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 i n soils and media adjusted to different water potentials (psi) by mean s of different concentrations of a salts mixture. Maximal removal of 2 ,4-D from solution in contact with soil occurred at the highest psi of -0.1 MPa, corresponding to soil at field capacity, and degradation de creased progressively down to psi = -5.5 MPa with no breakdown at psi = -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 sensiti ve to dry conditions than the community of culturable aerobic heterotr ophs (''heterotrophs''). The size of the ''heterotrophic'' bacterial c ommunity able to grow at each value of psi decreased as psi decreased, but large numbers were able to grow at psi = -5.5 MPa, whereas 54-D-d egrading bacteria were only recovered in appreciable numbers at the hi ghest moisture contents. Numbers of 2,4-D-degrading fungi also decreas ed with decreasing moisture; significant numbers were found only in so ils with psi = -1.4 MPa (slightly higher than the permanent wilting po int, -1.5 MPa) or greater. In contrast, numbers of ''heterotrophic'' f ungi were constant between -0.1 and - 5.5 MPa. 2,4-D-degrading microbe s were isolated on media of high water activity and representative iso lates tested for their ability to degrade 2,4-D at various values of p si. A unicellular bacterium degraded 2,4-D most rapidly at psi = -0.1 MPa, exhibited a prolonged lag phase and reduced degradation rate at p si = -1.4 MPa, and was unable to break down the herbicide at moisture values of psi = -2.7 MPa or less. Filamentous microbial isolates were able to remove 2,4-D at psi = -5.5 MPa when supplied with an alternati ve source of C and energy, but degradation was more rapid as the moist ure content increased. Fungi were most tolerant of dry conditions; at psi = -5.5 MPa a fungal isolate degraded 64% of the herbicide in 50 da ys, while an actinomycete removed 33% in the same time.