Objective  The objective of this study was to assess the effect of different organic materials coupled with optimized nitrogen (N) application on soil organic carbon (SOC) and its labile fractions, as well as the formation and stabilization mechanism of soil aggregates in calcareous aquic soil.

  Method  Based on a long-term experiment, the effects and mechanisms of SOC sequestration were studied in four treatments. They are conventional N application (CP_N), optimized N application (OF_N), straw combined with optimized N application (S_N) and straw biochar combined with optimized N application (BC_N).

  Result  BC_N enhanced SOC hugely among the four fertilization treatments, reaching 18.68 g kg^-1, followed by S_N, CF_N and OF_N treatment. Compared with CF_N treatment, organic materials combined with optimized N application could significantly increase the contents of microbial biomass carbon (MBC), dissolved organic carbon (DOC), particle organic carbon (POC) and easily oxidized organic carbon (EOC) by 128.26%, 96.62%, 219.72% and 45.42%, respectively. At the same time, the proportion of aggregates > 0.25 mm and the mean weight diameter (MWD) were significantly increased. The proportion of aggregates > 0.25 mm of S_N and BC_N were increased by 132.44% and 91.45%, and the MWD values were increased by 89.79% and 66.87%, respectively. This is mainly due to the increase in soil exchangeable Ca^{2 +} , Mg^{2 +} , K ⁺ and iron-aluminum oxides content, as well as the decrease in soil exchangeable Na ⁺ , which were positively correlated with the formation and stability of macroaggregates. However, there was no significant difference on the measured indicators between the OF_N and CF_N treatment, but the amount of N input was significantly reduced (261 vs. 480 kg N hm^-2 yr^-1).

  Conclusion  Although there were differences in the effects of long-term (12-years) straw and straw biochar applied with equal carbon combined with optimized N input on the enhancement of SOC and improvement of soil aggregate structure, they were significantly better than those of N application alone. The results could provide theoretical support for co-regulation strategy of carbon and N to promote soil health in calcareous fluvo-aquic soils of the North China Plain.