Long-Term Organic Fertilization and Straw Return Affect Shajiang Black Soil (Vertisol) Nutrient Availability, Aggregate Stability, and Crop Yield with the Winter Wheat–Summer Maize Double-Cropping System in Northern China

Long-term fertilization affects soil nutrient levels and aggregate stability, eventually altering crop yield. However, their responses to organic fertilizer application and straw returning are still unclear, particularly as the contributions of soil nutrient levels and aggregate stability on crop yields remain poorly quantified. Therefore, topsoil samples (0–20 cm) were collected from six fertilization treatments in a long-term (13-year) Shajiang black soil field experiment with no fertilization (CK), chemical fertilization (NPK), 50% NPK plus pig manure (50%NPKP), 50% NPK plus cattle manure (50%NPKC), 70% NPK plus pig manure with straw return (70%NPKPS), and 70% NPK plus cattle manure with straw return (70%NPKCS). We examined the characteristics of crop yield, soil nutrient levels, and soil aggregate stability parameters, including under different long-term fertilization treatments. The results show that long-term fertilization significantly influenced the distribution of soil nutrients and soil aggregates in Shajiang black soil. Compared to CK, organic fertilizers and straw returning significantly increased the soil organic matter (SOM), total nitrogen (TN), and total phosphorus (TP) contents but decreased soil pH, respectively, indicating the best strategies for improving soil fertility. Compared to the CK and NPK treatments, long-term organic fertilization and straw returning significantly increased the mean weight diameter (MWD) and geometric mean diameter (GMD) values and significantly decreased the fractal dimension (Dm) and mean weight-specific surface area (MWSSA) values, with the 70%NPKCS treatment showing the most pronounced effect of improving aggregate stability. A redundancy analysis revealed that SOM and TN exert significant effects on aggregate stability. Furthermore, a stepwise regression analysis showed that SOM and TN were positive factors affecting the yields of wheat and maize, while MWD and pH were negative factors affecting wheat yield, demonstrating that high crop yields are derived from soils with limited stability and high fertility. Thus, our findings indicate that the integrated application of cattle manure with straw returning was the most effective strategy to promote soil nutrient accumulation, improve aggregate stability, and enhance crop yield, albeit with the potential risk of soil acidification, which requires management in the Shajiang black soil (Vertisol) region of Northern China.