For this reason, a discrepancy could be observed in ED values predicted by Model 1 and Model 2 for ATP inhibition after 3 h of incubation of both soils (Table 2). However after 20 days of incubation, ED values predicted by Model 1 and Model 2 were the same, because the asymptote value was very low (tends to zero) and the fitted curves of both models were very similar. In this case, both models practically described a full inhibition of ATP with the Cd concentration.
The ED values for the inhibition of ATP were higher in the cultivated soil than in the uncultivated soil (Table 2), and for the latter soil, the trend of ED values in reference to the incubation time was inverse, as predicted by Model 1. The characteristics of both soils influenced their native microbial population and thus the resistance of the microbial populations to heavy metal (Degens and others 2001).