Within-field variability in optimum nitrogen rate for corn linked to soil moisture availability

Understanding the interaction between yield response to N and other growth-limiting factors is essential to improving spatially dependent N fertilizer applications. Our objective was to evaluate the impact of soil moisture variability on the economically optimum N rate (EONR) for corn (Zea mays L.). Corn grain yield response to N was determined at each of 10 locations along a hillslope in central Pennsylvania (2005-2007). Volumetric soil water content (θ_p, 0-90-cm depdi) was also determined at each location approximately weekly between mid-June and August. The mean EONR (n = 10) was 117, 140, and 169 kg N ha^-1 in 2005, 2006, and 2007, respectively, while the range in EONR among locations each year was 147, 69, and 146 kg N ha^-1, respectively. The mean θ_p (n = 10) was between 15 and 25 cm in 2005, 23 to 37 cm in 2006, and 21 to 27 cm in 2007. While there was greater temporal change in the mean θ_p in 2006, the smaller variability among locations during the drier growing seasons of 2005 and 2007 was more relevant to differences in EONR. The EONR in 2005 and 2007 was strongly related (r² = 0.74 and 0.71, respectively) to the change in θ_p at each location as determined during a defining drying and wetting cycle in June and July. During the relatively wetter 2006 growing season, the EONR was not related to changes in θ_p. Greater water availability during drier growing seasons increased the EONR.