Effects of urease and nitrification inhibitors on the efficient use of urea for pastoral systems

The effects of a urease inhibitor [N-(n-butyl) thiophosphoric triamide (nBTPT- trade name Agrotain (R))] alone and a combination of nitrification inhibitor dicyandiamide (DCD) and nBTPT, defined as a double inhibitor (DI), on urea fertilizer use efficiency in pastoral systems were investigated. The treatments comprised urea alone, urea with nBTPT, urea with DI at a low rate (LDI), and urea with DI at a high rate (HDI). Each treatment had three replicates and was applied at two rates - 30 and 60 kg nitrogen (N) ha(-1) to three trial sites (North Island-1, North Island-2 and South Island) in New Zealand during the 2005-2006 seasons. The trials at North Island-1 and South Island received six applications of fertilizers, giving total N application rates of 180 and 360 kg N ha(-1) over an18-month period. The North Island-2 trial received these treatments five times during a one-year period, giving total N application rates of 150 and 300 kg N ha(-1). Soil samples collected after the first fertilizer application from the South Island trial indicated that compared to urea alone, urea with nBTPT and urea with the DI treatments applied at the 60 kg N ha(-1) rate exhibited a significantly slower release of ammonium-N during the first two weeks. Nitrate-N production was only partially delayed by DCD applied in the DI treatment during the first week. On individual site and over all sites, urea applied with nBTPT and, to a lesser extent, urea applied with the DI treatment consistently produced significantly higher pasture dry matter (PDM) and nitrogen response efficiency (NRE), relative to pastures receiving urea alone at the two N rates. Across all three sites at the 30 kg N ha(-1) rate, over the entire period, urea with nBTPT produced 20,441 kg DM ha(-1) compared to 18,383 kg DM ha(-1) produced by urea alone, representing an increase of 11.2% over urea alone. At the 60 kg N ha(-1) rate, the increase in PDM by urea with nBTPT was 8.3% over urea alone. PDM yields from urea with LDI and HDI treatments were slightly lower than PDM yields of pastures receiving urea with nBTPT, but significantly higher than those of urea alone. The NRE of individual sites and over all sites were significantly higher for the treatments of urea + nBTPT, or urea + DI, relative to those pastures receiving urea alone. NRE values dropped at the higher urea rate (60 kg N ha(-1)). Compared to urea alone, urea with nBTPT or urea with the DI treatments also showed an improvement in N uptake in pasture herbage. However, these improvements in pasture N uptake were only significant for HDI and LDI treatments at 60 kg N ha(-1). This suggests that applying urea with nBTPT alone has the most potential to improve the efficiency of urea fertilizer use. The DI treatments may have other environmental benefits, like reduction in N2O emission and NO3 (-) leaching, though the agronomic benefits appear unlikely to be greater than those achieved by using urea with nBTPT alone.