Microbial Degradation of Pesticides in Tropical Soils
103
bacterium essentially to water-soluble metabolites, which were not identified
(Raghu et al., 1976).
4.4. FACTORS INFLUENCING PERSISTENCE OF PESTICIDES
Soil properties known to influence the microbial activities and thereby the
persistence of pesticides in tropical areas include moisture, organic matter content, redox status, acidity, temperature, sorption-desorption, and mineral constituents. Also, there is evidence that the persistence of a pesticide is altered
significantly in the presence of another pesticide.
4.4. 1. Soil Moisture
Soil moisture has a profound and direct effect on the proliferation of microorganisms and their activities. Water that competes for adsorption sites with
pesticides determines the amount of pesticides available for microbial attack.
Moreover, the aeration of the soil is closely linked with its moisture levels.
Following heavy rain or flooding as in rice culture, the bulk of the soil is reduced
and anaerobic (obligate or facultative) microorganisms become predominant. It
is now well known that soil flooding accelerates the degradation of at least some
persistent pesticides as discussed under Section 4.3.1. The order of persistence
of pesticides in aerobic systems generally follows the order: chlorinated hydrocarbons> organophosphtes = carbamates (Kearney et al., 1969). But this
generalization may not always be applicable to anaerobic systems because of
the susceptibility of several, if not all, chlorinated hydrocarbon pesticides to
anaerobic biodegradation, with persistence curves as low as or lower than those
of selected organophosphate and carbamate pesticides (Fig. 4.3). For instance,
HCH, a chlorinated hydrocarbon, almost completely disappeared within 20 days
of its application to microbially active soils under flooded conditions (Siddaramappa and Sethunathan, 1975), but degradation of carbofuran, a carbamate, in
these soils was relatively slow during the same period (Venkateswarlu et al.,
1977). This emphasizes an urgent need for a systematic evaluation of the persistence curves of more pesticides from different groups under both aerobic and
anaerobic conditions before generalizations on pesticide stability in the environment are made.
In many tropical areas characterized by intermittent heavy rain and dry
seasons, soils are subjected to alternate periods of flooding and drying with
concomitant increases in the activities of anaerobic and aerobic microorganisms,
respectively. Such alternate reduction and oxidation cycles in the soil could
provide a favorable environment for more extensive destruction of organic com-
103
bacterium essentially to water-soluble metabolites, which were not identified
(Raghu et al., 1976).
4.4. FACTORS INFLUENCING PERSISTENCE OF PESTICIDES
Soil properties known to influence the microbial activities and thereby the
persistence of pesticides in tropical areas include moisture, organic matter content, redox status, acidity, temperature, sorption-desorption, and mineral constituents. Also, there is evidence that the persistence of a pesticide is altered
significantly in the presence of another pesticide.
4.4. 1. Soil Moisture
Soil moisture has a profound and direct effect on the proliferation of microorganisms and their activities. Water that competes for adsorption sites with
pesticides determines the amount of pesticides available for microbial attack.
Moreover, the aeration of the soil is closely linked with its moisture levels.
Following heavy rain or flooding as in rice culture, the bulk of the soil is reduced
and anaerobic (obligate or facultative) microorganisms become predominant. It
is now well known that soil flooding accelerates the degradation of at least some
persistent pesticides as discussed under Section 4.3.1. The order of persistence
of pesticides in aerobic systems generally follows the order: chlorinated hydrocarbons> organophosphtes = carbamates (Kearney et al., 1969). But this
generalization may not always be applicable to anaerobic systems because of
the susceptibility of several, if not all, chlorinated hydrocarbon pesticides to
anaerobic biodegradation, with persistence curves as low as or lower than those
of selected organophosphate and carbamate pesticides (Fig. 4.3). For instance,
HCH, a chlorinated hydrocarbon, almost completely disappeared within 20 days
of its application to microbially active soils under flooded conditions (Siddaramappa and Sethunathan, 1975), but degradation of carbofuran, a carbamate, in
these soils was relatively slow during the same period (Venkateswarlu et al.,
1977). This emphasizes an urgent need for a systematic evaluation of the persistence curves of more pesticides from different groups under both aerobic and
anaerobic conditions before generalizations on pesticide stability in the environment are made.
In many tropical areas characterized by intermittent heavy rain and dry
seasons, soils are subjected to alternate periods of flooding and drying with
concomitant increases in the activities of anaerobic and aerobic microorganisms,
respectively. Such alternate reduction and oxidation cycles in the soil could
provide a favorable environment for more extensive destruction of organic com-