Effect of Lead and Cadmium Concentrations on The Population Density of Bacteria in The Soil Contaminated in Tikrit City

Abstract

Soil contamination with heavy metals such as lead (Pb) and cadmium (Cd) has become a growing environmental concern due to their persistence, toxicity, and adverse effects on soil microbial communities. This study aimed to determine some physical and chemical properties of the soils of the studied areas, and to evaluate the effect of varying concentrations of lead and cadmium on the density of soil bacteria in different polluted sites within Tikrit city. Four samples were collected from different areas of Tikrit Governorate. Sample collection began on winter (December 2023) and summer (May 2024). The modelling process began by taking samples of four environmental sites: a home garden (S1), an agricultural field (Al-Bu Ajil) (S2), a rea near the generator(S3), and a river bank area (S4). Soil textures ranged from loam (S1), loam and clay (S2, S3), to sandy loam (S4). Field capacity values showed slight variation, with the highest value in S3 (42%) and the lowest in S4 (29%). pH ranged between 6.1 and 6.9, indicating a near-neutral environment. Electrosalinity (EC) was significantly higher in S3 (221.4 dS/m), indicating potential salinization compared to the other samples. In terms of organic matter, the highest percentage was recorded in S1 (0.82%) and the lowest in S4 (0.66%). Nitrogen concentration was highest in S1 (11.8 ppm), while phosphorus and magnesium were highest in S3 (1.7 and 3.6 ppm, respectively). Potassium and calcium also reached their highest levels in S3. As for heavy metals, the highest concentration of lead was recorded in S3 (30 ppm), while cadmium ranged from 1.5–2.0 ppm. Cobalt and molybdeium varied between samples, with S3 predominating in most elements. Iron, zinc, and copper concentrations were relatively similar, with S3 and S2 slightly outperforming each other in some values. Lead (Pb) and cadmium (Cd) showed varying effects on bacterial growth at concentrations (0.5, 1, 2) mg/L, especially at the highest concentration (2 mg/L). Pb showed varying effects; bacterial growth continued in S3 (11 CFU/ml), S2 (5 CFU/ml), and S1 (2 CFU/ml), with dense growth observed in S4. Cadmium (Cd) completely stopped growth at all sites, indicating high toxicity at this concentration. The results indicate that physical and chemical properties influence bacterial growth. The effect of heavy metals on bacterial growth varies depending on the type of metal and the environmental location.