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Kinetics of Microbial Decomposition of 2,4-D in Soil: Effects of Herbicide Concentration¹

ABSTRACT

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.

Key Words: (2,4-dichlorophenoxy)acetic acid • pesticide • persistence

¹ Contribution of the Dep. of Agronomy, Colorado State University, Ft. Collins, CO 80523. Published with the approval of the Colorado State Exp. Stn. as Sci. Paper Ser. no. 2724.

² Research Associate/Assistant Professor, Dep. of Biology, Box 3AF, New Mexico State University, Las Cruces, NM 88003; and Associate Professor, Dep. of Agronomy, Colorado State Univ.; respectively.

Received for publication May 21, 1981.