Metabolism of the urease inhibitor n-( n -butyl)thiophosphoric triamide (nbpt) in soils

Abstract

An experiment was conducted to relate the effectiveness of N-( n-butyl)thiophosphoric triamide (NBPT) and its oxon analog N-( n-butyl)phosphoric triamide (BNPO) in controlling urea hydrolysis in soils to their corresponding soil concentrations. Both compounds were applied to an acid soil (pH 4.9) and to the same soil that had been neutralized (pH 7.1) by long-term liming or by the recent application of Ca(OH) 2. Hydrolysis of urea applied with the inhibitors was monitored along with the disappearance of the compounds themselves. Both compounds controlled urea hydrolysis much more effectively in the neutral soils than in the acid soil. HPLC analysis of soil extracts demonstrated that both compounds disappeared more rapidly in the acid soil, and that the compounds disappeared at similar rates for both neutral soils, indicating that pH governed disappearance rates in these soils. Disappearance rates were generally first-order for both compounds, although NBPT disappeared at an accelerated rate at low concentrations, presumably due to its simultaneous conversion to BNPO. The effectiveness of both compounds in controlling urea hydrolysis was closely related to the concentrations of BNPO found in the soil. BNPO was generally maintained at higher concentrations following NBPT application than when BNPO was applied directly to soil.