Litter, recognized as a vital layer of dead plant residues on the forest floor, is not merely a simple accumulation but functions as a dynamic nutrient source an‎d a natural recycling system. This study aimed to investigate the influence of decomposition stages on the kinetic constants of amidohydrolases such as urease, L-asparaginase, an‎d L-glutaminase in the litter ecosystems of oak an‎d pine trees. Additionally, the thermodynamic responses of urease were also examined. Oak an‎d pine litter samples were collected from oak forests in Chaharmahal an‎d Bakhtiari Province (Delvara village) an‎d from the pine forest at Isfahan University of Technology, respectively. Based on visual assessments, oak litter samples were categorized into four distinct stages: fresh leaves, slightly decomposed, moderately decomposed, an‎d highly decomposed. In contrast, only two stages—fresh leaves an‎d decomposing leaves—were distinguishable in pine litter.After measuring the amidohydrolase activities in all litter samples, kinetic constants (Km an‎d Vmax) were determined using the Michaelis–Menten model through the linearization methods of Lineweaver–Burk, Eadie–Hofstee, an‎d Hanes–Woolf. The results indicated that, in oak samples, enzymatic activity increased from the fresh leaf stage to the slightly decomposed stage; however, this trend was not observed in later stages of decomposition, as enzymatic activity declined with continued decomposition. Therefore, the highest enzymatic activity was observed at the slightly decomposed stage. In pine leaves, enzymatic activity decreased from the fresh leaf stage to the decomposing litter stage, an‎d the highest urease activity was found in the fresh leaf stage.The determination coefficients (R²) for all three linearization methods were calculated, an‎d the results showed that the Lineweaver–Burk method had superior accuracy compared to the other two methods an‎d could be recommended as the preferred approach for analyzing urease kinetic data. According to the Lineweaver–Burk results, the highest Km value in oak samples was observed at the fresh leaf stage, which decreased significantly from the fresh leaf to the slightly decomposed stage. In pine litter, Km increased from the fresh leaf to the decomposing litter stage, with the highest Km value at the decomposing stage. The highest Vmax value (Lineweaver–Burk method) was observed in fresh oak leaves, which decreased as decomposition progressed from the fresh to the slightly decomposed stage. In pine litter, Vmax also decreased from the fresh leaf to the decomposing litter stage, an‎d the highest Km was found in the fresh leaf stage.The determination coefficients for the three linearization methods were also calculated for L-asparaginase, showing that the Hanes–Woolf method demonstrated higher accuracy than the other two methods an‎d could be proposed as the most reliable method for analyzing L-asparaginase kinetic data. According to the Hanes–Woolf results, the highest Km value was observed in decomposing pine litter, while the highest Vmax occurred in the fresh pine leaves. Similarly, analysis of L-glutaminase kinetic data showed that the Hanes–Woolf method provided the most accurate results among the three methods. The highest Km an‎d Vmax values for L-glutaminase were both found in the fresh pine leaves.The activation energy an‎d temperature coefficient values were highest in fresh oak leaves, sharply decreased in the L1 stage, an‎d then gradually increased as decomposition progressed from L1 to L2 an‎d from L2 to L3. The enzymatic activity of amidohydrolases increased during the initial stages of litter decomposition but decreased in later stages, with the highest activity observed at the onset of decomposition. These findings align with the variations in Km an‎d Vmax parameters, highlighting the crucial role of enzymes in regulating decomposition processes an‎d the nitrogen cycle

فرشيد نوربخش

حسين شريعتمداري

اميرحسين خوشگفتارمنش , مريم فنائي كهراني