Microbial Degradation of Pesticides in Tropical Soils
105
1972). In recent years, there has been serious concern over the significant formation of soil-bound residues from soil-applied pesticides, especially under anaerobic conditions. The practical feasibility of accelerating the complete destruction of soil-bound residues of pesticides by returning the anaerobic system to
aerobic conditions or vice versa merits intensive study.
4.4.2. Organic Matter
The importance of organic matter, native or added, in influencing the persistence of pesticides in the environment is well established. Microbial activity
in a soil is very intimately associated with its organic matter content. Most
tropical soils have low organic matter content due to its rapid mineralization.
The addition of organic materials--a common practice to raise fertility, especially
of tropical soils--increases the microbial activity and in turn accelerates the
degradation of several pesticides. The higher the organic matter content in the
soil, the more rapid was the degradation of chlorinated hydrocarbon insecticides,
DDT, endrin, heptachlor, and four isomers (<<, J3, 8, 'Y) of HCH in flooded
Philippine soils (Castro and Yoshida, 1974). The addition of organic sources
such as rice straw and cellulose to flooded soils with low organic matter content
further accelerated the degradation of these insecticides, but the stimulatory effect
of added organic matter was less pronounced in soils with high native organic
matter content. Similarly, degradation of J3- and 'Y-HCH (Siddaramappa and
Sethunathan, 1975), endrin (Gowda and Sethunathan, 1977), and carbofuran
(Venkateswarlu and Sethunathan, 1979) in flooded soils proceeded much faster
in the presence of rice straw than in its absence. The effect of added organic
matter on the degradation of parathion in flooded soils was not always stimulatory
and was governed by the pathway of its degradation (Sethunathan, 1973; Rajaram
and Sethunathan, 1975). Thus the addition of organic sources to flooded soils
stimulated the nitro group reduction of parathion, but its hydrolysis in a flooded
soil that was inoculated with a parathion-hydrolyzing enrichment culture was
almost completely inhibited by organic matter amendment. The factor inhibiting
parathion hydrolysis was a water-soluble and low-molecular-weight substance(s)
formed during anaerobic decomposition of organic matter in flooded soil (Rajaram and Sethunathan, 1976; Rajaram et al .• 1978), possibly aliphatic and/or
phenolic acids which accumulate in flooded soils amended with organic matter.
4.4.3. Redox Conditions
The redox status of a soil as an important parameter in the environmental
fate of pesticides was demonstrated very recently. The redox potentials of most
soils especially rich in native organic matter content drop within a few days after
105
1972). In recent years, there has been serious concern over the significant formation of soil-bound residues from soil-applied pesticides, especially under anaerobic conditions. The practical feasibility of accelerating the complete destruction of soil-bound residues of pesticides by returning the anaerobic system to
aerobic conditions or vice versa merits intensive study.
4.4.2. Organic Matter
The importance of organic matter, native or added, in influencing the persistence of pesticides in the environment is well established. Microbial activity
in a soil is very intimately associated with its organic matter content. Most
tropical soils have low organic matter content due to its rapid mineralization.
The addition of organic materials--a common practice to raise fertility, especially
of tropical soils--increases the microbial activity and in turn accelerates the
degradation of several pesticides. The higher the organic matter content in the
soil, the more rapid was the degradation of chlorinated hydrocarbon insecticides,
DDT, endrin, heptachlor, and four isomers (<<, J3, 8, 'Y) of HCH in flooded
Philippine soils (Castro and Yoshida, 1974). The addition of organic sources
such as rice straw and cellulose to flooded soils with low organic matter content
further accelerated the degradation of these insecticides, but the stimulatory effect
of added organic matter was less pronounced in soils with high native organic
matter content. Similarly, degradation of J3- and 'Y-HCH (Siddaramappa and
Sethunathan, 1975), endrin (Gowda and Sethunathan, 1977), and carbofuran
(Venkateswarlu and Sethunathan, 1979) in flooded soils proceeded much faster
in the presence of rice straw than in its absence. The effect of added organic
matter on the degradation of parathion in flooded soils was not always stimulatory
and was governed by the pathway of its degradation (Sethunathan, 1973; Rajaram
and Sethunathan, 1975). Thus the addition of organic sources to flooded soils
stimulated the nitro group reduction of parathion, but its hydrolysis in a flooded
soil that was inoculated with a parathion-hydrolyzing enrichment culture was
almost completely inhibited by organic matter amendment. The factor inhibiting
parathion hydrolysis was a water-soluble and low-molecular-weight substance(s)
formed during anaerobic decomposition of organic matter in flooded soil (Rajaram and Sethunathan, 1976; Rajaram et al .• 1978), possibly aliphatic and/or
phenolic acids which accumulate in flooded soils amended with organic matter.
4.4.3. Redox Conditions
The redox status of a soil as an important parameter in the environmental
fate of pesticides was demonstrated very recently. The redox potentials of most
soils especially rich in native organic matter content drop within a few days after