Evaluation of Urease Inhibitors in Reducing Ammonia Volatilization and Increasing Grain Yield in Drill-Seeded, Delayed-Flood Rice.

Urea is the most commonly used nitrogen (N) source in Arkansas rice production. Losses as ammonia can occur if fields are not flooded in a timely manner. If a flood cannot be established in a timely manner, a urease inhibitor is recommended. Recently, research has reported that relative humidity (RH) below the critical relative humidity (CRH) of urea can limit volatilization. The objectives of this study were to investigate the effectiveness of the urease inhibitor NBPT (N-(n-butyl) thiophosphorictriamide) at inhibiting ammonia volatilization, increasing grain yield, and to evaluate in-chamber RH as compared to in-field conditions. Untreated urea, Agrotain-treated urea, and Arborite-treated urea were investigated as to their influence on ammonia volatilization and rice grain yield. Ammonia volatilization data were collected using semi-open static chambers with an acid trap during a 20-d period. Dataloggers were included in-chamber and adjacent to chambers to measure temperature, RH, and to determine CRH. The grain-yield study was comprised of the three fertilizer sources, three application timings (10, 5, and 1 d prior to flooding), and two N application rates (101 and 50 kg N ha^-1).  Ammonia-N losses at the end of the 20-d period were 15.8, 4.3, and 3.8% from untreated urea, Agrotain-treated urea, and Arborite-treated urea, respectively. Agrotain- and Arborite-treated urea did not differ during the 20-d study, but both had significantly less ammonia-N loss than untreated urea; however, grain-yield differences were not observed based on N source or application timing. In chamber RH was greater than CRH, while RH was less than CRH in-field during the 20-d study. This study revealed a situation where the ammonia losses in the semi-open static chamber were not reflective of in-field and provided an explanation for the discrepancy between measured ammonia-N losses between N sources compared to the lack of grain-yield differences based on N source.