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Kinetics of the Microbial Degradation of 2,4-D in Soil: Effects of Temperature and Moisture¹

ABSTRACT

The effects of soil moisture tension and temperature on the kinetics of the degradation of (2,4-dichlorophenoxy)acetic acid (2,4-D) in an Ascalon sandy loam were studied under laboratory conditions to develop a simulation model. Degradation occurred by a slow, first-order reaction (slow phase) which, under some conditions, was followed by a rapid, first-order reaction (fast phase). The optimum temperature and moisture tension were 27°C and 0.1 bar, respectively. Degradation rates under optimum conditions were 0.230 and 2.234 µg g^–1 soil for the slow and fast phases, respectively. At temperatures above the optimum, no fast phase was observed. The activation energy (E_A) values increased from 22.96 to 45.46 kcal mole^–1, with increasing soil moisture tension in the range from 0.1 to 1.0 bar. The rate of decomposition of 2,4-D decreased with increasing soil moisture tension for temperatures between 20 and 35°C. This decrease was a result of the reduced activity of the 2,4-D-degrading microorganisms arising from decreased water availability and increased 2,4-D solution concentration.

Key Words: (2,4-dichlorophenoxy)acetic acid • activation energy • rate constants • herbicide • pesticide

¹ Contribution from the Dep. of Agronomy, Colorado State University, Fort Collins, CO 80523. Published with the approval of the Colorado State Experiment Station as Scientific Paper Series no. 2693. Research conducted as part of Western Regional Research Project, W-82.

² Research Associate, Dep. of Biology, New Mexico State University, Las Cruces, NM 88003, and Associate Professor of Agronomy, Colorado State University, Fort Collins, respectively.

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