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N. Sethunathan et al.
short residual life of less than 3 to 6 months in flooded soils. Thus, more rapid
degradation of DDT, methoxychlor, and heptachlor in four Philippine rice soils
occurred under flooded than under nonflooded conditions (Castro and Yoshida,
1971). In some soils, these insecticides almost completely disappeared within
30 days of flooding; during this period the loss from nonflooded soils was
negligible. Only nonsterile soil samples exhibited the striking capacity to degrade
these insecticides (Castro and Yoshida, 1974). Furthermore, washed cell suspensions of C. sphenoides, isolated from -y-HCH-amended flooded soil, also
metabolized DDT, methoxychlor, and heptachlor under anaerobic conditions
(Sethunathan and Yoshida, 1973a). However, DDD, the major product formed
by reductive dechlorination of DDT both in flooded soils (Castro and Yoshida,
1974) and in bacterial culture (Sethunathan and Yoshida, 1973a), resisted further
degradation. Despite the extreme instability of DDT in anaerobic ecosystems,
its use even in such systems still poses serious problems of environmental contamination from its more persistent metabolite, DDD, which is also used as an
insecticide.
Endrin, a widely used insecticide until recently and now banned in many
tropical countries, decomposed rapidly in one of four Philippine soils under
flooded conditions (Castro and Yoshida, 1974). But endrin showed relatively
low persistence and reached low levels within 55 days of flooding except in a
sandy soil of eight selected Indian soils (Gowda and Sethunathan, 1976). Most
rapid degradation occurred in an acid sulfate Pokkali soil. Six stable products
were formed from endrin in most soils except in acid sulfate Kari soil, where
five products were formed, and in sandy soil where three products were formed.
More rapid degradation of endrin in three soils under fooded than under nonflooded conditions led to the formation of six compounds in flooded soils and
four compounds in nonflooded soils (Gowda and Sethunathan, 1977). The degradation of endrin in flooded soils is both chemical and biological, but biological
degradation is much faster and more extensive, with distinct formation of six
products in nonsterile soils and three products in sterile soils (Gowda and Sethunathan, 1977). Microorganisms capable of degrading endrin have been isolated from several terrestrial and aquatic ecosystems, including marine environments in tropical Hawaii (Matsumura et ai., 1971; Patil et ai., 1970), but not
yet from flooded soils. Although endrin is relatively short-lived in microbially
active flooded soils, its degradation products could persist in the environment.
It would be incorrect to conclude that all pesticides are unstable in flooded soils
and other anoxic systems since aldrin, dieldrin, and chlordane, for example,
resisted degradation under both flooded and nonflooded conditions (Castro and
Yoshida, 1971).
The herbicides 2,4-D, 2,4,5-T, and picloram degraded fairly rapidly under
both water regimes but only in nonsterile soils, a finding attributed to microbial
participation (Yoshida and Castro, 1975). Of interest here is the rapid breakdown
N. Sethunathan et al.
short residual life of less than 3 to 6 months in flooded soils. Thus, more rapid
degradation of DDT, methoxychlor, and heptachlor in four Philippine rice soils
occurred under flooded than under nonflooded conditions (Castro and Yoshida,
1971). In some soils, these insecticides almost completely disappeared within
30 days of flooding; during this period the loss from nonflooded soils was
negligible. Only nonsterile soil samples exhibited the striking capacity to degrade
these insecticides (Castro and Yoshida, 1974). Furthermore, washed cell suspensions of C. sphenoides, isolated from -y-HCH-amended flooded soil, also
metabolized DDT, methoxychlor, and heptachlor under anaerobic conditions
(Sethunathan and Yoshida, 1973a). However, DDD, the major product formed
by reductive dechlorination of DDT both in flooded soils (Castro and Yoshida,
1974) and in bacterial culture (Sethunathan and Yoshida, 1973a), resisted further
degradation. Despite the extreme instability of DDT in anaerobic ecosystems,
its use even in such systems still poses serious problems of environmental contamination from its more persistent metabolite, DDD, which is also used as an
insecticide.
Endrin, a widely used insecticide until recently and now banned in many
tropical countries, decomposed rapidly in one of four Philippine soils under
flooded conditions (Castro and Yoshida, 1974). But endrin showed relatively
low persistence and reached low levels within 55 days of flooding except in a
sandy soil of eight selected Indian soils (Gowda and Sethunathan, 1976). Most
rapid degradation occurred in an acid sulfate Pokkali soil. Six stable products
were formed from endrin in most soils except in acid sulfate Kari soil, where
five products were formed, and in sandy soil where three products were formed.
More rapid degradation of endrin in three soils under fooded than under nonflooded conditions led to the formation of six compounds in flooded soils and
four compounds in nonflooded soils (Gowda and Sethunathan, 1977). The degradation of endrin in flooded soils is both chemical and biological, but biological
degradation is much faster and more extensive, with distinct formation of six
products in nonsterile soils and three products in sterile soils (Gowda and Sethunathan, 1977). Microorganisms capable of degrading endrin have been isolated from several terrestrial and aquatic ecosystems, including marine environments in tropical Hawaii (Matsumura et ai., 1971; Patil et ai., 1970), but not
yet from flooded soils. Although endrin is relatively short-lived in microbially
active flooded soils, its degradation products could persist in the environment.
It would be incorrect to conclude that all pesticides are unstable in flooded soils
and other anoxic systems since aldrin, dieldrin, and chlordane, for example,
resisted degradation under both flooded and nonflooded conditions (Castro and
Yoshida, 1971).
The herbicides 2,4-D, 2,4,5-T, and picloram degraded fairly rapidly under
both water regimes but only in nonsterile soils, a finding attributed to microbial
participation (Yoshida and Castro, 1975). Of interest here is the rapid breakdown