Objectives Long-term and excessive application of chemical fertilizers adversely affects soil structure and the distribution of carbon and nitrogen (N) nutrients. We analyzed the influence of nitrogen fertilizer application rates on soil aggregate size distribution and associated carbon and nitrogen contents, with the aim of providing a theoretical support for mitigating soil structure degradation and enhancing carbon and nitrogen sequestration.

Methods A long-term experiment,initialed in 2011 in Yuzhou, Henan Province, was used in this research. In 2024, soil samples were collected from the 0−100 cm profile at 20 cm intervals in five N application rate treatment plots: no N control (N0), low (120 kg/hm², N120), moderate (N 180 kg/hm², N180), moderate high (N 240 kg/hm², N240), and high (N360 kg/hm², N360). The composition, stability, along with their organic carbon and total nitrogen contents were analyzed.

Results Across the 0−100cm soil profile, the proportion of >2 mm, 2−0.25, and <0.25 mm aggregates was 25.59%−38.75%, 36.35%−42.54%, and 20.78%−34.57%, respectively. The proportion of >2 mm aggregates was significantly (P<0.001) and positively correlated with the mean weight diameter (MWD), geometric mean diameter (GMD), and percentage of >0.25 mm aggregates (R>0.25). N120 treatment increased MWD, GMD, and R>0.25 while reducing the unstable aggregate index (Elt), whereas N360 treatment decreased these stability indices. With increasing soil depth up to 100 cm, MWD, GMD and R>0.25 gradually increased, while Elt decreased. The aggregate total N and organic carbon content were all the highest in 0−20 cm soil layer. Compared with N0, nitrogen application increased aggregate organic carbon contents in the 0−60 cm soil layer, and N240 treatment demonstrated significant effect, but did not showed the improvement effect on the 60−100 cm soil layer. The low-nitrogen treatment reduced the organic carbon content of aggregates in the 60−80 cm soil layer, while high-N treatment enhanced organic carbon content in aggregates in the 80−100 cm soil layer. N240 treatment increased total N content of all sizes of aggregates in the 0−60 cm soil layer. while N360 resulted the highest aggregate total nitrogen content. In the 60−80 cm soil layer, the total N content in >2 mm aggregates increased with the increase of N application rate, that in 2−0.25 mm aggregates reached peaks at N180 treatment, while that in <0.25 mm aggregates did not changed significantly. In the soil layer of 80−100 cm, the total N content in >2 mm aggregates increased with the increase of nitrogen application rate. The total nitrogen content of aggregates 2−0.25 mm showed no significant difference among treatments, while the total nitrogen content of aggregates <0.25 mm reached the highest under the N360 treatment. The correlation results showed that the MWD, GMD, and R>0.25 of aggregates were negatively correlated with the organic carbon and nitrogen contents in aggregates.

Conclusions Long-term nitrogen fertilization increased the proportions of >2 mm aggregates and reduced the proportion of <0.25 mm aggregates. Application of N 120 and 240 kg/hm² resulted the highest aggregate stability, while 180 kg/hm² led to the lowest stability. The application of N 240 kg/hm² most effectively enhanced aggregate organic carbon content in 0−60 cm soil layer and total aggregate N content in the 0−20 cm soil layer. In contrast, N 360 kg/hm² decreased aggregate stability but increased carbon content in all sizes of aggregates within 80−100 cm soil layer. Overall, applying nitrogen at 240 kg/hm² is moderate rate that improve aggregate stability and promotes the sequestration of nitrogen and carbon, thereby helping to alleviate degradation of soil structure.