Accelerated microbial degradation of carbofuran in soil

Accelerated microbial degradation is an agricultural problem that arises when plant protectants are repeatedly applied to the same soil resulting in reduced efficacy or failure of the compound. The purpose of my doctoral research was to study accelerated microbial degradation in soils using the soil-incorporated insecticide and nematocide carbofuran (2,3-dihydro-2,2-dimethylbenzofuran-7-yl methylcarbamate) and combining traditional and molecular methods. In the first study, a closely timed sequence of treatments with analytical carbofuran was used to induce accelerated microbial degradation in four soils that had no previous exposure to carbofuran or other carbamate pesticides. Actual degradation rates of 14C-carbonyl carbofuran (inactivation) and 14C-ring carbofuran (mineralization) to 14CO₂ were measured over time to determine developmental patterns of accelerated microbial degradation. For all soils, accelerated microbial degradation occurred after one or two treatments. In the second study, treatments with analytical carbofuran were discontinued and the recovery patterns were determined. Recovery patterns of carbofuran inactivation were observed in three soils and recovery patterns of carbofuran mineralization were observed in two soils. To address present and future ecotoxicological concerns, the potential effects of these carbofuran treatments on the soil microbial community structure were assessed using a terminal restriction fragment length polymorphism analysis. Each soil and treatment group were analyzed ten months after the last treatment with analytical carbofuran. No significant changes were observed between treatment groups of each soil. Finally, to explore the potential of molecular methods to rapidly predict the development of accelerated microbial degradation, the presence of the mcd gene was determined in 28 soils from five continents with and without treatment histories of carbamate pesticides using polymerase chain reaction. The mcd gene codes for carbofuran hydrolase. In addition, carbofuran inactivation rates of these soils were determined using 14C-carbonyl carbofuran. Accelerated degradation rates were observed in 64% of the soils, and the mcd gene was detected in 36% of the soils. Of the soils where the mcd gene was detected, all but one showed accelerated degradation rates. The absence of the mcd gene in the remaining soils with accelerated degradation rates suggests that other mechanisms are responsible for the loss of carbofuran efficacy.