Hillslope variability in corn response to nitrogen linked to in-season soil moisture redistribution

Spatial variability of corn (Zea mays L.) yield within a field is often identified as the primary criterion to justify site-specific nitrogen (N) management; yet, observed yield variability may be unrelated to N supply. The objective of this study was to characterize the spatial variability in economic optimum N rate (EONR) for corn. Ten plot locations were selected in 2005 along a 300-m toposequence of a field in central Pennsylvania. At each location, two replications of six N treatments (0, 56, 112, 168, 224, and 280 kg N ha ^-1) were broadcast applied at planting as NH₄NO ₃. Soil water content (0- to 90-cm depth) was recorded approximately weekly at each location between 5 June and 2 September. The quadratic-plateau response was selected as the most appropriate grain yield response function for 9 of 10 locations and for the field-mean response. The EONR ranged from 47 to 188 kg N ha^-1 among the nine locations, whereas EONR for the mean response was 137 kg N ha^-1. At four of nine locations, observed EONR deviated from field-mean EONR by 40 to 50 kg N ha^-1. The relationship between EONR and the change in soil profile water content (0-90 cm) between 30 June and 25 July (representing the driest and wettest soil conditions early in the growing season) was the defining relationship in this study (r² = 0.92; P > F < 0.0001). Successful site-specific N management depends on an evaluation of the spatial variability in EONR and the corresponding causal factors.