Developing Nitrogen Use Efficiency Performance Criteria to Optimize Wheat Yield and Evaluate Site-Specific Management.

Increased nitrogen (N) fertilizer additions to modern agricultural cropping systems will be necessary to feed a growing world population. However, greater nitrogen use efficiency (NUE) is required if agroecosystems are to continue to provide certain ecosystem services (e.g., greenhouse emission reductions or mitigation). Site-specific N fertilizer management has been reported as an important strategy to increase NUE in modern cropping systems. However, site-specific management requires knowledge of the spatial variability in soil properties, crop physiological response, and drivers of N cycling. The Palouse region of eastern Washington, USA is characterized by complex soil fertility and crop productivity patterns but cropping systems are typically managed uniformly. The overall research goals were to investigate the role of variable rate N and seeding of winter wheat (Triticum aestivum) for optimizing yield-water-NUE relationships across heterogeneous landscapes. Data from field plot studies conducted at the Cook Agronomy Farm, near Pullman, WA during the 2010, 2011 and 2012 winter wheat harvest years were used to develop NUE based performance classes. The field experiments were from a randomized complete block split plot N rate x seeding rate trial with nitrogen fertilizer rate (0, 40, 80, 120, 160 kg ha^-1) as the main plot and seeding rate (80, 165, 250, 335 seeds m^-2) as the subplot imposed across three landscape positions each year. Soil and crop physiology based nitrogen use efficiency components and indices were determined according to Huggins and Pan (1993). Discriminant analysis was used to evaluate NUE components and indices that could be used to not only predict but also evaluate soil and crop performance to aid in site-specific N fertilizer and seeding rate management decisions for the region.