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Pesticides are amongst the most widespread pollutants; they have a diverse effect on the organisms in the environment such as plants, humans, animals, and microorganisms. Even though the use of pesticides increases overall yield, their misuse can lead to their accumulation in the environment which could affect non-target organisms such as rhizospheric bacteria. The present study aims at evaluating the non-target effects of selected pesticide son growth parameters, and on the cultured rhizospheric bacterial community of Vicia faba plants, together with assessing the effect of pesticides on cultured N2 fixing bacteria. Soil was treated with the common 3 pesticides (Confidor (Imidacloprid), Bogiron (Difenoconazole) and, Durspan (Chlorpyrifos)) used by Palestinian farmers to assess the changes in the microbial community of the rhizosphere at two time points (45 and 75 days post-treatment) using two dosages (low dose and high dose). The residual concentrations of pesticides in soil were measured by GC-MS and the technique of enumeration of specific groups of rhizospheric microbes was employed by using two different media (RIM, dRSM); the presence of N2 fixing bacteria was carried out by inoculation on nitrogen free medium. Cultured bacteria on RIM and dRSM were isolated and identified according to colony morphology and biochemical testing. Plant parameters were not affected significantly by Confidor and Bogiron compared with the control, whereas Durspan affected both plant roots, which appeared thicker than control roots, and plant leaves, which presented brown dots on the surface. Moreover, roots’ length was significantly decreased (18.67 cm shorter, p= 0.031) after the application of Durspan after 75 days when compared to the control. The three pesticides studied differed widely in their soil degradation behavior, where Bogiron showed a slower degradation than Confidor and Durspan. The average colony forming unit (CFU) counting of cultured rhizospheric bacteria on both media was different between both time points of post pesticide treatments for all treatments, and there was significant difference for same treatments between both doses. All treatments showed a significant decrease in the CFU at the first time point (Confidor low dose (3.03 E+ 07 less < 0.001), Confidor high dose (1.44 E+ 07 less, p= 0.044), Bogiron low dose (2.45 E+ 07 less, p < 0.001), Bogiron high dose (3.25 E+ 07 less, p=0.000), Durspan low dose (3.29 E+ 07 less, p < 0.001), Durspan high dose (2.85 E+ 07 less, p < 0.001) when compared with the control. The decrease in the CFU at the fast time point is no longer detected at the second time point for all treatments with no significant change compared to the control. The data showed that the application of low dose of Confidor (1.92 E+ 08 more, p = 0.006), and Bogiron (2.61 E+ 08 more, p < 0.001), at both time points increased the growth of cultured rhizospheric bacteria.
There was a significant reduction in N2 fixing bacterial groups observed after all the pesticide treatments at the first time point (Confidor low dose (1.33 E+ 07 less, p= 0.051), Confidor high dose (2.00 E+ 07 less, p= 0.006), Bogiron low dose (1.81 E+ 07 less, p= 0.011), Bogiron high dose (1.79 E+ 07 less, p= 0.012), Durspan low dose (2.42 E+ 07 less, p= 0.002), Durspan high dose (2.24 E+ 07 less, p= 0.003)) compared with the control with maximum reduction associated with the application of Durspan. At the second time point there was no significant difference after application of both doses of Confidor and Bogiron compared to the control, but a significant reduction after application of both dosages of Durspan (3.82 E+ 07 less, p= 0.014). Thirty-nine separate and different colonies were counted on both media (RIM, and dRSM) which were isolated and identified based on their morphological characteristics and biochemical tests. Twelve isolates were confirmed to belong to Pseudomonas sp, four isolates were identified to belong to Actinobacteria sp according to their morphology, three isolates were confirmed to belong to Bacillus sp (two of them Bacillus cereus and one Bacillus subtilis). Two isolates were identified as Staphylococcus sp according to their morphology and cell staining, the remaining 18 isolates still need to be identified. Most of the isolates are able to solubilize phosphate and fix nitrogen. This study indicates that pesticides show non-target effects on active microbial populations that attend important ecosystem functions by enhancing or inhibiting their growth. Confidor and Bogiron enhance the growth of cultured rhizospheric bacteria with time, while Durspan seems to inhibit the growth of cultured rhizospheric bacteria with time. These results highlight the need to further study and understand of how the use of pesticides in agriculture can affect the environment, and the need to establish guidelines for their use. This study recommends avoiding the use of Durspan as insecticide on Vicia faba.
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