Objectives Applying organic materials can improve soil structure and enhance the fertility retention and supply capacity of farmland. We have studied the mechanism of long-term combined application of organic materials combined with chemical fertilizers affects soil aggregate stability, providing a theoretical basis for organic material application techniques.

Methods The long-term location experiment was established in 2007 at the Dryland Farmland Ecological Field Scientific Observation and Research Station in Luoyang, Henan Province. This study selected five treatments, including no fertilization (CK), chemical fertilizer only (F), chemical fertilizer + straw (FS), chemical fertilizer + organic fertilizer (FM), and chemical fertilizer + straw + organic fertilizer (FSM). In May 2025, soil samples from the plough layer were collected during the wheat maturity period to analyze soil aggregate distribution, structural stability, nutrient content, and organic nitrogen components.

Results Compared with the CK and F treatments, the application of straw or the combined application of straw with organic fertilizer significantly elevated the proportion of >2 mm aggregates, while reducing the proportions of aggregates within the size ranges of 0.053−0.25 mm and <0.053 mm. The mean weight diameter (MWD), geometric mean diameter (GMD), and mass fraction of aggregates larger than 0.25 mm (R_0.25) under the FS and FSM treatments exhibited increases of 14.9%, 24.0%, 5.9%, and 17.6%, 27.5%, 6.6%, respectively, compared with those under the CK treatment. Moreover, the FSM treatment showed increases of 10.9%, 16.0%, and 4.1% over the F treatment. The FSM treatment produced the greatest increases in soil organic carbon (SOC), total nitrogen (TN), available phosphorus (AP), total phosphorus (TP), and available potassium (AK). Long-term application of organic materials reduced the C/N, C/P, and N/P ratios of soil aggregates. The application of organic materials significantly increased the total acid hydrolyzable nitrogen (TAN) content of aggregates of various sizes, with the FSM treatment showing the highest value (average 709.4 mg/kg), which was 9.7% – 55.5% and 4.5% – 47.9% higher than those of the CK and F treatments, respectively. The FM treatment followed, with increases of 15.5% – 34.6% and 8.5% – 30.0% compared with the CK and F treatments, respectively. The concentrations of acid-hydrolyzable ammonium nitrogen (ANN) and amino sugar nitrogen (ASN) exhibited significant increases across all fertilization treatments within the >2 mm and 0.25–2 mm aggregate fractions, with the FSM treatment yielding the most favorable outcomes (an average ANN of 144.2 mg/kg and an average ASN of 137.0 mg/kg). The acid-hydrolyzable amino acid nitrogen (AAN) content under the FS and FM treatments was higher by 36.3% and 91.0%, respectively, than that under the F treatment, but only within the >2 mm aggregate fraction. Regarding acid-hydrolyzable unknown nitrogen (HUN), the FSM treatment enhanced it by 95.2% and 68.9% within the >2 mm and 0.053–0.25 mm aggregate fractions, respectively, compared with the CK treatment. Random forest analysis revealed that TN in the >2 mm and 0.053–0.25 mm aggregates, along with AK in the 0.25–2 mm aggregates, exerted significant influences on MWD, GMD, and R_0.25, with contributions ranging from 15.4% to 23.3%. Additionally, AK, AP, C/P, and TP in aggregates of other sizes contributed 5.9% to 11.8%. Redundancy analysis (RDA) revealed that the first two axes of the organic nitrogen components in soil aggregates of various sizes accounted for more than 68.0% of the variance, indicating a strong regulatory effect of soil physicochemical properties. The key influencing factors and their explanatory powers differed among aggregates of different sizes, with C/N, TN, AP, and AK being the main driving factors for each size.

Conclusions The combined application of organic and inorganic fertilizers, especially the use of chemical fertilizers in combination with corn straw returning and organic fertilizers, can significantly increase the total nitrogen, available phosphorus, and available potassium contents of aggregates of various sizes, reduce the C/N, C/P, and N/P ratios of aggregates, and create a favorable microenvironment for microorganisms. The increase in the content and proportion of acid-hydrolyzable total nitrogen, acid-hydrolyzable ammonium nitrogen, acid-hydrolyzable amino sugar nitrogen, and acid-hydrolyzable unknown nitrogen promotes the proportion of >2 mm aggregates, enhances the MWD, GMD, and R_0.25 of soil aggregates, and facilitates the mineralization of organic nitrogen, effectively improving the nitrogen supply capacity of the soil. Under the tested soil conditions, C/N, TN, AP, and AK play a decisive role in aggregate stability and organic nitrogen composition. C/N and AK significantly affect organic nitrogen in >2 mm aggregates, TN significantly affects organic nitrogen in 0.25−2 mm aggregates, and AP has a significant impact on organic nitrogen in 0.053–0.25 mm and <0.053 mm aggregates.