Effect of Combined Application of Wood Vinegar Solution and Biochar on Saline Soil Properties and Cotton Stress Tolerance

Biomass pyrolysis by-products, such as biochar (BC) and wood vinegar (WV), are widely used as soil conditioners and efficiency enhancers in agriculture. A pot experiment was conducted to examine the effects of WV, both alone and in combination with BC, on soil properties in mildly saline soil and on cotton stress tolerance. The results demonstrated that BC and WV application, either individually or together, increased soil nutrient content. The combined application was more effective than the individual applications, resulting in a 5.18–20.12% increase in organic matter, a 2.65–15.04% increase in hydrolysable nitrogen, a 2.23–58.05% increase in effective phosphorus, and a 2.71–29.38% increase in quick-acting potassium. Additionally, the combined application of WV and BC led to greater improvements in cotton plant height, net photosynthetic rate (Pn), leaf nitrate reductase (NR), superoxide dismutase (SOD), and catalase (CAT) activities compared to the application of BC or WV alone. The enhancements in this study varied across different parameters. Plant height showed an increase of 14.32–21.90%. Net photosynthetic rate improved by 13.56–17.60%. Leaf nitrate reductase increased by 5.47–37.79%. Superoxide dismutase and catalase showed improvements of 5.82–64.95% and 10.36–71.40%, respectively (p < 0.05). Moreover, the combined treatment outperformed the individual applications of WV and BC, resulting in a significant decrease in MDA levels by 2.47–51.72% over the experimental period. This combined treatment ultimately enhanced cotton stress tolerance. Using the entropy weight method to analyze the results, it was concluded that the combined application of WV and BC could enhance soil properties in mildly saline soils, increase cotton resistance, and hold significant potential for widespread application.