EFFECT OF INOCULANT STRAIN AND ORGANIC-MATTER CONTENT ON KINETICS OF 2,4-DICHLOROPHENOXYACETIC ACID DEGRADATION IN SOIL

We monitored rates of degradation of soluble and sorbed 2,4-dichlorophenoxyacetic acid (2,4-D) in low-organic-matter soil at field capacity amended with 1, 10, or 100-mu-g of 2,4-D per g of wet soil and inoculated with one of two bacterial strains (MI and 155) with similar maximum growth rates (mu(max)) but significantly different half-saturation growth constants (K(s)). Concentrations of soluble 2,4-D were determined by analyzing samples of pore water pressed from soil, and concentrations of sorbed 2,4-D were determined by solvent extraction. Between 65 and 75% of the total 2,4-D was present in the soluble phase at equilibrium, resulting in soil solution concentrations of ca. 8, 60, and 600-mu-g of 2,4-D per ml, respectively. Soluble 2,4-D was metabolized preferentially; this was followed by degradation of both sorbed (after desorption) and soluble 2,4-D. Rates of degradation were comparable for the two strains at soil concentrations of 10 and 100-mu-g of 2,4-D per g; however, at 1-mu-g/g of soil, 2,4-D was metabolized more rapidly by the strain with the lower K(s) value (strain MI). We also monitored rates of biodegradation of soluble and sorbed 2,4-D in high-organic-matter soil at field capacity amended with 100-mu-g of 2,4-D per g of wet soil and inoculated with the low-K(s) strain (strain MI). Ten percent of total 2,4-D was present in the soluble phase, resulting in a soil solution concentration of ca. 30-mu-g of 2,4-D per ml. Rates of degradation in the high-organic-matter soil were lower than in the low-organic-matter soil, presumably as a result of lower rates of desorption and microbial growth.