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Floyd M. Ashton
changes in the soil that may alter the degradation of organic molecules by
nonbiological means. This also appears to be true for methyl bromide fumigation.
Sterilization by gamma irradiation is considered to be less destructive to the
chemical and physical characteristics of soils than any other method. Fumigation
and specific microbial inhibitors have also been used. Microbial enrichment
techniques as well as isolation and identification of the microorganisms responsible for the degradation of the herbicide can also provide valuable information.
5.3.1. Biodegradation
This discussion of the biodegradation of herbicides will be primarily limited
to microbe-induced changes in the soil, the major site of herbicide biodegradation
in the environment. Previous presentations in this seminar have covered other
aspects of the biodegradation of pesticides. The microbial degradation of pesticides has already been described in Chapter 3. However, insecticides have been
used as examples. The other significant site of biodegradation, the higher plant,
has been previously described in Chapter 2, and the biodegradation of pesticides
by animals has been reviewed in Chapter 1. The ingestion of herbicides by
nonsoil animals under natural conditions is limited and usually restricted to
grazing animals. Several reviews have been published on the biodegradation of
herbicides (Ashton and Crafts, 1973; Casida and Lykken, 1969; Frank, 1970;
Frear and Shimabukuro, 1970; Hamaker, 1972; Kaufman, 1974; Kaufman and
Kearney, 1976; Kearney, 1970; Kearney and Kaufman, 1975; Meikle, 1972).
Herbicides: Chemistry, Degradation, and Mode of Action (Kearney and Kaufman, 1975) is probably the most comprehensive and presents detailed information
on the degradation of many herbicides by plants, animals, and microorganisms.
The rate of microbial degradation of herbicides under field conditions depends upon a number of interacting environmental conditions. In general, the
conditions that promote the growth of the microorganisms responsible for the
degradation accelerate the rate, and those that inhibit the growth of these microorganisms reduce the rate. These soil factors include temperature, pH, cation
exchange capacity, fertility, structure, type, moisture content, organic matter,
O2, and CO2 , as well as numerous other parameters.
Microorganisms degrade herbicides by a number of biochemical reactions,
including oxidation, reduction, hydrolysis, hydroxylation, decarboxylation,
deamination, dehalogenation, dethioaction, dealkylation, dealkyoxylation,
dealkythiolation, and conjugation with normal metabolites, usually sugars, amino
acids, or peptides (i.e., glutathione) (Kaufman, 1974; Kaufman and Kearney,
1976; Kaufman et at., 1976; Kearney and Kaufman, 1975; Sheets and Kaufman,
1970). Specific pathways of degradation and the intermediates formed by the
biodegradation of herbicides have been presented in numerous reviews (Ashton
and Crafts, 1973; Frear and Shimabukuro, 1970; Hamaker, 1972; Kaufman,
Floyd M. Ashton
changes in the soil that may alter the degradation of organic molecules by
nonbiological means. This also appears to be true for methyl bromide fumigation.
Sterilization by gamma irradiation is considered to be less destructive to the
chemical and physical characteristics of soils than any other method. Fumigation
and specific microbial inhibitors have also been used. Microbial enrichment
techniques as well as isolation and identification of the microorganisms responsible for the degradation of the herbicide can also provide valuable information.
5.3.1. Biodegradation
This discussion of the biodegradation of herbicides will be primarily limited
to microbe-induced changes in the soil, the major site of herbicide biodegradation
in the environment. Previous presentations in this seminar have covered other
aspects of the biodegradation of pesticides. The microbial degradation of pesticides has already been described in Chapter 3. However, insecticides have been
used as examples. The other significant site of biodegradation, the higher plant,
has been previously described in Chapter 2, and the biodegradation of pesticides
by animals has been reviewed in Chapter 1. The ingestion of herbicides by
nonsoil animals under natural conditions is limited and usually restricted to
grazing animals. Several reviews have been published on the biodegradation of
herbicides (Ashton and Crafts, 1973; Casida and Lykken, 1969; Frank, 1970;
Frear and Shimabukuro, 1970; Hamaker, 1972; Kaufman, 1974; Kaufman and
Kearney, 1976; Kearney, 1970; Kearney and Kaufman, 1975; Meikle, 1972).
Herbicides: Chemistry, Degradation, and Mode of Action (Kearney and Kaufman, 1975) is probably the most comprehensive and presents detailed information
on the degradation of many herbicides by plants, animals, and microorganisms.
The rate of microbial degradation of herbicides under field conditions depends upon a number of interacting environmental conditions. In general, the
conditions that promote the growth of the microorganisms responsible for the
degradation accelerate the rate, and those that inhibit the growth of these microorganisms reduce the rate. These soil factors include temperature, pH, cation
exchange capacity, fertility, structure, type, moisture content, organic matter,
O2, and CO2 , as well as numerous other parameters.
Microorganisms degrade herbicides by a number of biochemical reactions,
including oxidation, reduction, hydrolysis, hydroxylation, decarboxylation,
deamination, dehalogenation, dethioaction, dealkylation, dealkyoxylation,
dealkythiolation, and conjugation with normal metabolites, usually sugars, amino
acids, or peptides (i.e., glutathione) (Kaufman, 1974; Kaufman and Kearney,
1976; Kaufman et at., 1976; Kearney and Kaufman, 1975; Sheets and Kaufman,
1970). Specific pathways of degradation and the intermediates formed by the
biodegradation of herbicides have been presented in numerous reviews (Ashton
and Crafts, 1973; Frear and Shimabukuro, 1970; Hamaker, 1972; Kaufman,