Carbofuran microbial degradation

Felsot, A., J.V. Maddox, and W. Bruce. 1981. Enhanced microbial degradation of carbofuran in soils with histories of Furadan use. Bull. Environ. Contam. Toxicol. 26 781-788. 

In a small watershed, carbofuran was applied to corn seed farrows at a rate of 5.03 kg/ha active ingredient. Carbofuran was stable up to 166 d, but after 135 d, 95% had disappeared. The long lag time suggests that carbofuran degradation was primarily due to microbial degradation (Caro et al., 1974). 

Surface Water Sharom et al. (1980) reported that the half-lives for carbofuran in sterilized and nonsterilized water collected from the Holly Marsh in Ontario, Canada were 2.5 to 3 wk at pH values of 7.8-8.0 and 8.0, respectively. The half-lives observed in distilled water were 2 and 3.8 wk at pH values of 7.0-7.2 and 6.8, respectively. They reported that chemical degradation of dissolved carbofuran was more significant than microbial degradation. 

Read, D.C., Enhanced microbial degradation of carbofuran and fensulfothion after repeated applications to acid mineral soil, Agric. Ecosyst. Environ., 10, 37,1983. 

Carbofuran is soluble in water and moderately persistent in soil (half-life 30-120 days). Carbofuran is degraded by chemical, photochemical, and microbial processes. Hydrolysis is more rapid in alkaline conditions. Carbofuran breaks down in sunlight. Carbofuran has a high potential for leaching into groundwater. Carbofuran is mobile in sandy loam, silty clay, and silty loam soils. In surface water, carbofuran is subject to hydrolysis, particularly under alkaline conditions. Hydrolysis of carbofuran (half-lives) in water is 690, 8, and 1 weeks at pH values of 6, 7, and 8, respectively. As in soils, photodegradation and microbial transformation may also contribute to degradation. Carbofuran is not volatile and does not adsorb to sediment or particles. 

The microbial metabolism of pesticides has often been subdivided into 2 distinct classes. The first of these is termed simply "catabolism". This process often results in the mineralization of some portion of an organic compound via enzymatic pathways to simple products of universal currency (C02, NH3). In some cases, one portion of the molecule may be mineralized and another portion may accumulate in soil. This is true for the soil microbial degradation of carbofuran (42,43). Therefore, catabolism should not be equated with mineralization or complete destruction of a pesticide. It should be pointed out, however, that mineralization of a pesticide in soil Is nearly always a consequence of microbial activity (44). The key to understanding catabolism is that it is primarily a process driven by the microbial quest for energy. Therefore, catabolism has come to be equated with utilization of a pesticide as an energy source and thus a growth substrate (40,41). Catabolism is most commonly linked to the conversion of pesticides into carbon skeleton... 

Research on control failures during the late 1970 s and early 1980 s focused on carbofuran, a compound that had earlier provided excellent control and had captured a considerable share of the rootworm insecticide market (e.g. 20% of treated acres in Iowa in 1977)(9). Although some studies found no correlation between prior use of carbofuran and an enhanced rate of carbofuran degradation (10.11) both monitoring of field residues (12-14) and laboratory degradation tests (15-17) conclusively demonstrated enhanced microbial degradation as the cause of decreased carbofuran persistence in fields with histories of carbofuran use. Thus, a... 

Felsot et al. (1) demonstrated that the disappearance rate of carbofuran from soil was affected by previous treatments with the insecticide in 1981. Since then, the phenomenon now generally described as enhanced microbial degradation has been intensively examined, particularly for insecticides used to control corn... 

Implications of Mobility on the Availability and Degradation of Pesticides in Soil. Repeated application of 2,4-dichlorophenol, p-nitrophenol, and salicylic acid (as observed in current studies) and carbofuran phenol (20) has induced enhanced microbial degradation of their parent compounds. Rf values of these hydrolysis products indicate intermediate to high mobility in soils. The p-nitrophenol, 2,4-dichlorophenol, and salicylic acid were utilized as energy sources by microbes, and their availability in soil may contribute to the induction of rapid microbial metabolism. Carbofuran phenol did not serve as a microbial substrate but also enhanced the degradation of its parent compound, carbofuran (20). Carbofuran phenol is freely available in anaerobic soils, but the significance of its availability is yet to be understood. 

Any factors that stimulate the growth of soil microorganisms or that increase the availability of pesticides in soil will enhance the degradation of the chemical. Felsot et al. (1981) found that the persistence of carbofuran was inversely correlated with microbial activity in corn-cropped soils. Tu and Miles (1976) reported that 88% of parathion was lost from soil in 7 months diazinon, 92% lost in 20 weeks paraoxon, 100% hydrolyzed in 12 hr mala-thion, 50-90% lost in 24 hr and carbofuran, 50% lost in 3-50 weeks. 

Cumulative plots of carbofuran mineralization in a history and a nonhistory soil are presented in Figure 1. The initially accelerating rate of C02 production, indicative of a microbial response (e.g., enzyme induction, population growth), is characteristic of enhanced degradation. Comparison of the mineralization of u-carbonyl and 14C-ring labelled carbofuran demonstrates an important consideration in this type of assay the location of the u label in the insecticide is critical for this type of assay to provide useful information. Although the carbonyl C was almost completely evolved as CO2, the ring C was only slowly mineralized. 

Response of Microbial Populations to Carbofuran in Soils Enhanced for Its Degradation... 

The size of Che microbial populations able to rapidly degrade carbofuran in soils enhanced for its degradation were enumerated by means of substrate addition and fumigation. Use of these techniques followed unsuccessful attempts to enumerate the population using plate or direct counts in the enhanced soils. Overall biomass size declined following application of carbofuran. No biomass suppression was observed in the non-enhanced soils and implies this suppression may be related to the formation of metabolites such as carbofuran-phenol or methylamine. In the enhanced soils, 6% of applied pesticide was initially incorporated into biomass carbon. This contrasts with 0.87% incorporation in the non-enhanced soils. After 15 days there was complete loss of Che pesticide at this time the biomass contained 2% of the applied material. 

We are developing an understanding of how microbial populations in enhanced soils respond to carbofuran. Our efforts have centered around two sets of soils in which enhanced carbofuran degradation has been demonstrated (5). We have estimated the microbial population changes in these two systems. 

The substrate addition method for biomass estimation relies on the fact that the biomass response to the introduction of the pesticide is reflected in the response to the glucose. Fumigation relies on the conversion of the specifically labeled biomass to C-C02- From both estimations it is clear that the size of microbial biomass that can use the carbon in the carbofuran side-chain is small. Karns et al. (11) has shown that carbofuran may serve as nitrogen source for some organisms. Our estimates would overlook this N utilization. It is likely that organisms that utilize methylamine, the primary degradation product, for nitrogen would overlook this source of carbon. Data on bacterial use of methylamine is, however, limited. 

Measurements of the incorporation of 14C-labeled pesticides tend to support the concept of limited microbial growth from pesticide degradation. In order for a pesticide to support support substantial growth, concomitant levels of mineralization and incorporation into biomass should be observed. Studies with 14C-2,4-D, labeled in either the acetic acid or ring structures show that more 14C from the acetic acid is incorporated into biomass than from ring-labeled, depending upon soil type and the time after addition of the 14C to the soil (Table 1). Even lower amounts of carbofuran were incorporated into biomass than were observed for 2,4-D. There was not a strong relationship between mineralization and incorporation into biomass. 

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