Field Evaluation of Urease Inhibitors in Direct-Seeded , Delayed-Flood Rice on a Silt-Loam Soil

Urea is the most widely used nitrogen (N) fertilizer in direct-seeded, delayed-flood rice production. Urea fertilizer is susceptible to loss as ammonia gas to the atmosphere if not flooded in a timely manner. A urease inhibitor [i.e. N-(n-butyl) thiophosphorictriamide; NBPT] is recommended when a timely flood cannot be established. Recent research has indicated that when relative humidity (RH) is less than the critical relative humidity (CRH) of urea then ammonia volatilization can be limited. Also, recent field research in rice at the Rice Research and Extension Center (RREC), Stuttgart, Ark., using NBPT in comparison to untreated urea, has not produced significant yield differences. The objectives of this study were to (i) investigate the effectiveness of NBPT at inhibiting ammonia volatilization, (ii) investigate the effectiveness of NBPT at increasing grain yield, and (iii) determine the effect of RH in relation to CRH when comparing volatilization data to yield data. The study was comprised of two parts: i) an early-season ammonia volatilization study and ii) a grain yield study. Untreated urea, Agrotain-treated urea, and Arborite-treated urea were investigated. The volatilization study was conducted using semi-open static chambers with an acid trapping solution over a 20-d period. Dataloggers were included both in the chamber and in ambient conditions to measure temperature, RH, and for CRH determination. The yield study investigated the 3 fertilizer sources, 3 application timings (10, 5, and 1 day prior to flooding), and 2 N application rates (90 and 45 lb N/acre). Cumulative N-losses at the end of the measurement period were 15.8%, 4.3%, and 3.8% from urea, Agrotain-treated urea, and Arborite-treated urea, respectively. Agrotain-treated urea and Arborite-treated urea did not differ during the 20-d study but both had statistically less ammonia-N loss than untreated urea. However, there was no difference in rice grain measured between N