Use of phenylphosphorodiamidate and N-(n-butyl)thiophosphorictriamide to reduce ammonia loss and increase grain yield following application of urea to flooded rice

Ammonia (NH₃) volatilization is an important mechanism for nitrogen (N) loss from flooded rice fields following the application of urea into the floodwater. One method of reducing losses is to use a urease inhibitor that retards the hydrolysis of urea by soil urease and allows the urea to diffuse deeper into the soil. The two chemicals that have shown most promise in laboratory and greenhouse studies are phenylphosphorodiamidate [PPD] and N-(n-butyl)thiophosphorictriamide [NBPT], but they seldom work effectively in the field. PPD decomposes rapidly when the pH departs from neutrality, and NBPT must be converted to the oxygen analogue [N-(n-butyl)phosphorictriamide, NBPTO] for it to be effective. Our field studies in Thailand showed that NH₃ loss is markedly reduced when PPD is added with the algicide terbutryn. The studies also showed that a mixture of PPD and NBPT was even more effective than either PPD or NBPT alone. It appears that initially PPD inhibited urease activity, and during this time at least part of the NBPT was converted to NBPTO; then as the activity of PPD declined, NBPTO inhibited the hydrolysis of urea. The combined urease inhibitor treatment reduced NH₃ loss from 15 to 3% of the applied N, and increased grain yield from 3.6 to 4.1 t ha^–1.