Thermal sensitivity of enzyme activity in tropical soils assessed by the Q10 and equilibrium model

We studied the responses of soil microbial biomass, respiration and enzyme activities to temperature in three Mexican soils. Soils were incubated at temperature range of 15-550 A degrees C at 50 (wet) and 10 % (dry) of their water holding capacity. Soils were assayed for their adenosine triphosphate (ATP) content, CO2-C evolution and acid and alkaline phosphomonoesterase, phosphodiesterase, beta-glucosidase, urease, and protease activities. Thermal responses of soil enzyme activities were fitted to both the equilibrium model (EM) and the classical Q(10) model to describe the effects of temperature on enzyme activity. Total organic C, Fe speciation, available P, and inorganic N contents were also determined in all soils and at all temperatures. The results showed that the ATP content, CO2-C evolution, and enzyme activities of soil increased within the temperature range 15-65 A degrees C, and at higher temperatures, soil enzyme activities were more resistant than the ATP content and CO2-C evolution. The effects of temperature were more drastic in wet than in dry soil, although the two soil series displayed similar trends. The enzyme activities showed a good fit to the EM, making this model suitable for determining activation energy, enthalpy of inactivation, and equilibrium inactivation temperatures of soil enzymatic reactions. The results of the EM indicated that soil chemical parameters may not affect the activation energy of enzyme reactions, but may influence the enzyme resistance to inactivation at higher temperature. The results also indicated that the EM model can be used for predicting the impact of high temperatures on soil enzyme activities.