Ammonia Volatilization from Surface-applied Urea as Affected by Several Phosphoroamide Compounds

In an attempt to reduce ammonia (NH₃) losses from surface applied urea to ultimately increase plant N use efficiency, a field experiment was conducted to compare the effects of six phosphoroamide urease inhibitors on NH₃ volatilization from urea surface applied to no-till (NT) and conventionally tilled (CT) soils. Urea prills (200 kg N ha⁻¹) with and without phosphoroamide inhibitors (4.0 kg a.i. ha⁻¹) were broadcast on the surface of microplots. Ammonia evolution was measured using a semiopen-static system in which the volatilization chambers were moved periodically to compensate for soil environmental changes induced by the enclosures. Environmental and soil conditions affected the cumulative and daily losses of N. Losses were greater and more rapid under warm, moist soil conditions than hot and dry conditions. The low specific humidity during the hot, dry conditions prevented complete dissolution of prills until 6 d after application thus reducing hydrolysis and NH₃ volatilization. Phenylphosphorodiamidate (PPD) most consistently reduced cumulative NH₃ losses on NT and CT. Ammonia loss with the addition of PPD was 10 to 54% of the NH₃ loss with unamended urea in three of four tests. Trichloroethyl phosphorodiamidate and N-(diaminophosphinyl)-cyclohexane were the only other compounds to significantly reduce NH₃ losses compared to unamended urea, but these compounds were effective in only one-half the tests. Both field and laboratory studies showed significantly greater NH₃ losses from urea applied to residue covered soil than to bare soil. Addition of PPD or thiophosphoric triamide to residue reduced N losses to 43 and 39% of applied urea-N, respectively. The effectiveness of PPD in retarding NH₃ losses was decreased as the soil pH increased from 5.6 to 7.2. Joint contribution of the Arkansas and Purdue Agric. Exp. Stn. Purdue Journal no. 11237.