2,4-D DEGRADATION AND 2,4-D DEGRADING MICROORGANISMS IN SOILS1

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Abstract
In laboratory studies on Cecil loamy sand and Webster sandy clay loam, I examined the influence of soil-water tension and soil temperature on 14C-2,4-D degradation and the formation of nonextractable 14C-residues, and I correlated the degradation rate with the growth rate of 2,4-D degrading microorganisms. 14C-2,4-D rapidly mineralized in the two soils maintained at 0.1 and 0.33 bar of soil-water tension. Because the total amounts of metabolites in the solvent extracts never exceeded 5% of the total 14C activity, the disappearance rate of extractable 14C essentially represented the disappearance rate of extractable 14C-2,4-D. Extractable 14C in the 14C-2,4-D treated soils maintained at 1 bar and below disappeared rapidly, and at the same time nonextractable 14C rapidly formed, whereas extractable 14C in soils maintained at 15 bars disappeared at much slower rates, and the formation of nonextractable 14C was also slower. After 14 d of slow disappearance, however, extractable 14C in the Cecil soil held at 15 bars started disappearing at a constant rate of 3.6% of applied 14C per day. The disappearance rates in soils incubated at 35°C were generally smaller than those incubated at 25°C. Even though the initial most probable number (MPN) of 2,4-D degrading microorganisms in the Cecil soil was one fourth of that in the Webster soil, the organisms propagated more rapidly in the moist Cecil soil (0.33 bar) than in the moist Webster soil in response to the application of 10 μg 2,4-D/g of soil. As a result, 2,4-D in the Cecil soil degraded at a faster rate. The growth rates of 2,4-D degrading organisms in dry soils (15 bars) were small, in spite of the rapid degradation occurring in the Cecil soil after 14 d of incubation. In addition to forming nonextractable 14C residues in humus components, as much as 4.2% of applied 14C could be associated with microbial mass.