Kinetics of Microbial Decomposition of 2,4-D in Soil: Effects of Herbicide Concentration1
- L. W. Parker and
- K. G. Doxtader2
The kinetics of the degradation of (2,4-dichlorophenoxy) acetic acid (2,4-D) in Ascalon fine sandy loam (Typic Ustoll) under laboratory conditions was studied in relation to herbicide concentration to test the feasibility of employing either the first-order or Michaelis-Menten equation to model the degradation of 2,4-D. At 27°C, the herbicide degraded through two first-order reactions: an initial slow rate (slow phase) followed by a fast rate (fast phase). The first-order rate constants (k values) for the slow phase decreased as 2,4-D concentrations were increased from 1.3 to 10.2 µg g−1 soil, and remained constant above the latter concentration. The k values for the fast phases of decomposition were independent of herbicide concentration. The duration of the slow phase increased linearly from 11 to 28 d as the herbicide concentration was increased from 1.3 to 25 µg g−1 soil.
When 2,4-D degradation was measured during short incubation periods (instantaneous rates) to preclude growth of 2,4-D-decomposing organisms, the rates conformed to the Hane's linearization of the Michaelis-Menten equation (r2 = 0.93). Apparent substrate inhibition occurred above herbicide concentrations of 40 µg g−1 soil. Instantaneous rates were measured for soil samples previously incubated for various times with 2,4-D. Two significant peaks in activity (rate) were observed, one between concentrations of 4 and 9 µg g−1 soil, and the other between 20 and 40 µg g−1 soil. These data suggest the presence of two enzyme systems, differing with respect to the concentration at which substrate inhibition occurred.Please view the pdf by using the Full Text (PDF) link under 'View' to the left.
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