Kinetics and thermodynamics of urea hydrolysis under the coupling of nitrogen application rate and temperature

This study aimed to determine the coupled effects of temperature and urea application rate on kinetic and thermodynamic parameters to supplement the mechanism of urea hydrolysis and modify the Arrhenius model to improve the prediction accuracy of urea content. Laboratory experiments were conducted for sandy loam soil under different temperatures (T) (288, 293, 298, and 308 K) and urea application rates (F) (247, 309, 371, and 433 mg kg^-1). Urea content was determined daily through high-performance liquid chromatography. Results showed that the interaction between temperature and urea application rate had a significant effect on reaction rate (K_u) and half-life (H_1/2), whereas no significant effect on activation degree (lgN), activation free energy (ΔG), activation enthalpy (ΔH), and activation entropy (ΔS). The new K_u(T)-2 model with a determination coefficient (R²) = 0.990 was more accurate than the Arrhenius model with R² = 0.965. The new U(T, F) model with a mean absolute percentage error (MAPE) = 3.62% was more accurate than the traditional U(T) model with a MAPE = 6.38%. The effects of T and F were observed mainly during the preparatory stage and the most critical transition stage of the chemical reaction, respectively. The findings ΔH > 0, ΔG > 0, and ΔS < 0 indicated that urea hydrolysis was endothermic and controlled by enthalpy. These results supplemented the mechanism of urea hydrolysis and improved the prediction accuracy of urea content.