Long-Term Cotton Straw Returning May Increase Soil Secondary Salinization Risk in Some Xinjiang Cotton Fields: Evidence of Threshold-like Responses and Spatial Heterogeneity

Background: Cotton straw returning (CSR) is widely implemented in Xinjiang to achieve in situ residue utilization and ameliorate saline–alkali soils. However, its long-term efficacy in improving soil fertility without inducing secondary salinization remains poorly understood. Methods: This study evaluated the effects of different CSR durations on soil physicochemical properties and cotton yield across four major cotton-growing regions of Xinjiang, China (Shawan, Wusu, Manas, and Shihezi). Cotton fields with different CSR durations were classified into four treatments: CK (0 years), T1 (5 years), T2 (10 years), and T3 (20 years). Soil bulk density (BD), exchangeable sodium percentage (ESP), and key soil fertility indicators were measured to assess comprehensive soil fertility and soil secondary salinization risk. Results: CSR generally enhanced soil physicochemical properties, but responses exhibited spatial heterogeneity. Soil BD decreased in Wusu and Shihezi but showed temporary increases in Manas and Shawan. SFI increased continuously with CSR duration in Shawan and Wusu, whereas it peaked at around 10 years in Shihezi and Manas. Cotton yield was highest under the 20-year treatment in Manas but peaked under the 10-year treatment in the other regions. Notably, prolonged CSR (>10 years) elevated ESP in some areas, suggesting an increased risk of secondary salinization. Conclusions: A CSR duration of roughly 10 years appears optimal for balancing soil amelioration and salinity control in saline–alkali cotton fields of Xinjiang.