Effect of water availability on degradation of 2, 4-dichlorophenoxyacetic acid (2, 4-d) by soil microorganisms

We studied the effect of water availability on degradation of 2,4-D (2,4-dichlorophenoxyacetic acid) and on 2,4-D-degrading microorganisms in soils and media adjusted to different water potentials (ψ) by means of different concentrations of a salts mixture. Maximal removal of 2,4-D from solution in contact with soil occurred at the highest ψ of -0.1 MPa, corresponding to soil at field capacity, and degradation decreased progressively down to ψ = - 5.5 MPa with no breakdown at ψ = - 22 MPa. The community sizes of unicellular bacteria and fungi were determined in soils incubated for 60 days at various moisture contents. The community of microbes able to remove 2,4-D was much more sensitive to dry conditions than the community of culturable aerobic heterotrophs ("heterotrophs"). The size of the "heterotrophic" bacterial community able to grow at each value of ψ decreased as ψ decreased, but large numbers were able to grow at ψ = - 5.5 MPa, whereas 2,4-D-degrading bacteria were only recovered in appreciable numbers at the highest moisture contents. Numbers of 2,4-D-degrading fungi also decreased with decreasing moisture; significant numbers were found only in soils with ψ = - 1.4 MPa (slightly higher than the permanent wilting point, -1.5 MPa) or greater. In contrast, numbers of "heterotrophic" fungi were constant between -0.1 and - 5.5 MPa. 2,4-D-degrading microbes were isolated on media of high water activity and representative isolates tested for their ability to degrade 2,4-D at various values of ψ. A unicellular bacterium degraded 2, 4-D most rapidly at ψ = - 0.1 MPa, exhibited a prolonged lag phase and reduced degradation rate at ψ = - 1.4 MPa, and was unable to break down the herbicide at moisture values of ψ = - 2.7 MPa or less. Filamentous microbial isolates were able to remove 2,4-D at ψ = -5.5 MPa when supplied with an alternative source of C and energy, but degradation was more rapid as the moisture content increased. Fungi were most tolerant of dry conditions; at ψ = -5.5 MPa a fungal isolate degraded 64% of the herbicide in 50 days, while an actinomycete removed 33% in the same time.