Relative susceptibility of conventional and experimental nitrogen sources to ammonia loss from flooded rice fields

Urea, the most common N source in Asia, is prone to high loss as ammonia when applied to tropical flooded rice (Oryza sativa L.). Chemical or physical modifications of urea could offer potential in reducing ammonia loss. Two field studies were conducted to identify conventional and experimental N-containing sources loss prone to ammonia less than prilled urea. Relative susceptibility to ammonia loss was inferred from equilibrium ammonia vapor pressure, pNH₃. For the sources studied, ammonia formation and presumably loss were least for guanylurea sulfate (GUS) and sulfur-coated urea (SCU). The slow mineralization and acidifying effect of GUS resulted in negligible ammonia concentration in the floodwater. Amendment of urea with either 5 or 10% paraformaldehyde (ureaform) reduced pNH₃, but never by more than 55%. Coating urea with phosphate rock tended to be less effective than amendment with paraformaldehyde in reducing pNH₃. There was no significant difference in the pNH₃, and presumably ammonia loss, for urea phosphate (17-44-0), urea-urea phosphate (34-17-0), and urea. About 3 days after fertilization, the floodwater pH tended to become higher with NP sources than with urea. This elevation in pH was apparently due to the stimulation of algal photosynthetic activity by added P, and it may explain the failure of a phosphoric acid amendment to urea (urea phosphate) in reducing pNH₃. Ammonia disappearance from broadcast diammonium phosphate (DAP) and ammonium phosphate sulfate (16-20-0) was complete within 3 days after N application, whereas ammonia remained in floodwater for up to 7 days following broadcast application of urea and ammonium sulfate.