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Spatially variable N fertilizer application may reduce environmental impacts and increase the economic return of N fertilization. To achieve these benefits, N recommendations must account for within-field differences in the amount of N required to produce a unit of yield (unit N requirement, UNR). Component analysis was used to determine the sources of variation in the UNRs of winter wheat (Triticum aestivum L.) among landscape positions. The UNRs were divided into two components, N uptake efficiency (plant N/N supply) and N utilization efficiency (grain yield/plant N) observed in N rate trials (0-140 kg N ha-1 fall applications) established on footslope, south backslope, shoulder, and north backslope positions of two farms for 2 yr. Variation in the UNR among the 16 landscape positions studied was most associated with differences in N uptake efficiency (r = -0.80), although N utilization efficiency (r = -0.62) also contributed to the variation. Nitrogen uptake efficiency among landscape positions declined as more fertilizer was required to reach optimum yield (r = -0.56) due to low N fertilizer uptake efficiencies (delta plant N/delta N fertilizer). Nitrogen fertilizer uptake efficiency was related to the degree of apparent N loss (r = -0.87), indicating that N availability limited N uptake efficiency among landscapes. Overall, low N fertilizer uptake efficiencies ( 50%) indicate the need to reduce N losses and lower UNRs, particularly on north-facing back-slopes susceptible to N leaching. |
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