Abstract: Organic matter of modern and buried humus horizons was studied in the soils at the following archaeological sites: the Akberdinskoe II, Shipovskoe, and Okhlebininskoe II ancient settlements (Kara–Abyz culture, early Iron Age, 4th century BC to the cusp of the 2nd and 1st centuries BC). The archaeological sites are located in the forest–steppe zone (Iglino district, Republic of Bashkortostan). The morphological and physicochemical properties of the soils were determined. Spectrophotometry and ¹³C NMR spectroscopy were also used. The archaeological sites feature dark gray forest soils with loamy granulometric composition. The topside and buried humus horizons are 11–30 cm thick. Organic matter is represented by fulvate–humate, humate, and humate–fulvate humus types. Humic acids predominate in the group composition; the HA-2 fraction predominates among the humic acid fractions. The spectrophotometric data indicate that aromatic parts of humic acid molecules are more mature in buried humus horizons in comparison with modern humus horizons. According to the ¹³C NMR spectroscopy data, humic acid molecules in organic matter of modern humus horizons conform to the conditions of the chernozem soil formation type. Archaeological data indicate that the current intensification of the climate aridization trend in the forest–steppe zone of the Republic of Bashkortostan dates back to the middle of the 4th century BC and the cusp of the 2nd and 1st centuries BC. No changes in the soil formation type were registered in the soil cover of the archaeological sites at the time of the survey.