Atrazine is widely used for weed control, but its persistence and effects on soil microorganisms remain poorly understood in semi-arid Sudanese soils. This study evaluated the concentration-dependent effects of atrazine on culturable soil microbial groups and its biodegradation in Gerif silty clay loam soil collected from the Blue Nile Bank, east of Khartoum, Sudan. Soil samples were treated with four atrazine concentrations (0.678, 1.69, 3.39, and 5.08 mg g-1 soil) and incubated at 28 ± 1 °C for 150 days. Microbial populations were enumerated at 0, 15, 30, 60, 90, 120, and 150 days using selective culture media, while atrazine residues were quantified by gas chromatography. Atrazine significantly affected culturable microbial groups in a concentration- and time-dependent manner. Organic nitrogen-utilizing bacteria showed inhibition up to 73.6% at the highest concentration after 90 days, while fungal populations were inhibited by up to 70.0% at 3.39 mg g-1 soil after 15 days. Mineral nitrogen-utilizing bacteria were strongly suppressed during the early incubation period, with inhibition reaching 80.7% at 1.69 mg g-1 soil after 30 days, followed by partial recovery and stimulation at later stages. Microorganisms growing on nitrate agar were dominated by Mycobacterium spp., suggesting selective enrichment of atrazine-tolerant culturable taxa. Atrazine degradation began within 15 days and increased progressively during incubation; after 150 days, degradation reached 75.0% at 0.678 mg g-1 soil but only 54.2% at 5.08 mg g-1 soil. Kinetic analysis further showed that atrazine half-life in Gerif soil ranged from 69 to 125 days, with the longest half-life recorded at the highest concentration. These findings indicate that atrazine imposes selective pressure on culturable soil microorganisms and persists longer at higher concentrations in low-organic-matter Gerif soil. The results support optimized herbicide application rates, improved soil organic matter management, and microbial-based remediation strategies in semi-arid agroecosystems.