Fate of nitrogen (¹⁵N) fertilizer in tall fescue seed systems at different application timings

Optimal stewardship of fertilizer N is necessary to reduce the effects of reactive N on the environment. However, insufficient information on fertilizer fate and N use efficiency is available for tall fescue [Schedonorus arundinaceus (Shreb.) Dumort.] seed crops. Furthermore, changing weather patterns can lead to increasingly early spring N applications. A field trial using ¹⁵N-labeled urea applied at three spring timings (Early, Late, and Split) was implemented at three locations in western Oregon representing different soil drainage levels. We hypothesized that Early applications on the location having a poorly drained soil would lead to the highest fertilizer N losses and the lowest fertilizer N recovery efficiency (RE_N) at harvest. The RE_N was not affected by application timing or location and averaged 57%. Overall, 91% of fertilizer was recovered in the soil–plant system at harvest, with 24% remaining in the soil (0–45 cm), almost all in the organic pool, and 10% remaining in belowground (roots and crowns) biomass. Six months after harvest (i.e., 1 yr after application), total fertilizer recovery in the plant–soil system was unchanged. Spring fertilizer contributed only 31% of aboveground N content at harvest, indicating that N sources outside of spring fertilizer applications supplied most of plant N demand. Combined, our results suggest that fertilizer rates could be reduced and that historical N applications may be leading to elevated soil N and N cycling. Further, we show preliminary evidence that roots and crowns can play an important role in cycling fertilizer N in tall fescue seed systems.