Straw return increases crop production by improving soil organic carbon sequestration and soil aggregation in a long-term wheat-cotton cropping system

Straw return is a promising strategy for managing soil organic carbon (SOC) and improving yield stability. However, the optimal straw return strategy for sustainable crop production in the wheat (Triticum aestivumL.)-cotton (Gossypium hirsutumL.) cropping system remains uncertain. The objective of this study was to quantify the long-term (10 years) impact of C input on SOC sequestration, soil aggregation and crop yields in a wheat–cotton cropping system in the Yangtze River Valley. Five treatments were arranged with a single-factor randomized design as follows: no straw return (Control), return of wheat straw only (Wt), return of cotton straw only (Ct), return of 50% wheat and 50% cotton straw (Wh-Ch) and return of 100% wheat and 100% cotton straw (Wt-Ct). In comparison to the Control, the SOC content increased by 8.4 to 20.2% under straw return. A significant linear positive correlation between SOC sequestration and C input (1.42-7.19 Mg ha−1yr−1) (P<0.05) was detected. The percentages of aggregates of sizes >2 and 1–2 mm at the 0–20 cm soil depth were also significantly elevated under straw return, with the greatest increase of the aggregate stability in the Wt-Ct treatment (28.1%). The average wheat yields increased by 12.4–36.0% and cotton yields increased by 29.4–73.7%, and significantly linear positive correlations were also detected between C input and the yields of wheat and cotton. The average sustainable yield index (SYI) reached a maximum value of 0.69 when the C input was 7.08 Mg ha−1yr−1, which was close to the maximum value (SYI of 0.69, C input of 7.19 Mg ha−1yr-1) in the Wt-Ct treatment. Overall, the return of both wheat and cotton straw was the best strategy for improving SOC sequestration, soil aggregation, yields and their sustainability in the wheat-cotton rotation system.