Dissolved nitrogen and phosphorus in runoff from watersheds in conservation and conventional tillage

Dissolved N and P concentrations in surface and subsurface flows were measured from three corn-cropped watersheds in southwestern Iowa from 1974 through 1983. One watershed was tilled conventionally while the other two were till-planted, all on the contour. One of the till-planted watersheds was terraced, with underground pipe drains to remove excess water from the terrace channels. Each of the watersheds was fertilized at recommended N and P levels for optimum corn production. Losses of NO3-N, NH4-N, and PO4-P in surface runoff were low, representing less than 2% of the annual fertilizer application. However, NO3-N and NH4-N concentrations in surface runoff from the till-planted watersheds sometimes exceeded water quality standards. PO4-P in surface runoff from each of the watersheds always exceeded the water quality standard, and was especially high from the till-planted watersheds. NO3-N losses in subsurface flow represented more than 85% of the total NO3-N losses in both subsurface and surface flows. The highest annual NO3-N loss in subsurface flow was 74.4 kg/ha (66.4 pounds/acre) from the nonterraced, till-planted watershed in 1983. NO3-N concentrations in subsurface flow from this till-planted watershed have exceeded the water quality standard since 1977.