Soil properties and the ability of the urease inhibitor N-(n-BUTYL) thiophosphoric triamide (nBTPT) to reduce ammonia volatilization from surface-applied urea

A laboratory study was made to evaluate the ability of the urease inhibitor N-(n-butyl) thiophosphoric triamide (nBTPT) to reduce NH3 volatilization from surface-applied urea. Urea was amended with 0.0, 0.01, 0.058 or 0.28% nBTPT (w/w) and was applied, at a rate equivalent to 100kgN ha−1, to the surface of 16 grassland soils, selected to show a wide range of different chemical and physical characteristics. Daily NH3 loss was measured for 9 days, after fertilizer application, using ventilated enclosures at 13°C. The influence of soil properties on the effectiveness of nBTPT as a urea amendment was investigated using a modelling approach followed by stepwise multiple regression analysis. NH3 volatilization from unamended urea ranged from 5.8 to 38.9% of the N applied and was greatest from a soil with a high pH and low titratable acidity. The % inhibition of NH3 loss by nBTPT was highly dependent on soil type, being effective on some soils even at the 0.01% nBTPT level. The % inhibition at the 0.28% level ranged from 99.4 to 54.4%. Modelling the relationship between total NH3 loss and inhibitor concentration showed that the % nBTPT required to achieve a given % decrease in NH3 volatilization was constant for all soils. For example, 0.092% nBTPT was predicted to lower total NH3 loss by 90% from any given soil. The response to increasing inhibitor concentration in lowering NH3 volatilization was greatest in a soil with low organic matter content and high pH. Stepwise multiple regression analysis showed that 4 soil properties (titratable acidity, pH-H2O, urease activity and cation-exchange capacity) contributed significantly to explaining 90.6% of the variation in total NH3 loss. In all soils the % urea remaining at the end of the incubation was 2.6, 4.6, 10.9 and 25.5% for 0.00, 0.01, 0.058 and 0.28% nBTPT, respectively. The persistence of urea treated with nBTPT was particularly marked in soils with a high pH and could have physiological implications for plants. It is particularly interesting that nBTPT was most effective in soils which showed high NH3 volatilization from unamended urea.