Nitrogen in Subsurface Discharge from Agricultural Watersheds1

The nitrogen in subsurface discharge and surface runoff was measured from four agricultural watersheds on Missouri Valley deep loess near Treynor, lowa, from April 1969 through March 1974. The data showed that, with the agricultural management practices usad on the watersheds, the subsurface discharge of water ranged from 62 to 88% of the average annual stream flow. Nittate in subsurface discharge accounted for 84 to 95% of the total average annual soluble N discharged in stream flow. A tarraced watershed continuously cropped to corn (Zea mays L.) had reduced surface runoff, rheet-rill erosion, and associated nitrate-nitrogen discharges, but had increased subsurface discharge of water and soluble N as compared with two contoured corn watersheds. Nitrogen fertilizer applied at a high rate (448 kg ha-' year-') exceeding crop needs on the terraced and contoured corn watersheds, increased five- and threefold the average annual subsurface discharge of NO3*, respectively, as compared with a contoured watershed fertilized with N at a normal rate (168 kg ha-'year-'). To control the watershed discharge of N and subsequent pollution of meam flow from the lowa and Missouri deep loess hills requires N fertilizer application rates that do not exceed crop needs and using conservation practices that minimize soil erosion and deep percolation.