Modeling of the fine‐scale temperature response of arylsulfatase activity in soil

Abstract Fine‐scale (1.0–2.2 °C) temperature dependence of soil arylsulfatase activity (arylsulfate sulfohydrolase, EC 3.1.6.1) was measured at 0 to 75 °C in a Danish sandy, arable soil. Assays were done with field‐moist soil samples in the absence of toluene as plasmolytic agent – a procedure that primarily measures the extracellular enzymes. The aim was to evaluate the use of temperature models to describe the temperature response of soil arylsulfatase activity. In addition, we searched for increases in activity at high temperatures (e.g., 50–60 °C), which might be associated with unmasking (exposure) of intracellular enzymes. Arylsulfatase activities ranged from 1.1 to 60.3 μg p ‐nitrophenol (g dry weight soil) –1 h –1 , with an optimum temperature at 58.1 °C. The temperature response below 58.1 °C could be described by the Arrhenius equation ( r 2 = 0.978, n = 83) and the simple Ratkowsky equation ( r 2 = 0.977, n = 83). The expanded Ratkowsky equation, which covered the entire temperature range (0–75 °C), was less satisfactory ( r 2 = 0.958, n = 90) because the model underestimated the reaction rates near the optimum temperature. The activation energy ( E a ) calculated from the Arrhenius equation was 42.2 kJ mol –1 . This was higher than previously found for other soils (16.5–34.7 kJ mol –1 ), possibly due to the use of toluene in these studies. Further analysis of the temperature response showed that no increase in activity occurred due to potential unmasking of intracellular enzymes by disintegration of bacterial cell membranes at high temperatures. Thus, the use of high incubation temperatures did not facilitate the differentiation between intra‐ and extracellular enzyme activity.