Microbial Degradation of Pesticides in Tropical Soils
107
tropics and should favor pesticide loss through volatilization and increased microbial activity. Unfortunately, much less is known on the relationship of temperature to pesticide behavior in the tropical environments than in the temperate
zone. In one instance, however, 'Y-HCH was shown to undergo more rapid
decomposition in a flooded soil at 35°C than at 25°C (Yoshida and Castro, 1970),
possibly due to increased microbial activity and a rapid decrease in the redox
potential at the higher temperature (Cho andPonnamperuma, 1971). Clearly,
pesticides may undergo more rapid decomposition in a tropical soil than in a
temperate soil.
4.4.6. Sorption-Desorption
Sorption-
matter was the most important single factor responsible for the sorption of
parathion (Wahid and Sethunathan, 1978) and three isomers (n, ~, and 'Y) of
HCH (Wahid and Sethunathan, 1979a). In soils oxidized with H20 2 , clay and
free iron oxides were implicated in sorption. Up to a 2% organic matter level,
sorption of parathion was a result of an interaction between organic and inorganic
surfaces; beyond a 2% organic matter level, the role of inorganic soil constituents
was apparently masked, a finding attributed to "organic masking" of the inorganic
surfaces. Soils prereduced by flooding sorbed less lindane than the aerobic soil,
while desorption was not affected by soil anaerobiosis (Wahid and Sethunathan,
1980).
4.4.7. Mineral Constituents
Direct metabolism does not always account for widespread occurrence of
reductive dechlorination of DDT to DDD in anaerobic environments since this
reaction readily occurred even in oxygen-free sterile systems containing ferrous
compounds (Glass, 1972; Parr and Smith, 1974). Glass (1972) proposed a mechanism for conversion of DDT to DDD whereby electrons furnished by the reduced
organic substrate were transferred to DDT molecule via ferrous ion, thus initiating
a free radical reaction. The addition of ferrous sulfate to a flooded soil rich in
organic matter content led to more extensive metabolism of yet another insecticide, parathion, with the formation of six breakdown products including its
amino analog in amended soil and only three products in unamended soil; but
this effect was noticed also with other sulfate salts, viz., MgS04, MnS04, and
K2S04 (Rao and Sethunathan, 1979). Indeed sulfate rather than ferrous ion was
implicated in the extensive metabolism of parathion in flooded soils.
More in-depth studies with acid sulfate soils confirmed the role of sulfate
in parathion metabolism (Wahid and Sethunathan, 1979b). Parathion was rapidly
converted to aminoparathion in acid sulfate soil and in soils with low sulfate
107
tropics and should favor pesticide loss through volatilization and increased microbial activity. Unfortunately, much less is known on the relationship of temperature to pesticide behavior in the tropical environments than in the temperate
zone. In one instance, however, 'Y-HCH was shown to undergo more rapid
decomposition in a flooded soil at 35°C than at 25°C (Yoshida and Castro, 1970),
possibly due to increased microbial activity and a rapid decrease in the redox
potential at the higher temperature (Cho andPonnamperuma, 1971). Clearly,
pesticides may undergo more rapid decomposition in a tropical soil than in a
temperate soil.
4.4.6. Sorption-Desorption
Sorption-
matter was the most important single factor responsible for the sorption of
parathion (Wahid and Sethunathan, 1978) and three isomers (n, ~, and 'Y) of
HCH (Wahid and Sethunathan, 1979a). In soils oxidized with H20 2 , clay and
free iron oxides were implicated in sorption. Up to a 2% organic matter level,
sorption of parathion was a result of an interaction between organic and inorganic
surfaces; beyond a 2% organic matter level, the role of inorganic soil constituents
was apparently masked, a finding attributed to "organic masking" of the inorganic
surfaces. Soils prereduced by flooding sorbed less lindane than the aerobic soil,
while desorption was not affected by soil anaerobiosis (Wahid and Sethunathan,
1980).
4.4.7. Mineral Constituents
Direct metabolism does not always account for widespread occurrence of
reductive dechlorination of DDT to DDD in anaerobic environments since this
reaction readily occurred even in oxygen-free sterile systems containing ferrous
compounds (Glass, 1972; Parr and Smith, 1974). Glass (1972) proposed a mechanism for conversion of DDT to DDD whereby electrons furnished by the reduced
organic substrate were transferred to DDT molecule via ferrous ion, thus initiating
a free radical reaction. The addition of ferrous sulfate to a flooded soil rich in
organic matter content led to more extensive metabolism of yet another insecticide, parathion, with the formation of six breakdown products including its
amino analog in amended soil and only three products in unamended soil; but
this effect was noticed also with other sulfate salts, viz., MgS04, MnS04, and
K2S04 (Rao and Sethunathan, 1979). Indeed sulfate rather than ferrous ion was
implicated in the extensive metabolism of parathion in flooded soils.
More in-depth studies with acid sulfate soils confirmed the role of sulfate
in parathion metabolism (Wahid and Sethunathan, 1979b). Parathion was rapidly
converted to aminoparathion in acid sulfate soil and in soils with low sulfate