NITROGEN USE EFFICIENCY OF WINTER-WHEAT AMONG LANDSCAPE POSITIONS

Spatially variable N fertilizer application may reduce environmental i mpacts and increase the economic return of N fertilization. To achieve these benefits, N recommendations must account for within-held differ ences in the amount of N required to produce a unit of yield (unit N r equirement, UNR). Component analysis was used to determine the sources of variation in the UNRs of Hinter wheat (Triticum aestivum L.) among landscape positions. The UNRs were divided into two components, N upt ake 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 appli cations) established on footslope, south backslope, shoulder, and nort h backslope positions of two farms for 2 yr. Variation in the UNR amon g the 16 landscape positions studied was most associated with differen ces in N uptake efficiency (r = -0.80), although N utilization efficie ncy (r = -0.62) also contributed to the variation. Nitrogen uptake eff iciency among landscape positions declined as more fertilizer was requ ired 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%) and high N loss percentages (>50%) indicate the need to reduce N losses an d lower UNRs, particularly on north-facing backslopes susceptible to N leaching.