P concentrations and equilibrium P concentration (EPC₀) of tile drainage ditch sediments.  Surface sediments were collected from a total of 7 sites within 3 tile-fed drainage ditches (A, B, and C) that drain approximately 300 to 4,300 ha.  The sediments were sieved, analyzed for labile P and % organic matter (O.M.). The EPC₀  was determined after performing a P isotherm. The average exchangeable P was the greatest in sediments from the smallest watersheds within ditches C (2.85 mg kg ^–1) and B (1.87 mg kg ^–1), which also had the highest clay and % O.M.  Furthermore, the EPC₀ values for sediments from the small watersheds within ditches B and C demonstrated the potential to act as a P source two out of five sampling dates.  Sediments from the large watershed within ditches A and B that had a lower clay content also demonstrated the potential to act as a source, likely due to the % O.M.  The results from this study have indicated that sediment characteristics such as particle size and % O.M. are vital influences on the sink and source relationship between ditch sediments and water, buffering capacity, and P behavior with in agricultural catchments.