Objectives We studied the remediation effect of several amendment materials applied alone and in combination on agglomerates and maize yield, as well as the main mechanism of action, to propose practical measurements for saline-alkali soil improvement in Hetao Irrigation District of Inner Mongolia.

Methods A field experiment was conducted for two consecutive years in the saline-alkali soil of Hetao Irrigation District of Inner Mongolia, the test crop was maize. A total of 8 treatments were set up: CK (no remediation materials), T1 (desulfurized gypsum 15 t/hm²), T2 (new conditioner 15 t/hm²), T3 (cow dung organic fertilizer 7.5 t/hm²), T4 (desulfurized gypsum 7.5 t/hm² + new conditioner 7.5 t/hm²), T5 (desulfurized gypsum 7.5 t/hm² + cow dung organic fertilizer 3.75 t/hm²), T6 (new conditioner 7.5 t/hm² + cow dung organic fertilizer 3.75 t/hm²), T7 (desulfurized gypsum 7.5 t/hm² + new conditioner 7.5 t/hm² + cow dung organic fertilizer 3.75 t/hm²). At harvest of maize, topsoil samples were collected for measurement of physical and chemical properties, aggregate stability, organic carbon content, and maize yields were investigated. The main factors, and the relationships between soil property indicators and the aggregate characters were analyzed.

Results Compared with the control, all the treatments reduced soil bulk density, pH and salinity content, and increased available N, P, K, and organic carbon contents, and the organic carbon content of the new conditioner with desulfurized gypsum and cow dung organic fertilizer increased by 72.66% (P<0.05) compared to the application of new conditioner alone. Compared with the control, T7 treatment significantly reduced the content of <0.25 mm aggregates, and increased the MWD and GMD by 47.83% and 32.50%, respectively. The application of remediation materials increased the organic carbon content of aggregates in different particle sizes, and T7 treatment significantly increased the organic carbon content in aggregates >5 mm, 1−2 mm, 0.5−1 mm, with the largest increase in 1−2 mm. The application of remediation materials increased crop yield by 38.38% to 161.38%, and the effect of combined application on maize yield was more obvious, with the highest yield in T7 treatment. The redundancy analysis and random forest model found that soil SOC, AP, AK, pH and salinity significantly affected soil aggregate distribution and maize yield. The structural equation model showed that aggregate distribution and organic carbon had positive effect (P<0.05) on aggregate stability.

Conclusions Compared to individual remediation material, the combined application of desulfurized gypsum 7.5 t/hm² + new conditioner 7.5 t/hm² + cow dung organic fertilizer 3.75 t/hm² showed significantly better effect on improving soil pH, available nutrient contents, increasing the organic matter contents and thereby the stability of soil aggregates and maize yield, so is thought to be a measurement to maximize the synergistic effect of soil structure improvement, fertility improvement and crop yield increase in saline-alkali soil of Hetao irrigation area.