Prediction of nitrogen and phosphorus losses as related to agricultural drainage system design

The purpose of this study was to determine the effects of differing degrees of improved agricultural drainage on the long-term efflux of N and P from poorly drained soils common to the North Carolina Coastal Plain. The effect of using controlled drainage on efflux was also investigated. Computer simulations were used to predict the losses from 6 soils for a 20-year period.The simulations indicate that both drainage system design and management can have significant effects upon N and P efflux in drainage water. Drainage systems designed to give good subsurface drainage lost 17–35 kg ha⁻¹ year⁻¹ more NO₃-N than systems with poor subsurface drainage. Good subsurface drainage decreased total P lost by 0.2-0.4 kg ha⁻¹ year⁻¹ on the mineral soils studied.The increase in N lost because of installing a good subsurface drainage system can partially be offset by utilizing controlled drainage. Under the conditions simulated in this study, controlled drainage reduced the nitrate efflux by as much as 34%, but the reduction varies with soil and management conditions. Controlled drainage does, however, result in a small increase in P efflux under the conditions simulated.