106
N. Sethunathan et al.
flooding, and this drop is further accentuated in the presence of added organic
sources.
The extent of degradation of 13- and "V-HCH in different soils was related
to the redox potentials attained by the soils following flooding (Siddaramappa
and Sethunathan, 1975). The degradation of these isomers occurred only in
microbially active soils capable of attaining potentials of - 40 to - 100m V
within a few days after flooding; degradation of I3-HCH proceeded at a potential
lower than that required for "V-HCH degradation. These observations are in
substantial agreement with the reported molecular orbital calculations for the
isomers of HCH (Block and Newland, 1975). Likewise, endrin degradation was
favored by a potential of < -120 mV (Gowda and Sethunathan, 1976). Also,
free oxygen, potassium nitrate, and manganese oxide, all known for their exceptional capacity to stabilize the potentials at high levels even under flooded
conditions, retarded the degradation of "V-HCH in flooded soils (Yoshida and
Castro, 1970). This would explain the relatively long persistence of this and
related chlorinated hydrocarbon insecticides in aerobic nonflooded soils. Low
potentials would certainly favor the activity and proliferation of Clostridium sp.
and other anaerobes implicated in the degradation of HCH and related pesticides.
However, ring cleavage reactions are remarkably slow or virtually stopped at
low redox potentials.
4.4.4. Soil Acidity
The degradation of some pesticides, organophosphates and carbamates in
particular, is affected by the pH of the soil. Although most organosphosphates
undergo chemical hydrolysis under alkaline conditions, diazinon showed extreme
chemical instability in acid soils (Sethunathan and MacRae, 1969). Carbamates
are also rapidly hydrolyzed under alkaline conditions, as reported for carbofuran
in natural ecosystems (Seiber et al., 1978), whereas carbofuran persisted in acid
soils (Venkateswarlu et al., 1977). The persistence of organochlorine insecticides
is seldom affected by soil pH. However, HCH and DDT decomposed much
faster in alkaline soils (pH 9.5) than in soils with lower pH (Chawla and Chopra,
1967). In addition to the direct effects on the organic molecules, the pH may
also have indirect influence on their persistence, because of its effect on microbial
activities and sorption-desorption. The degradation of HCH isomers was slow
in an acid sulfate soil with extremely low pH even under flooded conditions, a
finding attributed to low microbial activity (Siddaramappa and Sethunathan,
1975).
4.4.5. Soil Temperature
Temperature is probably an important factor affecting the persistence of
pesticides in the soil, especially in the tropics. High temperatures exist in the
N. Sethunathan et al.
flooding, and this drop is further accentuated in the presence of added organic
sources.
The extent of degradation of 13- and "V-HCH in different soils was related
to the redox potentials attained by the soils following flooding (Siddaramappa
and Sethunathan, 1975). The degradation of these isomers occurred only in
microbially active soils capable of attaining potentials of - 40 to - 100m V
within a few days after flooding; degradation of I3-HCH proceeded at a potential
lower than that required for "V-HCH degradation. These observations are in
substantial agreement with the reported molecular orbital calculations for the
isomers of HCH (Block and Newland, 1975). Likewise, endrin degradation was
favored by a potential of < -120 mV (Gowda and Sethunathan, 1976). Also,
free oxygen, potassium nitrate, and manganese oxide, all known for their exceptional capacity to stabilize the potentials at high levels even under flooded
conditions, retarded the degradation of "V-HCH in flooded soils (Yoshida and
Castro, 1970). This would explain the relatively long persistence of this and
related chlorinated hydrocarbon insecticides in aerobic nonflooded soils. Low
potentials would certainly favor the activity and proliferation of Clostridium sp.
and other anaerobes implicated in the degradation of HCH and related pesticides.
However, ring cleavage reactions are remarkably slow or virtually stopped at
low redox potentials.
4.4.4. Soil Acidity
The degradation of some pesticides, organophosphates and carbamates in
particular, is affected by the pH of the soil. Although most organosphosphates
undergo chemical hydrolysis under alkaline conditions, diazinon showed extreme
chemical instability in acid soils (Sethunathan and MacRae, 1969). Carbamates
are also rapidly hydrolyzed under alkaline conditions, as reported for carbofuran
in natural ecosystems (Seiber et al., 1978), whereas carbofuran persisted in acid
soils (Venkateswarlu et al., 1977). The persistence of organochlorine insecticides
is seldom affected by soil pH. However, HCH and DDT decomposed much
faster in alkaline soils (pH 9.5) than in soils with lower pH (Chawla and Chopra,
1967). In addition to the direct effects on the organic molecules, the pH may
also have indirect influence on their persistence, because of its effect on microbial
activities and sorption-desorption. The degradation of HCH isomers was slow
in an acid sulfate soil with extremely low pH even under flooded conditions, a
finding attributed to low microbial activity (Siddaramappa and Sethunathan,
1975).
4.4.5. Soil Temperature
Temperature is probably an important factor affecting the persistence of
pesticides in the soil, especially in the tropics. High temperatures exist in the