Impact of Repeated Applications of Metalaxyl on Its Dissipation and Microbial Community in Soil

Abstract

Metalaxyl, an important phenylamide fungicide, is widely used for controlling fungal diseases caused by pathogens of the orders Peronosporales and Pythiales. Under laboratory conditions, metalaxyl was applied to soil samples at the recommended field rate (1×FR) and double of recommended field rate (2×FR) for two and three times. Soil subsamples were taken at 0, 1, 3, 7, 14, 28, and 45 days after the last application of metalaxyl for determination of metalaxyl residues and 7, 14, 28, and 56 days for enumeration of cultivable microorganisms and DGGE profile of soil microbial community. Soil incubation experiments revealed that metalaxyl was degraded faster in the third application than in the second application of the fungicide, half-lives of metalaxyl decreasing from 16.2 to 9.9 days for recommended field rate and 22.1 to 20.0 days for double of recommended field rate. Soil bacterial and fungal populations decreased in the first 14 days and then recovered to the control levels; population of actinomycetes did not alter in the first 28 days but increased at the end of the experiment after the second application. However, after the third treatment, temporary increase in soil bacteria population, nonsignificant inhibition effect on fungal population, and obvious stimulation effect on actinomycetes number were observed. DGGE results showed that successive inputs of metalaxyl altered the bacterial community structure. There were differences in the persistence and effects of metalaxyl on microbial community between the second and the third metalaxyl treatments.