Spatial Variability of Pedological Properties of Kirkuk Governorate Soils and the Impact of Agricultural Utilization

This study investigated the morphological, physical, chemical, fertility, and biological characteristics of soils in the Al-Riyadh district and the town of Al-Dibs, considering both cultivated and uncultivated soil profiles. The surface layers exhibited higher fertility and microbial activity compared to the subsurface layers. Clay content ranged from 340 to 440 g/kg in the various soil profiles, while silt content ranged from 320 to 420 g/kg, and sand content from 200 to 320 g/kg. Bulk density increased with depth from 1.32–1.42 MG/m³ in the surface layers to 1.60–1.63 MG/m³ in deeper layers, while total porosity decreased from 50.19–47.92% to 38.49–39.62%. The soil pH was neutral to slightly alkaline (7.32–8.11), and electrical conductivity ranged from 0.18 to 1.58 dS/m. Organic matter was highest in the surface layers (7.19–20.86 g/kg) and decreased with depth. The accumulation of calcium carbonate in the subsurface layers (192–324 g/kg) affected the pH and reduced nutrient mobility. Available nitrogen and phosphorus were highest in cultivated soils, for example, 30.6 mg/kg nitrogen and 10.14 mg/kg phosphorus in the topsoil layer (Ap) in Riyadh, and 40.4 mg/kg nitrogen and 16.14 mg/kg phosphorus in the topsoil layer (Ap) in Dibs, and decreased sharply with depth. Microbial populations peaked in the topsoil, with bacterial counts ranging from 4.66 to 9.00 × 10⁶ colony-forming units (CFU)/g of soil and fungal counts ranging from 9.33 to 15.00 × 10⁴ CFU/g of soil. Overall, the soils in the Dibs area exhibited higher fertility and greater microbial activity due to increased rainfall and organic matter accumulation. Agricultural practices contributed to improved nutrient availability and increased microbial populations in the topsoil, but did not fully compensate for climatic variations between sites. These results highlight the combined effects of environmental conditions and land use on soil properties, fertility, and biological activity.