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Aim:The aim of this study was to evaluate the toxicological effects of plastic composted soil on some nitrifying bacteria which are Nitrosomonas sp. and Nitrobacter sp. Methodology: Five plastic composted soil samples were collected from different locations within the Edo State Waste Management site located at Iyowa in Benin City which were merged to form a composite sample. Nitrosomonas sp. and Nitrobacter sp. were isolated from the soil samples. Plastic composted soil concentrations were prepared for LC 50 and EC 50 determination. Nitrobacter and Nitrosomonas acute toxicity test was carried out. Initial nitrite concentrations were determined and plates of Winograsky agar were immediately inoculated by spread plate techniques. Nitrite accumulation and utilization were also determined and inoculation by spread plate method was carried out from the various plastic composted soil concentrations after 1 h, 2 h, 3 h and 4 h time Original Research Article intervals. Plates were incubated at room temperature (28+/-2°C) for 24 h. The percentage inhibition of bacteria (Nitrosomonas sp. and Nitrobacter sp.) was determined using the probit regression analysis in Excel Microsoft.

This research studied the effects of pesticide application on the growth of soil nitrifying bacteria. Nitrosomonas sp. was isolated from the Soil samples gotten from five different locations within the University of Benin, Benin City and cultured using Winogradsky medium. The bacterial isolate was characterized and identified based on its cultural, morphological and biochemical characteristics. After 4 to 5 days of growth, Nitrosomonas Counts obtained from Winogradsky medium ranged from 4.8×10 3 to 9.2×10 3 cfu/g. There was an increase in bacteria growth at lower pesticide concentration which began to decrease as the pesticide concentration increased with time. The percentage nitrite accumulation gradually decreased with time, the LC50 values were higher than EC50 values which shows that the bacteria could survive and grow at lower pesticide concentrations but were completely inhibited at higher pesticide concentrations. It is therefore very important to examine pesticides so as to determine its toxicity to soil bacteria before application.