Effects of Substituting Organic Fertilizers for Chemical Nitrogen Fertilizers on Physical and Chemical Properties and Maize Yield of Anthropogenic-Alluvial Soil

To resolve issues in the traditional agricultural production of the Ningxia irrigated area, where the sole pursuit of yield through extensive application of chemical nitrogen fertilizers has resulted in a deteriorated soil structure, reduced quality of anthropogenic-alluvial soil, and limited improvement in crop yield per unit area, a fixed-site experiment on substituting organic fertilizers for chemical nitrogen fertilizers was performed at the comprehensive experimental base of the NingXia Academy of Agriculture and Forestry Sciences during 2021–2024. Using conventional fertilization (N, P2O5, and K2O application amounts of 450, 150, and 60 kg·ha−1, respectively) as the control (CK), treatments of substituting organic fertilizers for 15% (T1), 30% (T2), 45% (T3), and 100% (T4) of chemical nitrogen fertilizers were used to analyze their effects on soil physical and chemical properties, as well as the maize yield in anthropogenic-alluvial soil. Substituting organic fertilizers for chemical nitrogen fertilizers increased the content of water-stable macroaggregates and the mean weight diameter (MWD) stability parameter in the soil. In 2024, the treatments of substituting organic fertilizers for chemical nitrogen fertilizers significantly increased MWD by 24.18–30.22% compared to the CK treatment. The soil’s available nitrogen content significantly decreased under the T4 treatment by 8.25–20.50% compared to CK treatment during 2021–2024. The organic matter (OM) content showed an increasing trend with the proportion of substitution of organic fertilizers for chemical nitrogen fertilizers; in 2024, the T3 and T4 treatments significantly increased OM by 5.98% and 6.60%, respectively, compared to CK. Furthermore, the available phosphorus and potassium contents also exhibited an increasing trend with the proportion of substitution of organic fertilizers for chemical nitrogen fertilizers. Based on the full dataset method, it was calculated that the T1 treatment consistently improved the soil quality index (SQI) during 2021–2024, with an increase of 9.31–18.29% compared to CK. The T1 treatment increased maize yield by 9.90% and 16.93% in 2023 and 2024, respectively, compared to CK. A random forest model identified the available nitrogen as the most critical physical and chemical indicator affecting SQI, followed by the available potassium. Linear fitting between the SQI and yield showed a highly significant positive correlation (R2 = 0.6288, p < 0.01). Moreover, polynomial fitting of the proportion of substitution of organic fertilizers for chemical nitrogen fertilizers showed that SQI reached a maximum for a substitution proportion of 31.46%, while the maximum maize yield reached a proportion of 28.74%. Comprehensive analysis combining information and weight suggested an optimal proportion of substitution of organic fertilizers for chemical nitrogen fertilizers of 29.52%, achieving both an increase in SQI and maize yield in the anthropogenic-alluvial soil of the Ningxia irrigated area, while also achieving a rational utilization of organic fertilizer.