Growth-linked and cometabolic biodegradation: Possible reason for occurrence or absence of accelerated pesticide biodegradation

A study was conducted to relate the occurrence of accelerated pesticide biodegradation to the susceptibility of the pesticides to growth-linked degradation or cometabolism. The mineralization of 2,4-D was initially slow but then became rapid, and a second application was mineralized with no acclimation phase and more rapidly than the first. The numbers of 2,4-D-degrading micro-organisms increased markedly following its first application and then declined, but the population size increased after a second addition. Glyphosate was rapidly and extensively mineralized following the first and second applications to soil, and the abundance of organisms able to degrade it rose after the first addition and remained high before and following the second application. Propham (IPC) mineralization was detected only 15 days after its application but the degradation was rapid thereafter, and the second addition was rapidly and extensively mineralized with no acclimation phase. The population of propham-degrading micro-organisms was initially small, but increased markedy 10 days after the initial herbicide addition and was still large at the time of the second application. The rate of carbofuran biodegradation in the test soil was the same following the first and second applications, and the abundance of carbofuran-metabolizing microorganisms did not change appreciably as a result of soil treatment with the insecticide. Simazine mineralization was slow, although the rate was higher following the second addition; however, the number of simazine-degrading organisms did not increase appreciably. From 10 to 12% of the ¹⁴C from radiolabeled 2,4-D, propham, glyphosate or glucose was usually incorporated into the microbial biomass of soil but 0.82% or less of the ¹⁴C from simazine or ring- or carbonyl-labeled carbofuran was converted to biomass. It is suggested that pesticides that support microbial growth may be subject to accelerated biodegradation if the population remains large until the pesticide is applied again. Pesticides that do not support growth may not be subject to accelerated biodegradation.