Kentucky bluegrass recovery of urea-derived nitrogen-15 amended with urease inhibitor

Urea is a commonly used N fertilizer on turfgrass areas. Urease inhibitors represent a possible method of reducing volatile loss of NH₃ from treated turf. In this study, the recovery of urea-N by a Kentucky bluegrass (Poa pratensis L.) turf was determined in the field by using a ¹⁵N tracer. The urease inhibitor N-(n-butyl) thiophosphoric triamide (NBPT) also was evaluated as a fertilizer amendment to increase urea-N efficiency. The study was conducted on a Nicollet (fine-loamy, mixed, mesic Aquic Hapludoll) soil. Treatments included urea labeled with 5% ¹⁵N applied at the rate of 49 kg ha^-1 with NBPT at 0 (urea alone), 0.25, and 0.5% of the weight of N. Grass clippings were collected weekly for 5 wk. Shoots, thatch, and rootzone samples were collected at termination. Total urea-derived N recovered from 5 wk of clippings ranged from 7.5% from the areas treated with urea alone to 8.1% in the urea + 0.5% NBPT treated areas. The 0.25% NBPT treatment increased N recovery in the 0 to 7.5 cm depth of soil from 13.6 (urea without NBPT) to 22.4%. This NBPT rate also increased N recovery from the whole soil-plant system from 28.8 (urea without NBPT) to 45.0%, whereas the 0.5% NBPT rate either had no effect or numerically decreased N recovery. The increased N recovery at the low rate of NBPT was probably due to a reduction in NH₃ volatilization. The reducing effect of the higher NBPT treatment may have been due to delayed urea hydrolysis and to subsequent increased free-urea loss below the rooting zone after an unusual 130 mm rain during the fifth and sixth days after treatment.