Abstract
Nitrogen (N) fertilizer applications near the time of planting are important for upper US Midwest corn (Zea mays L.) production, but wet springs may result in significant N losses. Split applications may circumvent this problem and improve fertilizer uptake and use efficiency, but the relative contribution of N from the soil and fertilizer is poorly understood. A field study with six sites in Minnesota received N-15-labeled urea fertilizer in the first year and unlabeled urea in the second year to determine the effect of N rate and application timing on corn uptake and accumulation patterns of fertilizer-derived N (FDN) and soil-derived N (SDN) over two consecutive growing seasons. Corn responded positively to fertilization. The percentage of total N uptake as FDN was greatest closest to the time of application but decreased over time as SDN became the dominant N source. A split application (45 kg N ha(-1) at planting, 90 kg N ha(-1) at V4) significantly improved FDN uptake over the 135 kg N ha(-1) preplant treatment but did not improve total N uptake in the first year at any site. Fertilizer-N use efficiency ((FNUE)-N-15) using the isotopic method was 2.8%-43.3% across all sites at the end of the first year with the majority partitioned to the grain. At the end of the second year, approximately 2.2% of the first-year applied FDN was recovered in aboveground biomass. Fertilization ensures adequate N availability to the developing crop, but ultimately SDN contributed >= 61% of the total N uptake.