Objective Applying organic materials can improve soil structure and enhance the soil's capacity to retain and supply nutrients in farmland. We investigated the mechanisms by which long-term combined application of chemical fertilizers and organic fertilizers affects soil aggregate stability, providing a theoretical basis for organic material application techniques.

Method The long-term field experiment, located in Luoyang Dryland Farmland Ecological Field Scientific Observation and Research Station, Henan, began since 2007. Five treatments were selected for soil sampling in 2025 after wheat harvest,, including no fertilization (CK), chemical fertilizer alone (F), chemical fertilizer plus straw return (FS), plus manure (FM), and plus both straw return and manure (FSM). The soil aggregate composition were analyzed to estamite, structure stability, the contents of aggregate nutrients and organic N fractions were analyzed to calculate for explorition of mechanisms causing stability of aggregates.

Result Compared with the CK and F treatments, the application of straw/organic fertilizer significantly increased the proportion of >2 mm aggregate, while reduced the proportions of 0.053−0.25 mm and <0.053 mm aggregates. In the FS and FSM treatments, the mean weight diameter (MWD), geometric mean diameter (GMD) and percentage of >0.25 mm aggregates (R_0.25) increased by 5.9%～24.0% and 6.6%～27.5% compared to CK treatment, and by 3.5%～12.8% and 4.1%～16.0% compared to F treatment, respectively. The combined application of organic materials and chemical fertilizers significant altered nutrient contents within aggregates, the FSM treatment was the most effective in increasing organic carbon (SOC), total N (TN), available phosphorus (AP), total phosphorus (TP) and available potassium (AK). Long-term application of organic materials decreased the C/N, C/P and N/P ratios of soil aggregates. Furthermore, organic materials significantly increased total acid-hydrolyzable N (TAN) across all aggregate sizes. The FSM treatment exhibited the highest content (655.0～760.9 mg kg⁻¹), increasing by 9.7%～55.5% and 4.5%～47.9% compared with CK and F treatments, respectively, followed by the FM treatment (increasing by 15.5%～34.6% over CK, and 8.5%～30.0% over F treatment). Acid-hydrolyzable ammonium N (ANN) and amino sugar N (ASN) were significantly increased in >2 mm and 0.25-2 mm aggregates across all fertilized treatments, with FSM treatment showing the best performance (93.2～180.7 mg kg⁻¹ and 110.8～169.1 mg kg⁻¹). Notably, acid-hydrolyzable amino acid N (AAN) in FS and FM treatments increased by 36.3% and 91.0%, respectively, compared to F treatment only in >2 mm aggregates. For acid-hydrolyzable unknown N (HUN), FSM treatment increased by 21.3%～95.2% compared to CK. Random forest analysis indicate that TN in >2 mm and 0.053−0.25 mm aggregates, and AK in 0.25−2 mm aggregate, were the primary factors influencing MWD, GMD and R_0.25 (contribution: 15.4%～23.3%), followed by AK, AP, C/P and TP in other aggregate sizes (contribution: 5.9%～11.8%). Redundancy analysis (RDA) showed that the first two axes explained over 68.0% of the variation in organic N fractions, and soil physicochemical properties exerted a strong effect on them. Although the key impact factors and their explanatory rates vary among different soil aggregates sizes, C/N, TN, AP and AK were the primary driving factors for all fractions.

Conclusion The co-application of organic and inorganic fertilizers, particularly the co-application of chemical fertilizers with maize straw return and organic manure, significantly increased the contents of total N, available P, and available K across all aggregate size fractions. Concurrently, it reduced the C/N, C/P, and N/P ratios of the aggregates, thereby creating a favorable micro-environment for microbial activity. The increases in both the contents and proportion of total acid-hydrolyzable nitrogen, acid-hydrolyzable ammonium nitrogen, acid-hydrolyzable amino sugar nitrogen, and acid-hydrolyzable unknown nitrogen promoted the proportion of macroaggregates (>2 mm). This enhancement improved the mean weight diameter (MWD), geometric mean diameter (GMD), and the percentage of aggregates larger than 0.25 mm (R0.25). Furthermore, this process facilitated the mineralization of organic nitrogen and effectively improved the soil’s N supply capacity. Under the tested soil conditions, the C/N ratio, TN, AP, and AK were identified as key determinants of aggregate stability and organic nitrogen composition. Specifically, C/N and AK significantly influenced organic nitrogen in >2 mm aggregates; TN had a significant effect on organic nitrogen in the 0.25−2 mm fraction; whereas AP exerted a significant influence on organic nitrogen in the 0.053–0.25 mm and <0.053 mm fractions.