Phenoxy herbicide degradation in soils: Quantitative studies of 2,4-D- and MCPA-degrading microbial populations

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Microbial populations able to degrade 2,4-D (2,4-dichlorophenoxyacetate) and MCPA (4-chloro-2-methylphenoxyacetate) were enumerated by means of a most probable number (MPN) procedure in eight Natal soils not previously treated with these herbicides. Estimated 2,4-D-degrading populations ranged from 1.26 to 245.2 and MCPA-degrading populations from 0.34 to 1377 g −1 dry soil; in seven of the soils the populations of these organisms were less than 40 and 30 g −1, respectively. Such counts indicate that for the successful isolation of 2,4-D- or MCPA-degrading microorganisms from soil, at least 1 g dry weight of soil should be used for enrichment cultures. The 2,4-D-degrading organisms occurred among the aerobic soil bacteria detectable by plate count, at frequencies of only 1 in 30 × 10 3 to 1 in 36 × 10 6 and the MCPA-degrading organisms at frequencies of 1 in 5 × 10 3 to 1 in 133 × 10 6; the ease with which the herbicide-degrading organisms can be isolated from enriched soil cultures treated with 2,4-D or MCPA is evidence of their massive preferential proliferation in response to the herbicides. Log 2,4-D- and MCPA-degrading populations did not differ significantly in four soil samples, but in the others either the 2,4-D- or the MCPA-degrading population was dominant. The longer persistence of MCPA compared with that of 2,4-D could therefore not be ascribed to quantitative differences in the populations of MCPA- and 2,4-D-degrading soil microorganisms. No relationship was evident between the soil populations of 2,4-D- or MCPA-degrading microorganisms and aerobic soil bacteria, and variations of the three populations among the soil samples were not associated in any obvious way with the soil physical and chemical characteristics, except perhaps an association of the highest counts of herbicide-degrading organisms with a sugar cane soil of sandy texture and high C: N ratio.