Kinetics and thermodynamics of urea hydrolysis in the presence of urease and nitrification inhibitors

Abstract

Urease inhibitor [N-(n-butyl) thiophosphoric triamide (NBPT)] and nitrification inhibitor (NI) (3,4-dimethylpyrazole phosphate) have been used to reduce nitrogen (N) losses from urea-based fertilizers. This study evaluated the effect of temperature, NBPT, and NI on kinetic and thermodynamic properties of urea hydrolysis in six soils. Soils were amended (250 kg N·ha−1) with urea (UR), NBPT treated urea (URNBPT), or NBPT + NI treated urea (URDI), incubated at 5, 15, or 25 °C, and destructively sampled eight times during an 18 d incubation. We measured urea hydrolysis rate by the disappearance of urea with time and determined the rate constant (k; d−1) assuming first-order kinetics. Our results showed that k increased with temperature in the order of 0.07 (5 °C), 0.12 (15 °C), and 0.20 (25 °C) across soils and inhibitor treatments. In addition, k declined in the order of UR (0.19) > URDI (0.11) > URNBPT (0.08) across soils and temperatures. Although urease inhibitor, NBPT, increased the half-life of urea from 3.8 to 8.3 d across soil–temperature, the addition of a NI significantly reduced the half-life of NBPT treated urea by approximately 2 d across soil–temperature. Thermodynamics parameters showed that urea hydrolysis was nonspontaneous, and enthalpy and entropy changes were not significantly different among inhibitor treatments in five of the six soils. We conclude that the often-reported greater ammonia volatilization from URDI than URNBPT may not only be due to the persistence of ammonium in the presence of NI but also because NI reduced the inhibitory effect of NBPT on urea hydrolysis.