Soil microorganisms in cotton fields sequentially treated with insecticides

A crop protection system consisting of sequential treatments by six insecticides—dimethoate, monocrotophos, deltamethrin, endosulfan, cypermethrin, and triazophos—at recommended dosages in cotton fields in Punjab, India was investigated for its effects on nontarget soil microorganisms and their activities. Successive applications of the insecticides caused only short-lived adverse effects on the soil microorganisms. None of the insecticides used had any adverse effects on soil fungi as reflected by their total numbers. Significant change in Azotobacter numbers were observed after dimethoate, triazophos, and endosulfan treatment in 1998 soil. An increase of up to 71% in actinomycetes numbers was observed after deltamethrin treatment in the treated fields in 1995. Few short-term changes in iron-reduction capacity were observed after endosulfan and cypermethrin treatments. No adverse effect was observed on the soil respiration during all the experimental periods. The amount of residues detected in soil ranged from 8.5 to 42.0 ng g−1 dry wt. soil for organophosphorus insecticides and from nondetectable to 5.55 ng g−1 dry wt. soil for synthetic pyrethroids. It ranged between 7.3 and 35.6 ng g−1 dry wt. soil for endosulfan. On many occasions two or three insecticide residues were detected together; therefore, the effect observed on soil microorganisms and their activities was a multiresidue effect. In 1998, crop soil amounts of insecticide residues were generally more than those in 1995 and 1996. Persistence and dissipation patterns in soils with a history of exposure to the insecticides compared to the non-history soils were similar.