Nitrogen Management for Improved Nitrogen Use Efficiency in Dryland Cropping Systems.

An adequate N supply is required for optimum crop yield and profitability, but N use efficiency and profitability of production can decline if supply is not matched to crop demand, both in terms of amount and timing.  Nitrogen is lost from fields by leaching, denitrification, and ammonia volatilization, with risk increasing with amount of N available in excess of crop demand when conditions are conducive to loss. An effective management plan for dryland systems requires use of the right source, rate, time and place of application for the environmental conditions present, combined with healthy, vigorous crop growth. Crop N demand, soil N supply, and N losses to the air and water all vary greatly across years, fields, and topography in response to seasonal weather conditions, soil physical and chemical characteristics, crop genetics and agronomic practices. In dry environments, denitrification and leaching losses may be small, so residual N may persist though several cropping cycles, allowing the balance between N supply and crop demand to be viewed over multiple years.  In contrast, under wetter conditions, persistence of nitrate is limited, as it may be lost rapidly from the soil-crop system when soils are warm and moist. To maximize N used by the crop and minimize N moving to the air or water, it is important to avoid accumulation of nitrate in the soil solution during periods when crop uptake is low and nitrate may be lost from the system.  An understanding of the relationship among environmental conditions, patterns of N accumulation by the crop, and soil N dynamics must be used to develop nutrient management systems that optimize crop yield and quality and economics of production, while minimizing N movement to the air and water per unit of crop production.