Isotopic evaluation of in-season Y Drop applications for corn production

A common strategy for improving fertilizer N uptake efficiency by corn (Zea mays L.) is to synchronize application with crop N demand during the growing season, which can be done using the Y Drop system recently developed for surface dribble placement. A 2-yr field study was conducted using dual-labeled urea-ammonium nitrate (UAN) solution to compare fertilizer ¹⁵N uptake efficiency (F¹⁵NUE) for surface and subsurface sidedress applications to soils of contrasting fertility under either second-year corn or a corn–soybean [Glycine max (L.) Merr.] rotation. Besides placement, treatments were designed to allow comparison of ¹⁵N uptake from UAN applications made at planting vs. at the V9 growth stage. The importance of soil N supply was demonstrated by estimates of N derived from fertilizer that ranged from 17 to 47% in total aboveground biomass and never exceeded N derived from soil. Of the fertilizer N recovered in the crop at harvest, the majority originated from the ¹⁵N applied at sidedressing rather than at planting. The range in F¹⁵NUE was from 12 to 42% (26% on average) for grain and from 14 to 51% (31% on average) for total aboveground biomass, the only significant difference occurring when the subsurface treatment outperformed Y Drop placement under conditions conducive to urea N loss through NH₃ volatilization. Both methods of application offer the same fundamental advantage, in that fertilizer N is supplied during the period of maximal crop uptake.