94
N. Sethunathan et al.
D (Audus, 1951), it has become well established that the major, or frequently
the only, means of degradation for several pesticides in the environment is
microbial. In biological reactions essentially involving constitutive and adaptive
enzyme systems, stepwise degradation of pesticide molecules is accomplished,
often with the formation of inorganic end products; but, in nonbiological reactions, the degradation is less extensive and far from complete.
Microbial involvement in pesticide degradation is generally established by
(1) more rapid degradation in nonsterile than in sterile soil samples; (2) an initial
lag in the degradation followed by a more rapid loss attributed to the adaptation
of microorganisms; (3) the initial lag becoming shorter with successive additions
of pesticides due to the enrichment of pesticide-degrading microorganisms; (4)
evolution of 14C02 from ring- 14 C-labeled pesticides applied to biological systems;
and (5) final and conclusive demonstration of the degradation of pesticides in
isolated cultures of microorganisms. Using one or more of these methods, microbial participation has been demonstrated, with some degree of confidence, in
the degradation of some pesticides, insecticides in particular, in tropical soils.
4.3. 1. Chlorinated Hydrocarbon Insecticides
Although chlorinated hydrocarbon insecticides are used regularly in most
tropical countries, in-depth information on their metabolic fate in soils is not
available, with the exception of HCH.
4.3.1.1. Hexachlorocyclohexane
Commercial formulations of HCH generally contain
formulations of HCH, its entry, through persistence, into the food chain following
its intensive use has caused concern in Japan (Tomizawa, 1977). Considerable
work is available on the fate of HCH in tropical rice soils because of its continued
extensive use for controlling common rice insect pests. Extensive studies at the
International Rice Research Institute in the Philippines and elsewhere consistently
showed fairly rapid degradation of
The advantage of flooding the soil as an effective means of accelerating the
degradation of HCH isomers was demonstrated more convincingly by Yoshida
and Castro (1970). During 30-day incubation, "V-HCH decreased to very low
levels in all four soils under flooding while virtually no degradation occurred
under upland conditions. The degradation of all four isomers of HCH was much
faster in nonsterile than in sterile soils (MacRae et al., 1967; Castro and Yoshida,
1974). Also, the evolution of 14C02 from [14C]-"V-HCH from nonsterile samples,
N. Sethunathan et al.
D (Audus, 1951), it has become well established that the major, or frequently
the only, means of degradation for several pesticides in the environment is
microbial. In biological reactions essentially involving constitutive and adaptive
enzyme systems, stepwise degradation of pesticide molecules is accomplished,
often with the formation of inorganic end products; but, in nonbiological reactions, the degradation is less extensive and far from complete.
Microbial involvement in pesticide degradation is generally established by
(1) more rapid degradation in nonsterile than in sterile soil samples; (2) an initial
lag in the degradation followed by a more rapid loss attributed to the adaptation
of microorganisms; (3) the initial lag becoming shorter with successive additions
of pesticides due to the enrichment of pesticide-degrading microorganisms; (4)
evolution of 14C02 from ring- 14 C-labeled pesticides applied to biological systems;
and (5) final and conclusive demonstration of the degradation of pesticides in
isolated cultures of microorganisms. Using one or more of these methods, microbial participation has been demonstrated, with some degree of confidence, in
the degradation of some pesticides, insecticides in particular, in tropical soils.
4.3. 1. Chlorinated Hydrocarbon Insecticides
Although chlorinated hydrocarbon insecticides are used regularly in most
tropical countries, in-depth information on their metabolic fate in soils is not
available, with the exception of HCH.
4.3.1.1. Hexachlorocyclohexane
Commercial formulations of HCH generally contain
formulations of HCH, its entry, through persistence, into the food chain following
its intensive use has caused concern in Japan (Tomizawa, 1977). Considerable
work is available on the fate of HCH in tropical rice soils because of its continued
extensive use for controlling common rice insect pests. Extensive studies at the
International Rice Research Institute in the Philippines and elsewhere consistently
showed fairly rapid degradation of
The advantage of flooding the soil as an effective means of accelerating the
degradation of HCH isomers was demonstrated more convincingly by Yoshida
and Castro (1970). During 30-day incubation, "V-HCH decreased to very low
levels in all four soils under flooding while virtually no degradation occurred
under upland conditions. The degradation of all four isomers of HCH was much
faster in nonsterile than in sterile soils (MacRae et al., 1967; Castro and Yoshida,
1974). Also, the evolution of 14C02 from [14C]-"V-HCH from nonsterile samples,