Objectives Organic fertilizer is effective in improving soil structure and carbon and nitrogen retention. We studied the suitable substitution proportions of organic fertilizer based on long-term experiment, aiming to provide scientific basis for soil fertility improvement in the black soil region of Jilin Province.

Methods The 8-year positioning experiment located in Gongzhuling City, Jilin Province. Soil samples were collected without disturb from 0−20 cm and 20−40 cm soil layers in the treatment plots of no fertilizer (CK), only chemical fertilizer (M0), 25% organic fertilizer nitrogen substitutes chemical nitrogen (M25), 50% organic fertilizer nitrogen substitutes chemical nitrogen (M50), and 100% organic fertilizer nitrogen substitutes chemical nitrogen (M100). Dry sieving and wet sieving methods were employed to study the proportion of soil aggregates in different sizes, and the carbon and nitrogen content, the carbon-to-nitrogen ratio of each size soil aggregates.

Results Compared to CK, M0 treatment reduced the proportion of macroaggregates (R_>0.25) and aggregate stability in 0−20 cm soil layer. Compared to M0 treatment, M25, M50, and M100 treatments increased the proportions of 2−10 mm aggregates and R_>0.25, and decreased the proportions of 0.25−2 mm and <0.25 mm ones. The proportion of 7−10 mm aggregates in 20−40 cm soil layer was 3.56% to 9.92% lower than in the 0−20 cm soil layer, while the proportions of 0.5−5 mm aggregates were higher, and those of 5−7 mm and<0.5 mm aggregates remained relatively stable. Compared to M0, the M25 and M50 treatments reduced the proportion of R_>0.25, M100 treatment increased. Compared to CK, the M0 treatment increased the proportion of R_>0.25 in 0−20 cm by 0.13%, decreasing the mean weight diameter (MWD), geometric mean diameter (GMD), fractal dimension (D), organic carbon content (3.4%) and the C/N (21.29%) of water-stable aggregates. Compared to the M0 treatment, M25, M50, and M100 treatments did not increase the proportion of R_>0.25 significantly, but enhanced the C/N ratio by 6.85%, 33.02%, and 34.97%, respectively; M25 decreased the MWD and GMD, while M50 and M100 treatments were recorded higher MWD and GMD, and M25 and M50 treatments were recorded 9.18% and 16.08% higher organic carbon of aggregates than M0 respectively, while M100 was 5.36% lower. The organic carbon and total N content of water-stable aggregates in the 0−20 cm soil layer were significantly higher than in the 20−40 cm soil layer, with the highest content in the>2 mm, 1−2 mm, and 0.5−1 mm aggregates, and the lowest in the 0.053−0.25 mm aggregate. Compared to the CK treatment, M0 treatment decreased R_>0.25 by 10 percentage points, while increased 0.053−0.25 mm and <0.053 mm aggregates by about 2 percentage points in the 20−40 cm soil layer. Compared to the M0 treatment, organic substitution treatments increased R_>0.25 by 8.29−13.25 percentage points, decreased the 0.053−0.25 mm and <0.053 mm aggregates by 1.26−2.31 and 1.65−2.34 percentage points, respectively, and the increments in R_>0.25 were not significant different among treatments. Compared to the M0 treatment, organic substitution treatments increased aggregate stability to varying degrees. In the 0.25−0.5 mm and 0.053−0.25 mm aggregates, the organic carbon content increased with the increase of the organic substitution proportion.

Conclusions Substitution of chemical fertilizer with 50% of organic fertilizer showed the best effects in improving soil structure, promoting the accumulation of organic carbon and total nitrogen throughout the soil profile, accelerating the formation and stability of macroaggregates, and enhancing soil carbon sequestration capacity and fertility, providing a core strategy for the sustainable use of black soil and the development of green agriculture.