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TECHNICAL REPORTS

Surface Water Quality

Phosphorus Transport Pathways to Streams in Tile-Drained Agricultural Watersheds

^a Dep. of Natural Resources and Environmental Sciences, Univ. of Illinois at Urbana-Champaign, 1102 S. Goodwin Ave., Urbana, IL 61801
^b School of Public and Environmental Affairs, Indiana Univ., 1315 E. Tenth St., Bloomington, IL 47405
^c current address, Dep. of Crop and Soil Sciences, Michigan State Univ., 572 Plant and Soil Sciences Bldg., East Lansing, MI 48825

^* Corresponding author (gentryl{at}msu.edu)

Received for publication March 9, 2006. Agriculture is a major nonpoint source of phosphorus (P) in the Midwest, but how surface runoff and tile drainage interact to affect temporal concentrations and fluxes of both dissolved and particulate P remains unclear. Our objective was to determine the dominant form of P in streams (dissolved or particulate) and identify the mode of transport of this P from fields to streams in tile-drained agricultural watersheds. We measured dissolved reactive P (DRP) and total P (TP) concentrations and loads in stream and tile water in the upper reaches of three watersheds in east-central Illinois (Embarras River, Lake Fork of the Kaskaskia River, and Big Ditch of the Sangamon River). For all 16 water year by watershed combinations examined, annual flow-weighted mean TP concentrations were >0.1 mg L^–1, and seven water year by watershed combinations exceeded 0.2 mg L^–1. Concentrations of DRP and particulate P (PP) increased with stream discharge; however, particulate P was the dominant form during overland runoff events, which greatly affected annual TP loads. Concentrations of DRP and PP in tiles increased with discharge, indicating tiles were a source of P to streams. Across watersheds, the greatest DRP concentrations (as high as 1.25 mg L^–1) were associated with a precipitation event that followed widespread application of P fertilizer on frozen soils. Although eliminating this practice would reduce the potential for overland runoff of P, soil erosion and tile drainage would continue to be important transport pathways of P to streams in east-central Illinois.

Abbreviations: BDO, Big Ditch of the Sangamon River • DAP, di-ammonium phosphate • DRP, dissolved reactive phosphorus • EMC, Embarras River • LFK, Lake Fork of the Kaskaskia River • P, phosphorus • PP, particulate phosphorus • TP, total phosphorus

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