NITRATE LEACHING IN IRRIGATED CORN AND SOYBEAN IN A SEMI-ARID CLIMATE

Abstract

Nitrate-nitrogen leached from the root zone of land in intensive corn production is a major groundwatercontaminant in some of the intensively irrigated regions of the western Cornbelt, including central and western Nebraska.To obtain a clearer understanding of the amount and timing of nitrate leaching losses from irrigated crops, 14 monolithicpercolation lysimeters were installed in 1989-1990 in sprinkler irrigated plots at the University of Nebraskas WestCentral Research and Extension Center near North Platte, Nebraska. The lysimeters were used to provide a directmeasure of leachate depth from continuous corn and a corn-soybean rotation. Both cropping systems were sprinklerirrigated 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 theperiod of active uptake of water by crops played a major role in defining leaching patterns. Careful irrigation schedulingdid not eliminate leaching during the growing season. There was no significant difference in drainage depth betweencontinuous corn and the corn-soybean rotation. The average drainage depth among the lysimeters was 218 mm yr1. Thiswas more than expected, and in part resulted from above normal precipitation during several years of the study. No waterquality benefit was found for the corn-soybean rotation as compared to continuous corn. Nitrate-N concentration in theleachate from continuous corn averaged 24 mg L1, while that from the corn-soybean rotation averaged 42 mg L1. Totalyearly nitrate leaching loss averaged 52 kg ha1 for continuous corn and 91 kg ha1 for the rotation. This represents theequivalent of 27% and 105% of the amount of N fertilizer applied over the six years of study. In calculating N fertilizerneeds for corn in Nebraska, the recommended legume N credit of 50 kg ha 1 for a preceding crop of soybean may be toolow under irrigated production.