Synergistic effect of the amendment on soybean yield increase and soil remediation in soda saline–alkali land: focusing on the soil quality index

Understanding the relationship between the application of soil conditioners and soil quality is crucial for achieving sustainably high crop yields and the remediation of saline-alkali soils in agricultural systems. However, to date, there have been few reports on the use of soil quality index (SQI) to characterize soil quality restoration in different soil layers of saline alkali land. We conducted a field experiment over three growing seasons (2022 – 2024) and collected soil samples after soybean harvest in 2024. The following treatments were employed: conventional fertilization (CK), conventional fertilization plus 1 t ha-¹ citric acid (T1), conventional fertilization plus 1 t ha-¹ biochar (T2), and conventional fertilization plus 1.5 t ha-¹ organic fertilizer (T3). Based on the Soil Quality Index, a comprehensive evaluation was performed to assess variations in soil fertility, alkalization index, and crop yield across three soil depths: 0–10 cm, 10–20 cm, and 20–30 cm. The research results indicate that: Compared with the CK treatment, the application of amendments significantly increased the concentrations of available nitrogen, available phosphorus, and available potassium. Conversely, it reduced soil pH (0.95%-5.83%) and decreased levels of exchangeable sodium, exchangeable sodium percentage (10.88%-30.12%), and sodium ion (20.26%-37.15%). The amendments significantly enhanced the SQI through synergistic regulation of soil fertility and salinity. Specifically, the citric acid treatment (T1) yielded the most pronounced improvement in the 0–10 cm soil layer, whereas the organic fertilizer amendment (T3) was more effective at depths of 10–30 cm. The soybean yield increase effect of citric acid treatment (T1) is the best, with a yield increase rate of 42.67% (P < 0.05). In summary, citric acid (T1) optimizes nutrient availability by alleviating saline-alkali stress, laying a theoretical foundation for ecological restoration in alkaline soils and enhancing agricultural productivity.