NITROGEN LEACHING LOSSES FROM IRRIGATED ORCHARDGRASS ON SANDY SOILS

Irrigated pasture may provide an alternative forage source for livestock producers in Nebraska's Sandhills. As with any heavily fertilized crop, there is a significant potential for groundwater contamination by nitrate nitrogen (NO3-N) leached from the crop root zone. A 3-yr experiment was conducted in West Central Nebraska to evaluate wate and nitrate (NO3-) leaching losses form irrigated orchardgrass (Dactylis glomerata L.) seeded on a Valentine fine sand (Typic Ustipsamments). Three irrigation levels (slight deficit, slight excess, and excess) and four N amounts (0, 112, 224, and 336 kg ha-1) were applied. Deep percolation and NO3- leaching were measured with Duke-Haise trough-type extractors. All irrigation treatments had more deep percolation than expected, as the crop coefficient used for irrigation scheduling proved to be too large. The deficit irrigation treatment served as a "close management" treatment since percolation was minimal and yield was not reduced by water stress. During the first 2 yr of operation, in-season percolation losses averaged 7, 28, and 47 cm yr-1 for low, medium, and high irrigation levels, respectively. Total in-season NO3-N leaching loss for the same period ranged from 6 to 228 kg ha-1, depending on N and irrigation amount. Winter and early spring N leaching losses, as estimated by soil sampling, were a significant part of total N loss. Under reduced inseason drainage and an N rate commensurate with 80 to 85% of maximum production, a minimum annual N leaching loss of 35 kg ha-1 can be expected. Greater losses are probable under average water and N applications and average management skills.