Long-term human cultivation activities are the key factors of the vertical distribution and storage dynamics of soil organic carbon (SOC) in cropland. Based on a 45-year long-term field experiment, this study systematically compared SOC dynamics and carbon storage characteristics in soil profiles (0–200 cm) between cultivated land and adjacent natural forest. The findings reveal the hierarchical regulatory effects of tillage management on the soil carbon pool. The results show that: (1) Under both land use types, SOC content decreased exponentially with depth, but values in cultivated soils were 0.35–1.54% lower than in forest soils at each layer. SOC content in surface soil (0–78 cm) was significantly higher than in the subsoil (78–158 cm) and substratum layers (158–200 cm) (
p < 0.01). At equivalent depths, SOC in cultivated land was significantly lower than in forest land (
p < 0.01). Over 45 years, the SOC accumulation rate in the surface soil of cropland (0.07 g·kg
−1·yr
−1) was only half that of forest land (0.14 g·kg
−1·yr
−1). (2) The controls of soil physicochemical properties on SOC differed with land use: in forest soils, SOC correlated positively with clay content (r = 0.63,
p < 0.01), whereas in cultivated soils, SOC was primarily regulated by total nitrogen (r = 0.94,
p < 0.01) and sand content (r = 0.60,
p < 0.01) and negatively correlated with bulk density (r = −0.55,
p < 0.01) and pH value (r = −0.45,
p < 0.05). (3) Long-term tillage significantly reshaped soil profile structure, thickening the plough layer from 20 cm to 78 cm. Surface carbon storage reached 20.76 t·ha
−2, an increase of 11.13 t·ha
−2 compared with forest soil (
p < 0.01). However, storage decreased by 4.99 t·ha
−2 and 7.60 t·ha
−2 in the subsoil and substratum layers, respectively (
p < 0.01). The SOC storage increment rate was 50.95 t·ha
−2·yr
−1 higher than that of forest soil in the surface layer but 46.81 t·ha
−2·yr
−1 and 11.12 t·ha
−2·yr
−1 lower in deeper layers. These results confirm that cultivation alters soil structure and material cycling, enhancing carbon enrichment in surface soils while accelerating depletion of deeper carbon pools. This provides new insights into the vertical differentiation mechanisms of SOC under long-term agricultural management.
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