Evaluation of Methods for Characterizing Carbofuran Hydrolysis in Soil

The objective of this study was to develop a method that could be applied to investigations of soil/environmental factors influencing the spatial and temporal variability of carbofuran hydrolysis in field soils. Two factors were considered: (i) soil handling and pesticide application, and (ii) the method of data summarization. Three soil handling/pesticide treatments were evaluated: (i) sieved soil/sprayed pesticide application, (ii) injected pesticide application/sieved soil, and (iii) injected pesticide application/intact core incubation. This last method was developed to mimic field conditions where high localized concentrations of carbofuran (2,3-dihydro-2,2-dimethyl-7benzofuranyl methylcarbamate) occur as a result of banded application of granular carbofuran at planting time. For all three treatments sigmoidal 14CO2 production kinetics were observed. Several mathematical models for describing sigmoidal product appearance data were evaluated and a general saturation model was found to yield the best fit. Using parameter estimates obtained from this model in statistical tests, we found that the intact core soil treatment yielded significantly longer half lives for carbofuran degradation. Additional experimental evidence suggests that soil structure of the intact cores retarded diffusion of the 14CO2 produced from carbofuran degradation and influenced the kinetic pattern observed. BIODEGRADATION has been thoroughly documented as a dominant fateof pesticides in soils. Biodegradation is generally considered to be desirable from both an environmental (pollution) perspective as well as an agricultural (carryover) view, as long as pesticide efficacy is not affected. In recent years, however, reduced efficacy has been reported for several pesticides including the insecticide carbofuran (2,3-dihydro-2,2dimethyl-7-benzofuranyl methylcarbamate). The underlying mechanism commonly cited as responsible for this loss of efficacy is the enhanced microbial degradation in soils that have received repeated applications of carbofuran. Such soils have been referred to as problem soils (Kaufman et al., 1985). Carbofuran is a soil-incorporated insecticide used extensively, in the past, for control of the corn rootworm (Diabrotica sp.). Granular formulations of carbofuran are typically applied in the furrow at the time of planting and must persist for 30 to 60 d until the corn root worm larvae hatch (Felsot et al., 1985). Numerous studies including both field and laboratory incubations have documented rates of carbofuran degradation. Studies conducted prior to reports of loss of carbofuran efficacy describe apparent first-order rates of dissipation with reported half-lives ranging from 21 to 227 d, depending on soil type and incubation conditions (Caro et al., 1973; Getzin, 1973; Ou et al., 1982). More recent studies have focused on comparisons T.B. Parkin, USDA-ARS-National Soil Tilth Lab., 2150 Pammel Dr., Ames, IA 50011; D.R. Shelton, USDA-ARS-PDL, Bldg. 050, BARC-W, Beltsville, MD 20705; and J.A. Robinson, 7922-190-MR, The Upjohn Co., Kalamazoo, MI 49001. Contribution from the USDA-AR and the Upjohn Company, Kalamazoo, MI. Received 20 Dec. 1990. *Corresponding author. Published in J. Environ. Qual. 20:763-769 (1991). of carbofuran degradation in problem soils and soils without a previous history of carbofuran application (nonproblem soils). In incubations of field soils, Felsot et al. (1981, 1982, 1985) observed first-order kinetics of carbofuran hydrolysis in soils with a history of carbofuran application; however, in soils without previous carbofuran exposure, a lag phase was observed prior to degradation. It was not possible to calculate half-lives in the nonhistory plots because of the apparent lag; thus, rates were summarized as DT-50%; the time required for 50% of carbofuran disappearance. Read (1983) observed an accelerated rate of degradation between the first and second carbofuran applications to a soil. In both instances, kinetics of carbofuran degradation appeared to be sigmoidal. Chapman et al. (1986) also observed an accelerated rate of carbofuran degradation in soil pretreated with carbofuran, with apparent first-order kinetics. Kaufman et al. (1985) observed a sigmoidal kinetic pattern of CO2 production resulting from the hydrolysis of carbofuran added to adapted or problem soils; however, zero-order kinetics were observed in nonproblem soils. Similar results have recently been observed by Turco and Konopka (1990). Previous work has demonstrated that microbial growth in soil results in sigmoidal rates of degradation if physical/chemical mass transfer limitations do not control the overall rate (Focht and Shelton, 1987). Thus, the observation of sigmoidal kinetics is consistent with the hypothesis that the enhanced degradation of carbofuran observed in problem soils may be a direct result of microbial growth. Indeed, microorganisms capable of using carbofuran as a C and N source have been isolated from soil (Rajagopal et al., 1984; Karns et al., 1986; Ramanand et al., 1988; Chaudhry and Ali, 1988). Studies of the spatial and temporal variability of carbofuran degradation activity in field soils require that a quantitative approach be adopted for the summarization of kinetic data. Mathematical modeling is a useful data summarization tool, and the first-order model has been utilized for this purpose (Ou et al., 1982). However, the first-order model is not applicable for summarizing sigmoidal kinetic data. Several mathematical relationships exist for describing sigmoidal data; however, quantitative analysis of sigmoidal carbofuran degradation data using mathematical models has only recently been applied (Scow et al., 1990). The purpose of this study was to develop a protocol for performing large screening studies, which can be used to assess patterns of spatial and temporal variability of pesticide degradation activity in field soils. A primary requirement of such a protocol is that it must allow for the analysis of a large number of samples with minimum sample handling and preparation. Field studies are generally labor-intensive, and interpretations of field dissipation data is complicated by Abbreviations: MMF, Morgan-Mercer-Flodin; AIC, Akaike Information Criterion.