Toxicological Effects of Plastic Composted Soil on Nitrifying Bacteria

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 LC50 and EC50 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 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.

Results: The EC50 values for Nitrosomonas sp. are as follows; 0.52, 0.82, 1.11 and 1.23 with LC50 values of 27.47, 24.14, 19.74 and 16.73. It was observed that the EC50 values were very low which increased gradually with time, this suggest that there was a high percentage inhibition of Nitrosomonas sp. thereby altering and reducing the percentage nitrite accumulation by the bacteria. The EC50 values for Nitrobacter sp. are 52.00, 81.72, 111.31 and 123.13 and LC50 values are 25.04, 23.93, 15.94 and 13.39. The EC50 values for Nitrobacter sp. were high which indicates that there was high nitrite utilization by Nitrobacter sp. as the percentage inhibition gradually decreased with exposure time. The coefficient of determination showed that the bacteria response which could be either inhibition, utilization or accumulation greatly depends on the amount/concentration of the plastic contaminants present in the test soil sample.

Conclusion: The result from this study shows that the EC50 determination was more sensitive than the LC50 determination for Nitrosomonas sp. and that the LC50 determination was more sensitive than the EC50 determination for Nitrobacter sp. Also that the populations of Nitrosomonas sp. were more sensitive to the plastic contaminants than the populations of Nitrobacter sp. The results obtained from this study suggest that autotrophic transformation by nitrifying bacteria which enhances soil fertility may be hindered in an ecosystem polluted with plastics as nitrification processes will be altered.