Diversified Crop Rotation Improves Soil Quality by Increasing Soil Organic Carbon in Long-Term Continuous Cotton Fields

To explore the improvement effect of diversified crop rotation on soil quality in long-term continuous cotton fields (15 years), a field experiment was conducted in southern Xinjiang in 2024. With continuous cotton (C-C) as the control, four crop rotations, namely, cotton–maize (C-M), cotton–wheat (C-W), cotton–soybean (C-S), and cotton–peanut (C-P), were set up. The results showed that compared with C-C, the soil organic carbon (SOC) treated by C-P and C-S increased significantly by 11.76% and 3.38%, respectively, and the easily oxidized organic carbon (EOC) increased by 45.18% and 37.15%, respectively. The dissolved organic carbon (DOC) treated with C-S increased by 14.36%, while C-M decreased by 10.98%. The carbon pool index (CPI) of C-P was the highest in the 0–20 cm soil layer, which was 13.00% higher than that of C-C. The β-1, 4-glucosidase (BGL) activity of C-C at 0–20 cm and 20–40 cm was 144.70–387.26% and 48.01–71.32% higher than that of other treatments, respectively. The RuBisCo activity of C-P was 80.96% higher than that of C-C. The soil quality index was the highest for C-S, followed by C-P, which was 74.56% higher than that of C-C. In conclusion, the cotton–peanut rotation can effectively improve the soil quality of continuous cotton fields by increasing the organic carbon composition, enhancing the activity of carbon-fixing enzymes and bacterial diversity.