Reducing NH3, N2O and NO3--N losses from a pasture soil with urease or nitrification inhibitors and elemental S-amended nitrogenous fertilizers

A 3-month field experiment comparing nitrogen (N) losses from and the agronomic efficiency of various N fertilizers was conducted on a sandy loam (Typic Hapludand) soil at Ruakura AgResearch farm, Hamilton, New Zealand during October to December 2003. Three replicates of seven treatments: urea, urea + the urease inhibitor N-(n-butyl) thiophosphoric triamide (trade name Agrotain), urea + Agrotain + elemental sulphur (S), urea + double inhibitor [DI; i.e., Agrotain + dicyandiamide (DCD)], diammonium phosphate (DAP), DAP + S, each applied at 150 kg N ha(-1), and control (no N). After fertilizer application, soil ammonium (NH4+) and nitrate (NO3-) concentrations (7.5-cm soil depth), ammonia (NH3) volatilization, nitrate (NO3-) leaching, nitrous oxide (N2O) emission, pasture dry matter, and N uptake were monitored at different timings. Urea applied with Agrotain or Agrotain + S delayed urea hydrolysis and released soil NH4+ at a slower rate than urea alone or urea + DI. Urea applied with DI increased NH3 volatilization by 29% over urea alone, while urea + Agrotain and urea + Agrotain + S reduced NH3 volatilization by 45 and 48%, respectively. Ammonia volatilization losses from DAP were lower than those from urea with or without inhibitors. Total reduction in NO3- leaching losses for urea + DI and urea + Agrotain compared to urea alone were 89% and 47%, respectively. Application of S with urea + Agrotain reduced NO3- leaching losses by an additional 6%. Nitrous oxide emissions were higher from the DAP and urea alone treatments. Urea applied with DI and urea + Agrotain reduced N2O emissions by 37 and 5%, respectively, over urea alone. Compared to urea alone, total pasture production increased by 20, 17, and 15% for urea + Agrotain + S, urea + Agrotain, and urea + DI treatments, respectively, representing 86, 71, and 64% increases in N response efficiency. Total N uptake in urea + Agrotain, urea + Agrotain + S, and urea + DI increased by 29, 22, and 20%, respectively, compared to urea alone. These results suggest that the combination of both urease and nitrification inhibitors may have the most potential to reduce N losses and improve pasture production in intensively grazed systems.