Nitrogen Management in Dryland Cropping Systems

Management of fertilizer N in dryland cropping systems in the semi-arid Great Plains is important to the economic and environmental sustainability of these systems. As producers shift from the traditional tilled 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 optimum yields of dryland crops than N placement in drier environments, while placement becomes more important as rainfall increases. Soil testing is an accurate method of quantifying the residual soil nitrate-N level in the root zone. However, a combination of soil testing, fertilizer N experiences of the producer, and projected N requirement (expected yield) are the best factors producers can use in determining fertilizer N rates. If soil testing occurs early in the spring/summer fallow period preceding planting, a correction to the fertilizer N recommendation should be made to account for N mineralization that occurs between soil sampling and planting. This can prevent overfertilization. Dryland systems appear to have a soil-plant N buffer capacity that prevents inorganic N accumulation at fertilizer N rates that exceed optimal N requirements to meet crop needs. Recent research has reported N buffering in the range of 21 to 76 lb N/acre per yr for annually cropped dryland wheat production systems. This means that the application of from 21 to 76 lb N/acre per yr did not result in an accumulation of inorganic N in the soil. This concept should be evaluated on additional datasets, and, if found to be applicable to a range of conditions, it could have an effect on establishing environmentally safe fertilizer N rates for dryland cropping systems. IntroductionDryland winter wheat producers in the semi-arid western Great Plains are finding economic returns marginal for winter wheat-fallow (WF) systems and compliance with conservation plans difficult. This has stimulated an interest in intensive cropping systems that are more economical and that also allow retention of crop residue on the soil surface, making conservation compliance more manageable. These intensive cropping systems integrate summer crops such as corn, proso millet, sunflowers, forages, etc., into the rotation. Typical rotations are wheat-corn-fallow (WCF), wheat-corn-millet-fallow, wheat-sunflower-fallow, etc. With this shift in cropping system management, many questions have been asked regarding fertilizer N management. This paper is a review of N management of intensive cropping systems in the semi-arid western Great Plains. Any contemporary treatment of N management must also consider environmental factors. We have limited our environmental discussion to the relationship between N management and nitrate leaching. Literature SummaryFertilizer N management of dryland cropping systems is important to the economic and environmental sustainability of the semi-arid western Great Plains. As producers shift from traditional WF to systems that include summer crops in the rotation, fertilizer N management becomes more important because yield losses as a result of underfertilization become greater. Most of the early work on minimum- and no-till systems was focused on weed control and equipment needs to operate in surface crop residue. Study DescriptionThis review covers four main areas of fertilizer N management of intensive dryland cropping systems; (i) rates, (ii) placements, (iii) soil sampling for soil testing, and (iv) environmental impacts of N management. Nitrogen management research of intensive cropping systems has primarily emerged in the last decade. We have drawn on this recent work but have also relied on data from stubble-mulch tillage WF systems for parts of this review where appropriate. Research on environmental impacts of N management of intensive dryland cropping systems is limited. A concept of “soil-plant N buffering” has recently been proposed that could have an impact on our traditional concepts of fertilizer N management and nitrate leaching. We have attempted to summarize these diverse subjects into a review that will be helpful in guiding practicing agricultural professional and producers in making proper fertilizer N management decisions. Applied QuestionAre fertilizer N requirements greater for intensive cropping systems than for the traditional WF systems? Since intensive cropping systems result in as much as a 75% increase in annualized grain production and N removal, the fertilizer N needs of these systems are greater than for the WF. Fertilizer N requirements fkom 73 to 90% greater have been reported for intensive cropping systems than for WF. What is the most important, fertilizer N rate or placement? Fertilizer N placement has more effect on dryland crop yields in wetter regions of the western Great Plains than in drier regions. Subsurface N placement is essential and can result in substantially greater yields in higher rainfall regions than surface broadcast applications. Producers, however, have greater flexibility in choosing fertilizer N placements in the drier parts of the Great Plains. In Colorado, winter wheat grain yield differences due to changes in fertilizer N placement are in the 0 to 4 bu/acre range. The two treatments that outperformed other placements were broadcast preplant urea and a treatment of 30% of the UAN banded below the seed and 70% surface banded at planting. However, no differences in corn yields were observed as a result of different fertilizer N placements. Average corn grain yield increases were as high as 40 bu/acre due to the addition of 105 lb fertilizer N/acre. Winter wheat yields were increased as much as 13 bu/acre with 90 lb N/acre. Fertilizer N response in WCF systems was greater than in the WF system because of greater residual soil N depletion in the WCF. This points out that attention to using the proper fertilizer N rate is more important in more intensive cropping systems (WCF) than in WF. How should producers determine the proper fertilizer N rate? Every state/province in the western Great Plains publishes state specific fertilizer N recommendations, all of which do not agree with neighboring states. Obviously, the “final” fertilizer N recommendation model has not been developed for the range of conditions that exists in this geographic region. A combination of soil testing, fertilizer N experience by each producer, and expected crop N requirement (expected yield) are probably the best factors producers can use in determining fertilizer N needs. However, rainfall (plant available water) is the most limiting, and least predictable, growth factor. Producers should fertilize for average years while being flexible in their N management to take advantage of above average rainfall and production conditions when they occur. Topdressing of additional fertilizer N can be profitable and should be considered if the environmental/production conditions indicate there is an opportunity for higher than average yields. RecommendationsFertilizer N use in dryland cropping systems is an important production parameter that must be carefully managed to insure optimum economical return. Nitrogen rate is more important than placement in the drier regions of the semiarid western Great Plains. As rainfall increases, however, placement becomes more important and subsurface N placements can result in greater yields than surface broadcast applications. Producers should manage their fertilizer N inputs for the average year while maintaining flexibility to respond to higher than average production years when environmental and growing conditions warrant. When spring/summer soil moisture is high and producers anticipate higher than usual yield potential, topdressing additional fertilizer N is profitable. If applications are made too late or if sufficient precipitation does not occur to move the N into the root zone, positional unavailability can occur. The authors have observed this on summer crops but not on winter wheat.