Effect of Different Pb Levels on Bacterial and Fungal Populations During Soil Incubation

Municipal compost and sewage sludge which are used as amendments to improve soil structure and fertility، often contain Pb، Cd، Zn، Ni and Cu at levels that are toxic to soil microorganisms. Among these elements، Pb appears at relatively higher concentration. In this study، the impact of different levels of Pb as Pb (NO3) 2 on the population of soil microorganisms in soil were evaluated. Six levels of Pb including 0، 100، 200، 300، 400 and 500 mg Pb per kg soil were added at two replications to the pots containing 2 kg of soil and kept 25±2°C and 0. 7 FC soil moisture for six months. Bacterial and fungal cells numbers were enumerated using plate count method، on days 15th، 30th، 90th and 180th day during incubation. At each incubation period، bacterial number decreased with increasing Pb levels. The most adverse effect of Pb was observed at above 200 mg Pb kg-1. Fifteen days after incubation، the fungal population was significantly (p≤0. 05) declined at 400 and 500 mg Pb kg-1 compared to the control level (zero addition of Pb). By increasing of incubation time to 180 days، the fungal number cells showed no significant differences between Pb levels. It seems that the fungal population is more tolerant to Pb than the bacterial population in the tested soil.