Effects of carbofuran and the corn rhizosphere on growth of soil microorganisms

Biodegradation of carbamates and carbamothioates by microorganisms in soil has been widely reported; however, the ecology of pesticidedegrading microorganisms in soils and plant rhizospheres containing these pesticides has received less attention. Some bacteria capable of carbofuran (2,3-dihydro-2,3-dimethyl-7-benzofuranyl methyl carbamate) biodegradation in soil have been isolated and identified (Felsot et al. 1981 ; Chaudhry and Ali 1988). Reed et a1.(1987) identified several fungi and bacteria from soils with histories of pesticide application and observed that bacteria were able to metabolize the pesticides in pure culture more efficiently than were fungi. Bacteria and actinomycetes from soils with histories of carbamothioate herbicide use were found to utilize carbamothioates more efficiently than were isolates from nonhistory soils (Mueller et al. 1989). Lee (1984) found that biodegradation of EPTC (Sethyl dipropyl carbamothioate) was significantly affected by soil fungi. Interestingly, after storage bacterial isolates lost the ability to degrade EPTC. He concluded that fungi able to degrade EPTC may retain this ability in soil longer than do bacteria. Recent studies demonstrated that fungi along with bacteria detoxify herbicide wastes during bioremediation of contaminated soils (Felsot and Dzantor 1990). Microbial communities composed of several to numerous species are more likely to be responsible for pesticide biodegradation in soil and rhizosphere environments than are single species. Pesticides applied to soil at planting should persist during the development of plant roots. Therefore, a portion of the pesticide likely interacts with microorganisms in the rhizosphere. Few previous studies have considered the effect of plant roots on the activity of microorganisms in the presence of pesticides. A five-member microbial community from a wheat rhizosphere was able to metabolize the herbicide mecoprop [2-(4-chloro2-methylphenoxy) propanoic acid] in liquid culture even though individual members could not (Lappin et al. 1985). Biodegradation of (2,4-dichlorophenoxy)acetic acid by microorganisms was found to be several times higher in sugarcane rhizospheres than in surrounding soils (Sandmann and Loos 1984). Send reprint requests to R.J. Kremer, 144 Mumford Hall, Columbia, Missouri 65211 USA