Losses of fertilizer nitrogen after a winter fertilization in three managed pine plantations of the southeastern United States

Losses of fertilizer nitrogen (N) were compared between urea and urea treated with the urease inhibitor N‐(n‐Butyl) thiophosphoric triamide (urea + NBPT) after a surface application in winter at three thinned mid‐rotation (age 15–20 years) loblolly pine plantations in Virginia, South Carolina, and Florida. Treatments were labeled with 15N and applied to open chamber microcosms in January and February 2016. Fifteen days after application, microcosms were removed from the field to determine fertilizer N lost from each microcosm. Losses following fertilization with urea (24% to 50%) were greater (p ≥ .05) at all sites compared to urea + NBPT (12% to 22%). Fertilizer N losses were greater in Florida than in SC and Virginia although N loss following urea fertilization was still 25% in Virginia. The loss of fertilizer N was consistently lower on beds compared to interbeds for both urea (bed = 25%, interbed = 40%) and urea + NBPT (bed = 12%, interbed = 23%). This research highlights the value of using urea + NBPT to reduce fertilizer N losses after a winter application and the greater potential loss in the interbed on wetter sites.