102
N. Sethunathan et al.
water of rice fields ranged from 7.0 to as high as 9.5 (Seiber et al., 1978;
Siddaramappa et al., 1978). For carbaryl, evidence for microbial degradation
was more conclusive when a strain of Achromobacter sp. utilized it as a sole
carbon source with concomitant formation of I-naphthol, hydroquinone, catechol, and pyruvate (Sud et al., 1972).
Carbofuran phenol, the major product of carbofuran metabolism in flooded
soils, accumulated under continued anaerobiosis (Venkateswarlu and Sethunathan, 1979). Concomitantly, evolution of 14C02 from the aromatic ring in carbofuran was almost negligible, accounting for less than 0.9% of the ring_ 14 C
even after 40 days of flooding as compared to 27% released from the carbonyl14C (Venkateswarlu and Sethunathan, 1979).
Another insecticide, Baygon, was converted to 2-isopropyl phenol by a soil
bacterium, Pseudomonas sp. (Gupta et al., 1975).
4.3.4. Fungicides
Information on the fate of relatively few fungicides in tropical areas is
available, although their use is far less extensive than that of insecticides.
A widely used carboxanilide fungicide, carboxin, was converted to sulfoxide
in all five soils under nonflooded conditions, but further degradation to aminophenol, ammonium, and nitrite occurred only in two soils (Balasubramanya,
1977). Under flooded conditions, the reaction ceased at the sulfoxide stage. The
degradation of carboxin to sulfoxide proceeded in both sterile and nonsterile
soils and past sulfoxide only in nonsterile soils. However, a strain of Pseudomonas sp., isolated by the soil perfusion technique, converted carboxin to sulfoxide, sulfone, 2-(vinylsulfonyl) acetanilide, 2-(2-hydroxyethylsulfonyl) acetic
acid, aminophenol, ammonium, and nitrite in that order (Balasubramanya et al.,
1980). Likewise, oxycarboxin was decomposed by the same bacterium first to
2-(vinylsulfonyl) acetanilide and thereafter by the same pathways as for carboxin.
A soil bacterium, Pseudomonas /ragi, decomposed dexon through cometabolism to several products; the major product was identified as N,N-dimethylp-phenylenediamine (DMPDA); dexon applied to a nonftooded soil disappeared
in 60 days, and DMPDA was the major product (Karanth and Vasantharajan,
1973; Karanth et al., 1974).
In aerobic soils, thiram was converted to dimethylamine, carbon disulfide,
and an unidentified divalent sulfur compound (Raghu et al., 1974, 1975). The
unidentified compound was subsequently characterized as copper dimethyl dithiocarbamate (CuDDC2) (Kumarasamy and Raghu, 1976). DDC-a-aminobutyric acid was the major product of its metabolism by a strain of Pseudomonas
sp. The breakdown products of thiram, particularly CuDDC2, were highly fungitoxic, and this would explain, at least in part, the prolonged fungitoxicity of
thiram despite its instability in soil. Also, ziram was decomposed by a soil
N. Sethunathan et al.
water of rice fields ranged from 7.0 to as high as 9.5 (Seiber et al., 1978;
Siddaramappa et al., 1978). For carbaryl, evidence for microbial degradation
was more conclusive when a strain of Achromobacter sp. utilized it as a sole
carbon source with concomitant formation of I-naphthol, hydroquinone, catechol, and pyruvate (Sud et al., 1972).
Carbofuran phenol, the major product of carbofuran metabolism in flooded
soils, accumulated under continued anaerobiosis (Venkateswarlu and Sethunathan, 1979). Concomitantly, evolution of 14C02 from the aromatic ring in carbofuran was almost negligible, accounting for less than 0.9% of the ring_ 14 C
even after 40 days of flooding as compared to 27% released from the carbonyl14C (Venkateswarlu and Sethunathan, 1979).
Another insecticide, Baygon, was converted to 2-isopropyl phenol by a soil
bacterium, Pseudomonas sp. (Gupta et al., 1975).
4.3.4. Fungicides
Information on the fate of relatively few fungicides in tropical areas is
available, although their use is far less extensive than that of insecticides.
A widely used carboxanilide fungicide, carboxin, was converted to sulfoxide
in all five soils under nonflooded conditions, but further degradation to aminophenol, ammonium, and nitrite occurred only in two soils (Balasubramanya,
1977). Under flooded conditions, the reaction ceased at the sulfoxide stage. The
degradation of carboxin to sulfoxide proceeded in both sterile and nonsterile
soils and past sulfoxide only in nonsterile soils. However, a strain of Pseudomonas sp., isolated by the soil perfusion technique, converted carboxin to sulfoxide, sulfone, 2-(vinylsulfonyl) acetanilide, 2-(2-hydroxyethylsulfonyl) acetic
acid, aminophenol, ammonium, and nitrite in that order (Balasubramanya et al.,
1980). Likewise, oxycarboxin was decomposed by the same bacterium first to
2-(vinylsulfonyl) acetanilide and thereafter by the same pathways as for carboxin.
A soil bacterium, Pseudomonas /ragi, decomposed dexon through cometabolism to several products; the major product was identified as N,N-dimethylp-phenylenediamine (DMPDA); dexon applied to a nonftooded soil disappeared
in 60 days, and DMPDA was the major product (Karanth and Vasantharajan,
1973; Karanth et al., 1974).
In aerobic soils, thiram was converted to dimethylamine, carbon disulfide,
and an unidentified divalent sulfur compound (Raghu et al., 1974, 1975). The
unidentified compound was subsequently characterized as copper dimethyl dithiocarbamate (CuDDC2) (Kumarasamy and Raghu, 1976). DDC-a-aminobutyric acid was the major product of its metabolism by a strain of Pseudomonas
sp. The breakdown products of thiram, particularly CuDDC2, were highly fungitoxic, and this would explain, at least in part, the prolonged fungitoxicity of
thiram despite its instability in soil. Also, ziram was decomposed by a soil