Previous studies showed that the program of converted forestland from cropland (CFC), initiated by the Chinese government in 1999, has been a significant contributor to China’s efforts towards carbon neutrality. Here, the 20-year CFCs of two aspects (sunny and shady) and three positions (upper, middle, and lower) hillslopes, adjacent maize (Zea mays L.) cropland, and natural secondary Castanea mollissima forest (CCF) within southern Qinling-Daba Mountains region (Qinba) had been selected as the targets. The soil bulk density (SBD), soil organic carbon concentration (SOCC), total nitrogen (TN), and total phosphorus (TP) had been determined. The results showed that SBD increased with depth, and other parameters decreased, which varied largely with the aspect and position. The SOC stocks (SOCS) of CCF, cropland, and CFC were 152.81 ± 5.17, 168.19 ± 11.87, and 183.92 ± 35.69 Mg C hm⁻², respectively. The SOCCs of CCF, cropland, and CFC were 17.71 ± 4.38, 20.23 ± 5.28, and 21.89 ± 7.33 g kg⁻¹, respectively. The CFC increased the correlations between SOC and TP, and decreased the correlations between SOC and TN. The CFC shifted the relationships of lg SOC versus lg N:P and lg SOC versus lg TP from decreasing returns of cropland to isometric. Overall, the CFCs enhanced SOC, especially in the middle shady hillslopes within the southern middle-mountain of Qinba. In contrast, SOC levels decreased in the sunny upper hillslopes. We presented the evidence that hillslopes aspect and position had significant effects on SOC, which was regulated by soil phosphorus.