| With the increasing planting area and breeding scale of cotton in southern Xinjiang, the efficient decomposition of cotton straw in saline environments and the rational utilization of biogas slurry have become important issues restricting the sustainable development of local agriculture. This study conducted a 120 day indoor cultivation experiment in 2023. Five treatments were set up, including cotton stem full return (AS), low amount of biogas slurry (LB), high amount of biogas slurry (HB), cotton stem return with low amount of biogas slurry (AS+LB), and cotton stem return with high amount of biogas slurry (AS+HB). Destructive sampling was conducted on days 3, 6, 10, 20, 30, 40, 60, 80, 100, and 120, respectively, Analyzed the changes in the main physicochemical factors and key enzyme activities of soil under different treatments. The results showed that the soil organic matter treated with biogas slurry showed two stages: rapid increase (0-10 days) and slow accumulation (>10 days). High volume biogas slurry application increased soil organic matter, total nitrogen, available phosphorus, and available potassium by 25.15%, 35.26%, 79.25%, and 66.9%, respectively. At the same time, it increased soil catalase, alkaline phosphatase, and sucrase activities by 90.67%, 66.54%, and 44.25%, respectively. However, it also improved soil conductivity. Redundancy analysis (RDA) revealed that soil organic matter is the main controlling factor for soil enzyme activity changes, explaining 54.30% of variance variation. Therefore, the combination of straw returning and biogas slurry application can accelerate the decomposition of cotton straw, increase the organic matter and total nitrogen content of saline soil in cotton fields, and promote the increase of enzyme activity. However, excessive biogas slurry application may increase the risk of soil salinization. |