Objective This study aimed to explore the effects of long-term organic material return on the stability of black soil aggregates and the distribution of carbon (C) and nitrogen (N), thereby to provide a theoretical basis for soil structure improvement and efficient C and N management in black soil regions.

Method A 13-year continuous field location experiment was conducted with six treatments, namely no fertilization (CK₀), chemical fertilizer alone (CK), chemical fertilizer combined with straw (ST), chemical fertilizer combined with biochar (BR), chemical fertilizer combined with cow manure (CM), and chemical fertilizer combined with compost (CP). The wet-sieving method was used to classify soil aggregates by particle size, and the contents and stocks of organic C and total N in each particle size fraction were determined.

Result The application of organic materials significantly improved the soil aggregate structure. Compared with the CK0 treatment, the proportion of large-sized aggregates (> 0.25 mm) in all organic material treatments increased by 10.0% - 37.2%. Moreover, the CM and CP treatments showed the best effects with the increase of 32.0% and 37.2%. The addition of organic materials promoted the transformation of micro-aggregates to macro-aggregates, and the mean weight diameter (MWD) and geometric mean diameter (GMD) of soil aggregates increased by 13.3% - 27.0% and 12.1% - 29.4%. Organic material application also increased the soil organic C content, with the BR and CP treatments showing significant increasing trend with 29.5% and 21.8%. The BR, CM, and CP treatments significantly increased the organic C contents in large-sized aggregates, while only the BR treatment significantly increased the organic C content in the < 0.053 mm aggregates. The CM and CP treatments had prominent effects on increasing soil total Nn content, with the increasing trends of 27.2% and 28.7%. The distribution of aggregates and the contents of C and N jointly regulated the C-N stocks and their relative contribution rates. After the application of organic materials, the C-N stocks and contribution rates of small-sized aggregates (< 0.25 mm) decreased. In large-sized aggregates, the CM and CP treatments significantly improved the N stock.

Conclusion The study indicates that long-term organic material return will optimize the distribution characteristics of C and N in aggregates of different particle sizes by promoting the transformation of micro-aggregates to macro-aggregates. Biochar and compost exhibit unique advantages in C sequestration in small soil aggregates and N storage in large aggregates.