Degradation soil-applied herbicides

Seibert, K., F. Fuhr, and H.H. Cheng (1995). Experiments of the degradation of atrazine in the maize-rhizosphere. European Weed Research Society Symposium Theory and Practice of the Use of Soil Applied Herbicides. 

Walker, A., Welch, S.J. (1991) Enhanced degradation of some soil-applied herbicides. Weed Res. 31, 49-57. 

Enhanced biodegradation of pesticides has received considerable attention in recent years since it was first described by Audus (1) for the herbicide 2,4-D. Diphenamid, a soil applied herbicide, is also subject to enhanced biodegradation by soil microorganisms (2.31. Fungicides (4.51 and insecticides (6-91 are also subject to enhanced degradation by soil microorganisms. The carbamothioate herbicides are readily degraded by microbes (10.Ill and especially after repeated applications (12-191. ... 

Urea compounds are predominantly soil-applied herbicides and are thus metabolised in the microflora of the soil (Hill et al., 1955 Sheets and Crafts, 1957 Hance, 1967a, 1967b Geissbuhler, 1969). In soils with intensive microbial activity degradation is more rapid. 

Feng (1987) monitored the persistence and degradation of hexazinone in a silt loam soil 104 d after treatment of the herbicide. After 104 d, 66% of the hexazinone degraded via hydroxylation to form the major metabolite 3-(4-hydroxycyclohexyl)-6-(dimethylamino)-l-methyl-s-triazine-2,4(l//,3//)dione (30-50% yield). A minor metabolite, 3-cyclohexyl-6-(methylamino)-1-methyl-s-triazine-2,4(1//,3//)-dione, forming via demethylation, accounted for only 0-12% of the applied herbicide. Hexazinone and its metabolites were not detected in soils at depths of 15-30 cm. 

All herbicides degrade in soil, but at variable rates (Dawson et al, 1968 Rouchard et al, 2000). The rates of breakdown or deactivation of herbicides are related to a number of soil and environmental factors (Upchurch and Mason, 1962 Upchurch et al, 1966). Surface-applied herbicides volatilize at varying rates, dependent on their vapor pressure (Kearney et al, 1964). Some surface-applied herbicides also break down from ultraviolet light. 

Although the majority of an applied triazine herbicide remains in the surface soil where it controls weeds while degrading, soil movement and persistence has been well studied and documented. A variety of factors affect triazine runoff, including method of application, soil properties, type of tillage, and environmental conditions. Estimates of triazine amounts in runoff from agricultural fields vary widely, with the highest concentrations occurring in the first 2 months after application. 

One application of EPTC or butylate was sufficient to induce enhanced degradation of both herbicides applied the following year (19, 21). At Clay Center, EPTC degradation was most rapid in vernolate-history soil followed by EPTC-history soil (Figure 7). 

A number of substituted triazines are used as herbicides, and their biodegradation has been discussed in Chapter 10, Part 1. Treatment of soil contaminated with atrazine (2-chloro-4-(ethylamino)-6-isopropylamino-l,3,5-triazine) illustrated a number of significant features. Although the soil that was used had the potential for degradation, a laboratory experiment with Pseudomonas sp. strain ADP that had an established potential for atrazine degradation revealed important limitations. There was a substantial decline in the numbers of Pseudomonas sp. strain ADP and only limited mineralization. Supplementation with citrate or succinate increased the survival of the strain, and successful mineralization was dependent on the preservation of a carbon/nitrogen ratio >10 (Silva et al. 2004). The last would apply generally to substrates with a low C/N ratio such as triazines. 

The degradation rate of paraquat in certain soils can be slow, and the compound can persist for years — reportedly in a form that is biologically unavailable. But data are missing or incomplete on flux rates of paraquat from soil into food webs and on interaction dynamics of paraquat with other herbicides frequently applied at the same time. It seems prudent at this time to keep under close surveillance the residues of paraquat in soils in situations where repeated applications have been made over long periods of time (Summers 1980). 

In the biodegradation plots and In the Herbicide Storage Areas, high concentrations of herbicides were applied In a short time period and Incorporated Immediately Into the soil profile, and hence, the long persistence time. Nevertheless, these studies do show that the soil chemistry and the soil microbial populations can effectively combine to degrade massive concentrations of the phenoxy herbicides and that recovery of the sites occur as documented by the re-establlshment of the vegetative community. 

Metolachlor is applied as a mixture of eight different stereoisomers, only four of which have herbicidal activity (10). This implies that the other four isomers are applied as contaminants, with no additional benefit to crop production. Whether the chirality of metolachlor influences its degradation rate is unknown. This lack of information is mainly due to the difficulty of separation and analysis of its isomers. Because the stereochemistry of compounds plays an important role in their biological activity and degradation pathways, it is valuable to investigate the influence of stereochemistry on the rate of metolachlor degradation in soil. 

Pre-plant incorporated herbicides are applied before the crop is sown and are incorporated into the soil. Hence, they are also applied before weeds emerge. The reason for incorporation is usually because the herbicides are volatile and would be lost if they were not incorporated, or light unstable and they would be degraded if they remained on the soil surface. Volatility is a useful characteristic as it allows the redistribution of the compound throughout the soil following incorporation. 

The plant-microbe symbiosis may help facilitate the effective use of inoculants. For example, developed (brady)rhizobial strains or root-colonizing pseudomonads may be more effectively introduced into a contaminated soil environment when they are applied in conjunction with theirplant host. Kingsley etal. (1994) showed that inoculation of soil with a 2,4-D-degrader protected germinating seeds from the herbicidal effects of residual pesticide. Thus, plants may be used to help restore treated soils that contain residual but biologically active compounds. 

Soil. Herbicides and pesticides arc of course metabolized in the soil to which they are applied, and there are many reports of isolating degrading organisms from such sites. Little work has yet been presented where the biodegradation of these compounds has been successfully stimulated by a bioremediation approach,... 

Typically, only 0.01-10% of the mass of pesticide compounds applied to fields is detected in streams [91]. The remaining 90-99% of pesticides adsorb to soil, percolate into groundwater, or volatilize [79]. The major degradates of the most heavily used herbicides found in surface water have not been studied widely. Many chemical properties of pesticides affect the amounts transported to streams. In general, acetanilide herbicides are more soluble in water, and thus more mobile than are the triazines [92], The solubilities of sulfonated degradates of acetanilides (ethane sulfonic acid, or ESA), can be 10 times the solubility of the parent compound [93]. The greater mobilities of the degradates of the acetanilides (amide family) can explain these com-... 

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