Corn ¹⁵N uptake and partitioning in response to fertilizer application rate and timing

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 ¹⁵N-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 (F¹⁵NUE) 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.