Improving Fertilizer N Recovery and Mitigating NH3 Volatilization From Surface-Urea Applications in a Semiarid Climate.

Surface application of urea during the cold weather months (November-April) is a common management practice for dryland winter wheat grower in the semiarid northern Great Plains.  Urea applications during this time period are susceptible to ammonia volatilization, and management practices to mitigate losses and enhance N fertilizer recovery are needed.  The objective of this study was to determine effect of N-(n-butyl) thiophosphoric triamide (NBPT) on volatilization losses, fertilizer N recovery, and grain yield for urea application made in the late-fall, winter, and early spring.   Ammonia volatilization was quantified according to micrometeorological integrated horizontal flux method and using circular plots, a center mast and Leuning samplers.  A replicated small-plot fertilizer trial with 15N (5%) enriched N microplots was conducted in the same field where micrometeorological measurements of NH₃ loss were made in order to measure fertilizer N recovery and grain yield.  Ammonia volatilization losses from urea (100 kg N ha-1) without and with NBPT (0.1%) were equivalent to 13.4 and 5.9% of applied N for the late-fall, 13.0 and 3.0% of applied N for the winter, and 1.2 and 0.0% of applied N for early spring applications, respectively. Urea 15N recovery in wheat grain was significantly (P <0.05) greater for the spring (46.6%) vs. the fall (31.7%) and winter (31.1%) applications.  Addition of NBPT improved fertilizer N recovery, in particular, for the fall and winter applications.  Grain yield response to NBPT was affected by the timing of the urea application (timing x NBPT P<0.05).  NBPT did not improve grain yield for the winter and spring urea applications, but did improve grain yield (378 kg ha-1) for the fall application.   This found that surface-applied urea N recovery can be enhanced with NBPT, but the greatest improvements can be achieved by delaying N applications to the early spring in order to minimize fertilizer N immobilization.