Fate of fertilizer nitrogen applied to corn as estimated by the isotopic and difference methods

The percentage of applied fertilizer N taken into plants is often estimated by measuring the difference in plant N uptake between treated and check plots. This method has often overestimated plant N uptake. The objectives of this study were to (i) compare the recover of fertilizer N in corn (Zea mays L.) as calculated by the difference and isotopic methods and (ii) track fertilizer N in the plant-soil system using isotopic enrichment of N. Sixteen N plots (3 m x 3 m) were established on a Hecla fine-sandy loam (sandy, mixed, frigid Oxyaquic Hapludolls) and replicated four times in a completely random design. Corn received sidedressed band applications (15 cm from row and 5 cm deep) of (15)N-enriched and nonlabeled urea N at 135 kg N ha(-1) in 1993 and 1994. Plant uptake of fertilizer N as estimated by the isotopic and difference methods was 45% and 39% in 1993 and 40% and 22% for 1994, respectively. Nearly 42% and 36% of the applied labeled N was accounted for in the soil at the end of 1993 and 1994, respectively. The difference method did not overestimate plant N uptake because of high soil N availability. Lower corn yield potential in 1993 was the consequence of a cooler, shorter growing season. This climatic difference had less effect on the results generated by the isotopic method. Reasons for the N deficit in this investigation are speculative since no attempts were made to measure gaseous emissions; however, denitrification and/or leaching are thought to be the primary mechanisms.