Objectives This study investigated the effects of different land management and fertilization practices on soil organic carbon (SOC) content and aggregate stability, providing a scientific basis for optimizing land management and fertilization strategies.

Methods A localized field experiment was conducted in Yangling, Shaanxi, since 2013. In 2024 after maize harvest, soil samples were collected in bare land (F), fallow land (G, suspending crop planting while implementing soil management practices), and fertilization plots including unfertilized control (CK), applying N fertilizer only (N), N and P fertilizer (NP), N and K fertilizer (NK), P and K fertilizer (PK), NPK fertilizer (NPK), NPK+straw return (S+NPK), NPK + cow manure at the same total N input (M1+NPK), and at 1.5 times of total N input (M2+NPK) during wheat season, all the treatment had the same fertilizer input in maize season. The organic carbon content in bulk soil and different fractions, and aggregates of varying sizes were determined to evaluate soil fertility and aggregate stability while revealing their relationships.

Results Fallow land significantly enhanced soil fertility. Chemical fertilizer treatments were not effective in increasing SOC, while the combination of organic materials and NPK fertilizers significantly increased SOC content, and M2+NPK treatment showed the highest improvement, with the POC and MAOC increased by 249% and 60%, relative to that of CK. Bare land had the highest proportion of micro-aggregates (78.4%), showing the worst structure among the treatments, while NPK treatment demonstrated the highest values of GMD, MWD, and R_0.25, along with the lowest D value, showing the best soil structural stability. Pearson correlation analysis revealed that MAOC had stronger correlation with SOC than POC did. The PLS-PM model revealed that carbon content across aggregate sizes directly controlled POC and MAOC content, thereby indirectly regulating SOC content.

Conclusions The relatively high proportion of microaggregates in bare soil is unfavorable for organic carbon sequestration. Fallow promotes the accumulation of POC, not increases MAOC, indicating the lower carbon sequestration stability. NPK application enhances soil structural stability and facilitates carbon sequestration, but reduces the nutrient supply capacity of soils. Organic fertilizer outperforms straw return in enhancing soil organic carbon and its fractions, however, long-term application of cattle manure-based organic fertilizer in high rate may limit its benefits in carbon sequestration due to reduced aggregate stability. Although POC account dominant proportion on SOC, MAOC is more significantly correlated with SOC. Furthermore, POC and MAOC co-regulate SOC accumulation through direct contributions and aggregate-mediated indirect pathways.