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The effects of temperature and moisture regimes on kinetics of urea hydrolysis in soils of Hisar (Typic Camborthids) and Hansi (Typic Ustochrepts) were studied. The rate constant (K) for first-order reaction increased with temperature from 10 to 35�C. Energy of activation (Ea), activation free energy (�F*) and activation entropy (�S*) between 10 and 35�C were 33.8, 81.8 kJ mol-1 and -176.55 K-1I in a sandy loam soil, and 29.2, 81 -6 kJ mol-I and - 155.3 J K-I in a clay loam soil, respectively. The hydrolysis rate was not a linear function of moisture content but increased with field capacity (F.C.). The rate of hydrolysis in both the soils followed the order: 20% F.C. < 40% F.C. < 80% F.C. = 100% F.C. The rate for flooded soil was between those for 20% and 40% F.C. for sandy loam, and between 40 and 80% F.C. for the clay loam. The added urea was completely hydrolysed in 3 days at all moisture regimes except 20% F.C.; maximum NH4+-N was found at the third day, then decreased thereafter. The time at which NO2- first appeared varied with treatments, but then it persisted throughout the 24-day incubation in all treatments.

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