Effect of Pesticides on Bacterial and Fungal Populations in Ecuadorian Tomato Cultivated Soils

Abstract

Indiscriminate use of insecticides and fungicides leads to a combined contamination of pesticide residues in the soil environment and causes a severe threat to beneficial microbial activities. Hence, it is essential to assess the microbial populations of soils in the biosphere. In this study, two insecticides, cypermethrin and chlorpyrifos alone and in combination with two fungicides, mancozeb and carbendazim, were assessed for their effects on the bacterial and fungal populations in two tomato cultivated soils. Samples of soil-1 and soil-2 were collected from tomato cultivated fields of El Quienchi, Pichincha, Ecuador. Initially the physico-chemical characteristics of soils, e.g., soil pH, organic matter, total nitrogen, electrical conductivity, sand, silt and clay contents were detected, and then soil bacterial and fungal populations were determined. The influence of selected pesticides alone or in combination on the bacterial and fungal population was concentration-dependent; the populations were increased with increasing concentration of pesticides up to 5.0 kg ha−1 compared to the controls in 10-day incubated soils. The bacterial and fungal populations continued to increase up to 20 days, and then, gradually decreased after 30 and 40 days of incubation. The results clearly indicate that application of individual and/or mixtures of the pesticides in cultivation of tomato, at field application rates (2.5–5.0 kg ha−1), significantly improved the bacterial and fungal populations in soil-1 and soil-2. However, further increase in the dose of pesticides (7.5–10 kg ha−1) dramatically decreased the bacterial and fungal populations. On the other hand, insecticides in combination with fungicides showed negative effect on fungal populations.