Nitrate leaching in irrigated corn and soybean in a semi-arid climate

Nitrate-nitrogen leached from the root zone of land in intensive corn production is a major groundwater contaminant in some of the intensively irrigated regions of the wester Cornbelt, including central and western Nebraska. T obtain a clearer understanding of the amount and timing of nitrate leaching losses from irrigated crops, 14 monolithic percolation lysimeters were installed in 1989-1990 in sprinkler irrigated plots at the University of Nebraska's West Central Research and Extension Center near North Platte, Nebraska. The lysimeters were used to provide a direct measure of leachate depth from continuous corn and a corn-soybean rotation. Both cropping systems were sprinkler irrigated and used current best management practices (BMPs) in the region for water and nitrogen management. Leachate was collected from 1990 through 1998 and analyzed for nitrate-N concentration. Results for the period 1993-1998 are reported here. In the semi-arid climate of West-Central Nebraska, the interaction of rainfall patterns with the period of active uptake of water by crops played a major role in defining leaching patterns. Careful irrigation scheduling did not eliminate leaching during the growing season. There was no significant difference in drainage depth between continuous corn and the corn-soybean rotation. The average drainage depth among the lysimeters was 218 mm yr-1. This was more than expected, and in part resulted from above normal precipitation during several years of the study. No water quality benefit was found for the corn-soybean rotation as compared to continuous corn. Nitrate-N concentration in the leachate from continuous corn averaged 24 mg L-l, while that from the corn-soybean rotation averaged 42 mg L-1. Total yearly nitrate leaching loss averaged 52 kg ha-1 for continuous corn and 91 kg ha-1 for the rotation. This represents the equivalent of 27% and 105% of the amount of N fertilizer applied over the six years of study. In calculating N fertilizer needs for corn in Nebraska, the recommended legume N credit of 50 kg ha-1 for a preceding crop of soybean may be too low under irrigated production.