Huggins, David (USDA-ARS, 215 Johnson Hall, Washington State University, Pullman, WA, 99164; Phone: 509-335-3379; Fax: 509-335-3842; Email: dhuggins@wsu.edu)
D.R. Huggins *, R.E. Rossi, A.R. Kemanian, W.L. Pan
Improving N use efficiency has been identified as the primary agricultural means for decreasing nitrous oxide emissions. Quantifying field-scale variability in nitrogen use efficiency (NUE) is essential for developing management strategies that increase NUE. Our objectives were to assess field-scale variation in hard red spring wheat (HRS) NUE, yield-protein relationships and unit N requirements; and to evaluate the crop physiologic and environmental suitability of HRS production. Data from two plot-scale studies with tillage and N rate treatments were combined with one field-scale (13 ha) study to evaluate HRS performance. Measurements of grain yield, grain N, aboveground plant N, applied N, and pre- and post-harvest root-zone soil N were used to assess components of NUE including N retention, uptake and utilization efficiency. Plot-scale HRS data displayed characteristic yield-protein relations with increasing N supply and expressed a curvilinear relationship between NUE and grain protein concentration (GPC). Field application of plot-derived unit N requirements gave highly variable within-field responses of grain yield (1.3 to 3.8 Mg ha-1), GPC (106 to 179 g kg-1) and N uptake efficiency (12 to 48%). We concluded that: (1) N requirements and management strategies based on small-scale plot data cannot be extrapolated to more diverse field-scale conditions; (2) uniform field-scale applications of N are not likely to achieve field-scale goals of grain yield, GPC and NUE; (3) a large proportion of the field may not be suitable for HRS production unless site-specific N management strategies that improve NUE are devised; (4) NUE components and indices can be used to evaluate crop grain yield-GPC relations and to diagnose field areas with over or under application of N, poor N utilization or uptake efficiencies, and areas with significant N loss; and (5) devising N requirements and management strategies for HRS should use a combination of plot- and field-scale data.