Objective This study investigates the comprehensive effect of long-term fertilization on soil aggregate size distribution, physicochemical properties, and microbial characteristics in walnut orchards and clarifies its regulatory mechanisms, aiming to provide theoretical and practical support for scientific fertilization. Methods The walnut (Juglans regia L.) cultivar ‘Xiangling’ was selected as the research object. Five fertilization treatments were applied: (1) no fertilization (CK), (2) chemical fertilizer alone (T1), (3) combined application of chemical and organic fertilizers (T2), (4) combined application of chemical and bacterial biofertilizers (T3), and (5) combined application of chemical, organic, and bacterial biofertilizers (T4). Soil aggregate sizes were refined using the dry-wet sieving method, and soil aggregate structural stability, nutrient distribution, and microbial activity were systematically evaluated. Results Long-term fertilization significantly improved the stability and functionality of soil aggregates. The T4 treatment showed the optimal comprehensive effectiveness, significantly increasing the proportion of small- and medium-sized aggregates (1~0.25 mm), improving soil pH and moisture content, and enhancing microbial activity and enzymatic metabolic potential. Aggregate size affected microbial enrichment and enzyme activity distribution, with small and medium-sized aggregates demonstrating stronger ecological advantages in nutrient retention and functional enzyme expression. Although the T1 treatment showed limited improvement in microbial activity, its higher soil microbial biomass nitrogen to total nitrogen (SMBN/TN) ratio indicated a regulatory effect on nitrogen transformation. Conclusion The combined application of chemical, organic, and bacterial biofertilizers can optimize aggregate structure and activate carbon- and nitrogen-related enzyme activities, thereby establishing an efficient and stable nutrient cycling system. This significantly enhances soil ecological functions and production potential, making it a key approach for achieving high yield and high quality in walnut orchards while promoting sustainable soil management.
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