Nitrification inhibitor reduces the inhibitory effect of N-(n-butyl) thiophosphoric triamide (NBPT) on the hydrolysis of urea

Abstract

The addition of nitrification inhibitor (NI) with a urease inhibitor, N-(n-butyl) thiophosphoric triamide (NBPT), has been reported to offset the reduction of ammonia volatilization by NBPT. An incubation study was conducted to investigate the interaction between NBPT and NI (3,4-dimethyl pyrazole phosphate) on hydrolysis of urea in five soils with a range of physico-chemical properties. Untreated urea (UR), NBPT treated urea (URNBPT), or NBPT+NI treated urea (URDI) were surface-applied (250 kg N ha−1) to each soil. The soils were incubated (21 °C) and destructively sampled nine times during a 22-day period. Urea hydrolysis rate (k; d−1) was measured by the disappearance of urea with time and modeled with a first-order kinetic. The value of k was in the order of UR (0.321) > URDI (0.183) > URNBPT (0.151) across the five soils. While the urease inhibitor, NBPT, significantly reduced k in each soil, the addition of a NI with NBPT significantly decreased the ability of NBPT to inhibit urea hydrolysis by an average of 21% across the soils. We found that NI significantly reduced the half-life of urea by about 1 d when compared with NBPT alone. Principal component analysis showed that k did not depend on any of the soil properties, rather, it depended on the type of treatment. Net nitrification rate constant was significantly greater in UR than URNBPT in loam and clay soils but not different in sandy loam soils. We conclude that the often-reported increase in ammonia volatilization with the double inhibitor relative to NBPT alone may not only be due to the persistence of ammonium but may also be due to an increased rate of urea hydrolysis in the presence of a NI.