Microbial Degradation of Pesticides in Tropical Soils
93
in contrast to its reported residual life of 2-10 years in more aerobic upland soils
(Kearney et ai., 1969). Moreover, even repeated additions of this insecticide to
the same submerged field plot did not lead to any significant residue buildup in
the soil. 'V-HeH levels in the field soils that received a total of 48 and 12 kg!
ha in four crop seasons were comparable with values of 0.018 and 0.014 ppm,
respectively, when analyzed 30 days after the last application. Irrespective of
the dose of HeH applied, within 20-30 days after its application the residues
declined to less than 0.02 ppm in both soil and water. Likewise, accumulation
of diazinon was negligible upon its repeated applications to submerged rice fields
in the Philippines (Sethunathan et ai., 1971).
In a long-term field experiment, residues of DDT and dieldrin decreased to
20 and 30% of the original levels in the fall and winter months in an upland
soil under subtropical conditions in Taiwan; no accumulation of these residues
occurred with further additions in spring and summer (Talekar et ai., 1977).
Also, fonofos, phorate, and carbofuran, which decreased to 8, 0.4, and 32% of
the original levels in the fall and winter months, disappeared even faster during
the hot and rainy conditions of spring and summer. Application of carbofuran
and chlordimeform to the root zone in the reduced soil layer of flooded rice
paddies, in lieu of the conventional practice of broadcasting to the standing water
or spraying to the foliage, reduced the concentration of the insecticides in the
flood water and thereby the surface losses (Aquino and Pathak, 1976). Besides,
root-zone application of the insecticides increased their uptake by the rice plants
and stretched their efficiency in controlling brown planthoppers over a prolonged
duration. A meaningful conclusion from most field studies essentially involving
the monitoring of only the parent compounds, and not their degradation products,
is difficult since transport losses through volatilization are considerable under
tropical and subtropical conditions. Under flooded conditions, as in paddy fields,
substantial volatilization of pesticides such as HeH may occur even more readily
(Siddaramappa and Sethunathan, 1976). Also, in the tropics, increased loss of
soil-applied pesticides can occur through translocation from the roots to the
foliage and subsequent transpiration from the leaf surface, especially under conditions of excess water, as in paddy ecosystems. More recently, Siddararnappa
and Watanabe (1979) showed, by radioautography, fairly rapid translocation of
carbofuran in significant amounts to the foliage of rice seedlings after its application to flooded soils. However, actual measurements are not available to indicate the quantities involved in such losses under tropical field conditions.
4.3. MICROBIAL DEGRADA TlON
Pesticides applied to the environment are acted upon by several forcesphysical, chemical and biological-independently or in combination. Since the
classical demonstration of microbial breakdown of the hormonal herbicide 2,4-
93
in contrast to its reported residual life of 2-10 years in more aerobic upland soils
(Kearney et ai., 1969). Moreover, even repeated additions of this insecticide to
the same submerged field plot did not lead to any significant residue buildup in
the soil. 'V-HeH levels in the field soils that received a total of 48 and 12 kg!
ha in four crop seasons were comparable with values of 0.018 and 0.014 ppm,
respectively, when analyzed 30 days after the last application. Irrespective of
the dose of HeH applied, within 20-30 days after its application the residues
declined to less than 0.02 ppm in both soil and water. Likewise, accumulation
of diazinon was negligible upon its repeated applications to submerged rice fields
in the Philippines (Sethunathan et ai., 1971).
In a long-term field experiment, residues of DDT and dieldrin decreased to
20 and 30% of the original levels in the fall and winter months in an upland
soil under subtropical conditions in Taiwan; no accumulation of these residues
occurred with further additions in spring and summer (Talekar et ai., 1977).
Also, fonofos, phorate, and carbofuran, which decreased to 8, 0.4, and 32% of
the original levels in the fall and winter months, disappeared even faster during
the hot and rainy conditions of spring and summer. Application of carbofuran
and chlordimeform to the root zone in the reduced soil layer of flooded rice
paddies, in lieu of the conventional practice of broadcasting to the standing water
or spraying to the foliage, reduced the concentration of the insecticides in the
flood water and thereby the surface losses (Aquino and Pathak, 1976). Besides,
root-zone application of the insecticides increased their uptake by the rice plants
and stretched their efficiency in controlling brown planthoppers over a prolonged
duration. A meaningful conclusion from most field studies essentially involving
the monitoring of only the parent compounds, and not their degradation products,
is difficult since transport losses through volatilization are considerable under
tropical and subtropical conditions. Under flooded conditions, as in paddy fields,
substantial volatilization of pesticides such as HeH may occur even more readily
(Siddaramappa and Sethunathan, 1976). Also, in the tropics, increased loss of
soil-applied pesticides can occur through translocation from the roots to the
foliage and subsequent transpiration from the leaf surface, especially under conditions of excess water, as in paddy ecosystems. More recently, Siddararnappa
and Watanabe (1979) showed, by radioautography, fairly rapid translocation of
carbofuran in significant amounts to the foliage of rice seedlings after its application to flooded soils. However, actual measurements are not available to indicate the quantities involved in such losses under tropical field conditions.
4.3. MICROBIAL DEGRADA TlON
Pesticides applied to the environment are acted upon by several forcesphysical, chemical and biological-independently or in combination. Since the
classical demonstration of microbial breakdown of the hormonal herbicide 2,4-