Dinitroaniline Herbicides in Soils
1976
Helling, Charles S.
The behavior and fate of dinitroaniline herbicides in the soil environment is reviewed. This rapidly expanding class of herbicides is used particularly for pre-emergence control of annual grasses and small-seeded broadleaf weeds in cotton (Gossvpium hirsutum L.), soybeans [Glycine max (L.) Merr.], and peanuts (Arachis hypogaea L.). All dinitroaniline herbicides in use are 4-alkyl (or -sulfonyl)-2,6-dinitro-N,N-(mono- or disubstituted) anilines, characterized by very low water solubility and basicity. Volatilization from surface applications to moist soil is substantial for trifluralin, benefin, and profluralin, but is low for nitralin and oryzalin. In water and on dry soil, dinitroanilines exposed to sunlight degrade to photoproducts that are often polar and include dealkylated and cyclic intermediates. Such photodecomposition greatly reduces bioaccumulation of dinitroanilines by aquatic orgahisms. Physico-chemical behavior of dinitroanilines in soils or sediments is influenced by volatilization and by strong soil adsorption. Nitralin and oryzalin may leach slightly, but dinitroanilines are essentially immobile. Short range diffusion may be important for herbicide efficacy or loss from the soil surface. Both solution and vapor phase transport components affect trifluralin diffusion; the extent and balance of diffusion are strongly affected by soil temperature, moisture, and bulk density. Phytotoxicity apparently cannot be correlated with adsorption strength, but may be related to volatility. Physical loss from soil, unless incorporated, is by sediment runoff and by volatilization. Dinitroaniline herbicides degrade more rapidly in anaerobic, than aerobic soil. Metabolites are numerous and often resemble those produced by the photolytic mechanisms of dealkylation and cyclization. A gradually increasing quantity of residue becomes associated with soil organic matter, as bound residue. For normal use rates, persistence in soils ranges from about 2–3 months, for oryzalin and dinitramine, to 5–6 months for benefin and trifluralin. Persistence increases with decreasing soil temperature and moisture content; seasonal carryover sometimes occurs. Limited evidence suggests that dinitroanilines produce no lasting effect on soil microbial processes.
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