NITROGEN MANAGEMENT IN DRYLAND CROPPING SYSTEMS

Management of fertilizer N in dryland cropping systems in the semi-ari d Great Plains is important to the economic and environmental sustaina bility of these systems. As producers shift from the traditional tille d winter wheat (Triticum aestivium L.)-fallow (WF) cropping systems to those that include summer crops in the rotation, N management becomes more important because yield losses as a result of underfertilization become greater. Fertilizer N rate is more important in obtaining opti mum yields of dryland crops than N placement in drier environments, wh ile placement becomes more important as rainfall increases. Soil testi ng is an accurate method of quantifying the residual soil nitrate-N le vel in the root zone. However, a combination of soil testing, fertiliz er N experiences of the producer, and projected N requirement (expecte d yield) are the best factors producers can use in determining fertili zer N rates. If soil testing occurs early in the spring/summer fallow period preceding planting, a correction to the fertilizer N recommenda tion should be made to account for N mineralization that occurs betwee n soil sampling and planting. This can prevent overfertilization. Dryl and systems appear to have a soil-plant N buffer capacity that prevent s inorganic N accumulation at fertilizer N rates that exceed optimal N requirements to meet crop needs. Recent research has reported N buffe ring in the range of 21 to 76 lb N/acre per yr for annually cropped dr yland wheat production systems. This means that the application of fro m 21 to 76 lb N/acre per yr did not result in an accumulation of inorg anic N in the soil. This concept should be evaluated on additional dat asets, and, if found to be applicable to a range of conditions, it cou ld have an effect on establishing environmentally safe fertilizer N ra tes for dryland cropping systems.