Impact of long term pesticide usage on soil microbial activities and 14C-monocrotophos degradation

The effects of long term pesticide usage on soil microbial activities and degradation of 14C-monocrotophos was observed under cotton field conditions. The experimental field was divided into treated and untreated plots. Pesticides were applied to treated plots at weekly intervals as in common practice in Thailand. The total numbers of applications were 11, 16 and 16 for first, second and third crop seasons, during the three years from 1996 to 1998. Soil samples at depths of 0-15 cm and 15-30 cm were sampled before and after pesticide application for the first two crops, while in the third crop season only the surface layer of soil was taken. The samples were assessed for CO2 from respiration, soil microbial population, iron reduction capacity, and rates of nitrification. Soil biomass and microbial activities as measured from respiration and iron reduction decreased in the treated plots at both depths after each pesticide application over the three crop seasons, whereas samples from untreated plots at both depths did not show decreases. Repeated application of pesticides did not show any effect on nitrification rates of the first crop but there was inhibition in the second and third crops. Soil columns, treated with 14C-monocrotophos one week after last pesticide application, were harvested after 0, 3, 6, 9, 18, 24 and 30 months. Extractable residues of 14C were found only in the 0-15 cm layer. In treated and untreated plots, residues declined from 80.17 and 85.68 to 0.44% of the applied 14C within 6 months. The long term usage of pesticides did not affect the half-life of 14C-monocrotophos. Bound residues of 14C were found at the highest concentrations, 18.94 and 12.58% of that applied, at 6 months in treated and untreated plots, thereafter the binding decreased to 4.68 and 2.74% within 30 months. (author)