Abstract

Pesticides are widely utilized in agriculture to control plant diseases and enhance crop yield. However, their persistence as metabolite residues following degradation and their adverse effects on ecological communities are a global concern. This study evaluated the bioavailability and ecotoxicological impacts of organophosphate and alkyl-phenol pesticides on earthworms and nitrifying bacteria isolated from legume plant soil using standard methods. Bacterial isolates were cultured on Winogradsky medium and identified based on cultural, morphological, and biochemical characteristics. Molecular identification and PCR detection confirmed the nitrifying bacteria as Nitrosomonas europaea strain AL 954747.1 and Nitrobacter winogradskyi strain NR 074324.1, with percentage identities of 98.0 and 99.0, respectively.  Experimental concentrations (100 mg/l, 200 mg/l, 300 mg/l, 400 mg/l, 500 mg/l, 1000 mg/l, and 2000 mg/l) were derived from pesticide stock solutions prepared according to manufacturer’s instructions (100 mg in 1000 ml distilled water) and applied to Nitrobacter sp., Nitrosomonas sp., and to earthworms for 96 hours. Toxicity tests results indicated that at 100 mg/l organophosphate pesticide, Nitrosomonas sp. growth ranged from 75.67±6.13 to 40.32±3.20, and Nitrobacter sp. from 78.37±6.26 to 43.34±3.25 after 96 hours compared to controls of 100.54±7.34 and 105.29±7.68, respectively. While for alkyl-phenol pesticides at 100 mg/l, Nitrosomonas sp. growth ranged from 94.31±6.45 to 63.48±3.40, and Nitrobacter sp. from 92.56±6.61 to 62.59±3.50.  Generally, the order of growth of both nitrifying bacteria in various concentrations of both pesticides was Control > 100mg/l ˃ 200mg/l ˃ 300mg/l ˃ 400mg/l ˃ 500mg/l ˃ 1000mg/l ˃ 2000mg/l. The Organo-phosphorus and Alkyl-phenol pesticide toxicity on earthworms showed similar reductions. At 100 mg/l, numbers declined from 10 (24 hours) to 5 (120 hours); at 500 mg/l, from 6 (24 hours) to 1 (120 hours); and at 2000 mg/l, from 2 (24 hours) to 0 (72 hours), while controls remained at 10 throughout. These findings suggest that autotrophic transformation by nitrifying bacteria and earthworms, which supports soil fertility, may be impaired in ecosystems contaminated with these pesticides, potentially reducing or halting nitrification processes. It is therefore recommended that pesticides be applied strictly according to manufacturer instructions to prevent ecosystem disruption.