Corn Yield Response to Nitrogen and Soil Water Content Variability along a Hillslope.

A better understanding of the interaction of corn response to N fertilizer and soil water availability should improve N fertilizer use efficiency in corn (Zea mays L.) through spatially variable N applications. The objective of this study was to determine whether the within-field spatial variability in the economic optimum N rate (EONR) for corn could be attributed to the spatial and temporal variability in soil water availability. Ten plot locations were selected each year (2005-2007) along a 300-m toposequence in the same field in central Pennsylvania. At each location, increasing N rates were broadcast applied at planting as NH₄NO₃ or urea. Soil water content (0-90-cm depth) was recorded weekly at each location between 15 June and 15 August. Grain yield was determined at harvest. The EONR was determined at each location, and ranged from 47 – 188, 118 – 186, and 117 – 280 kg N ha^-1 in 2005, 06, and 07, respectively. Total June rainfall was 30, 128, and 83 mm in 2005, 06, and 07, respectively; consequently, mean soil water content (0-90 cm) on 23 June varied among year (25.5, 35.2, and 31.1 cm, respectively). In 2005 (driest year), EONR among the ten locations was positively related (r² = 0.88) to increased soil water availability during July. In 2006 (wettest year), EONR was unrelated to changes in soil water availability. In 2007, mean EONR was the greatest (204 kg N ha^-1, compared to 140 and 116 kg N ha^-1 in 2006 and 05, respectively), but EONR among locations was negatively correlated (r² = 0.78) to increased soil water availability during July. Soil water content during the early growing season varied considerably among years, which corresponded with differences in the EONR response to increased soil water availability.