Abstract

Crude oil contamination of soil is known to affect soil quality and productivity.  This study assessed selected physicochemical properties, particle-size distribution and microbial populations in polluted forest soil ecosystem of Ogbogu (OG) and Ahiahu (AH)’s unpolluted forest soil which served as control. Ogbogu and Ahiahu are located in Ogba/Egbema/Ndoni Local Government Area, Rivers State, Nigeria. Soil samples were collected at 0–15 cm and 15–30 cm depths and analyzed using standard methods. Results obtained were analysed using descriptive statistics and significance testing at the 5% level. Total organic carbon (TOC) ranged from 5.42 ± 0.04% to 9.85 ± 0.44%, with the highest value at OG 0–15 cm and the lowest at AH 15–30 cm. Total organic nitrogen (TON) ranged from 0.22 ± 0.02% to 0.42 ± 0.02%, available phosphorus from 8.49 ± 0.79 to 16.67 ± 0.95 mg/kg, and potassium from 63.45 ± 4.16 to 102.56 ± 5.11 mg/kg. Soil pH ranged from 5.28 ± 0.08 to 5.92 ± 0.09, indicating moderately acidic conditions, while electrical conductivity (EC) ranged from 585.44 ± 25.65 to 932.05 ± 35.78 µS/cm. Moisture content ranged from 13.85 ± 0.57% to 20.48 ± 0.79%, and bulk density ranged from about 1.38 to 1.65 g/cm³. TOC, phosphorus, potassium, EC and moisture content differed significantly (P < 0.05), whereas pH, TON and bulk density did not. Particle-size analysis showed a silt-dominated soil, with silt ranging from 62.50 ± 1.29% to 72.50 ± 1.29%, sand from 14.75 ± 0.96% to 20.75 ± 0.96%, and clay from 12.75 ± 0.50% to 16.75 ± 0.50%. Total heterotrophic bacterial counts ranged from 75.38 ± 23.92 to 92.88 ± 29.39, while reported bacterial densities ranged from 1.64 × 10³ to 7.54 × 10⁵ CFU/g. The highest mean count occurred at AH 0–15 cm (92.88 ± 29.39), whereas the highest reported bacterial density occurred at OG 15–30 cm (7.54 × 10⁵ CFU/g). These findings indicate substantial depth- and site-related variation in soil conditions and microbial abundance in the polluted forest environment. Nickel and cobalt were identified as important heavy-metal concerns. Therefore, continued hydrocarbon assessment and evidence-based bioremediation are recommended.