Variability in Corn Response to Nitrogen – Influence of Soil Moisture.

Corn yield response to N fertilizer applications has been observed to co-vary with soil moisture as related to topographic position. The objective of this study was to evaluate the variability in corn response to N under relatively wet and dry soil moisture regimes as determined by topography. A field experiment was conducted in 2007-2008 at ten sites in central PA using a randomized complete block design with increasing N rates applied at V6-V8 (N_V6) as the treatment factor. Field sites were selected from different landscape positions representing variable soil moisture regimes.  Soil and plant biomass samples were obtained at planting, V6, R1 and physiological maturity. Soil water content (0- to 90-cm depth) was recorded approximately bi-weekly at each location between V4 and black layer. A significant grain yield response to N was observed at seven of the ten sites. Overall, grain yield levels were higher in 2008 compared to 2007. At the medium soil moisture regime, the optimum N rate was higher in 2008 with a corresponding increase in yields, while the same trend was not observed under dry and wet soil moisture regimes. Measured profile level soil moisture content, soil NO₃-N and total N uptake data during the growing season will be used to explain the variability in corn response to N under varying moisture levels. A better understanding of the soil moisture redistribution to the depth of root zone in relation to landscape position should help in understanding the influence of water stress on N utilization by corn and thereby improve estimates of in-season N requirements.