20 Metabolism of Herbicides in Soils
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PHILIP C. K E A R N E Y Crops Research Division, U.S. Department of Agriculture, Beltsville, M d .
Soil microorganisms are responsible for the metabolic degradation of many organic pesticides. From products found in soils or in culture solutions of selected soil microorganisms, pathways of decomposition have been proposed for the phenylurea, phenylcarbamate, s-triazine, chlorinated aliphatic acid, and phenoxyalkanoic acid herbicides. Reactions associated with herbicide metabolism include N-dealkylation of the N,N-dimethyl-N'-phenylureas, ester or amide hydrolysis of the phenylcarbamates, side-chain degradation of the s-triazines, dehalogenation of the chlorinated aliphatic acids, and beta oxidation or ether cleavage of the phenoxyalkanoic acids. Enzymes responsible for the hydrolysis of the phenylcarbamates and dehalogenation of 2,2-dichloropropionic acid have been isolated and characterized.
T h e metabolic fate of organic pesticides in soils is currently of interest in relation to the over-all aspects of residues i n our environment. Several processes i n soils tend to dissipate herbicide residues—namely, volatilization, photodecomposition, leaching, adsorption, and microbial degradation. This paper concerns itself entirely with the microbiological aspects of decomposition. Specifically, it deals with recent biochemical studies on five major classes of herbicides: (1) the phenylureas, (2) the phenylcarbamates, (3) the s-triazines, (4) the chlorinated aliphatic acids, and (5) the phenoxyalkanoic acids. Primary emphasis is directed toward metabolic pathways elucidated i n soils or i n pure cultures of selected soil microorganisms. Where possible, specific details on the enzymes responsible for the initial decomposition reaction are discussed. Phenylureas
The phenylurea herbicides have the same general structure as fenuron, shown i n Figure 1. If a chlorine is substituted i n the 4-position of 250 Rosen and Kraybill; Organic Pesticides in the Environment Advances in Chemistry; American Chemical Society: Washington, DC, 1966.
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Metabolism of Herbicides
the ring, the compound is called monuron. If chlorines are placed i n the 3- and 4-positions of the ring, it is called diuron. Inspection of the Ν,Ν-dimethyl-N'-phenylureas might suggest that the classical urease type reaction would cleave this molecule directly to aniline, C0 , and d i methylamine. Although urease is widely distributed in microorganisms, the enzyme shows an absolute specificity for urea. Sumner (39) ex amined many substrates, including the substituted ureas and related compounds, but reported no hydrolysis of these various substrates by urease.
Downloaded by NANYANG TECHNOLOGICAL UNIV on September 25, 2017 | http://pubs.acs.org Publication Date: June 1, 1966 | doi: 10.1021/ba-1966-0060.ch020
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Figure 1. Chemical structure of 3-phenyl-l, 1-dimethylurea (fenuron) —a typical phenylurea herbicide
Recent reports from scientists working independently on phenylurea degradation in soils clearly show that dealkylation of the methyl groups probably precedes hydrolysis of the urea linkage. For example, Geissbuhler et al. (11) working with carbonyl labeled N'-(4-chlorophenoxy)phenyl-N,2V-dimethylurea ( chloroxuron ) identified the monomethyl and completely demethylated urea in soil extraction studies. The meta bolic pathway shown in Figure 2 was proposed. Demethylation of the herbicides monuron and diuron has also been reported recently. Decom position of diuron proceeds by removal of first one and then the other methyl group, followed by hydrolysis of the urea to aniline (7). A similar process must be occurring with monuron since the monalkyl, urea, and aniline derivatives have been detected in soils (38). Disappearance of the phenylurea herbicides from soils is caused partly by soil microbiological activity. Several bacteria (Xanthomonas sp., Sarcina sp., Bacillus, and two species of Pseudomonas) and fungi (species Pencillium and Aspergillus) have been reported to utilize monuron as a sole source of carbon in an agar medium (17). A bac terium of the Pseudomonas species isolated from a nonherbicide-treated Brookston soil was only capable of oxidizing monuron in the presence of exogenous growth factors (18). Metabolic products resulting from microbial decomposition of monuron by these soil microorganisms were not examined. Although a cell-free system capable of degrading the phenylureas has not yet been reported, an enzyme system from rat liver microsomes has been isolated which requires reduced triphosphopyridine nucleotide
Rosen and Kraybill; Organic Pesticides in the Environment Advances in Chemistry; American Chemical Society: Washington, DC, 1966.
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ORGANIC PESTICIDES IN THE ENVIRONMENT
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Downloaded by NANYANG TECHNOLOGICAL UNIV on September 25, 2017 | http://pubs.acs.org Publication Date: June 1, 1966 | doi: 10.1021/ba-1966-0060.ch020
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