Objectives The size distribution and chemical composition of soil organic carbon (SOC) in aggregates play pivotal roles in regulating nutrient storage, transformation, and extracellular enzyme-driven phosphorus cycling in paddy ecosystems. This study explores the impacts of different fertilization regimes on the proportion of soil aggregate size fractions and the corresponding chemical structure of organic carbon in paddy soil, aiming to provide a theoretical basis for scientific fertilization.

Methods A long-term field trial was initiated in 2011 under a rice-rape annual rotation in a subtropical paddy region. The experimental design included five treatments: a no-fertilizer control (CK), nitrogen-potassium fertilizer (NK), nitrogen-phosphorus-potassium fertilizer (NPK), organic manure (M), and combined organic-inorganic fertilization (NPKM). In May 2023, rapeseed grain and straw yields were recorded, and soil samples were collected for aggregate fractionation via wet sieving. Key analyses included soil phosphorus (P) fractions, extracellular enzyme activities (alkaline phosphatase, urease), and N, P, and K concentrations across aggregate size classes. The chemical structure of SOC was characterized using solid-state cross-polarization magic-angle spinning ¹³C nuclear magnetic resonance (NMR) spectroscopy.

Results Compared with NPK, NPKM treatment significantly increased soil organic matter, total and available nitrogen and potassium content, increased available phosphorus content in soil and across aggregate size classes, and had no significant effect on organic phosphorus; alkaline phosphatase and urease activities within the macroaggregates, microaggregates, and silt-clay aggregates were significantly increased by NPKM treatment (P<0.05). Under the conditions of this study, NPKM treatment obtained the raltive high rapeseed grain and straw yield, which was about 2−4 times those of CK and NPK treatments. Compared with the CK and NPK treatments, the NPKM treatment resulted in a decrease in the proportion of alkyl carbon in aggregate size fractions across aggregate size classes, and at the same time led to an increase in the proportions of aromatic carbon and carboxyl carbon. However, the proportion of alkoxy carbon increased only in the silt-clay size aggregates. The results of the correlation analysis revealed that the proportions of aromatic carbon and carbonyl carbon in large and small aggregate size fractions were positively correlated with the contents of soil available phosphorus and inorganic phosphorus, and also showed positive correlations with the activities of urease and alkaline phosphatase, respectively.

Conclusions Compared with the long-term no-fertilization treatment or the NPK chemical fertilizer treatment, the combined application of organic and inorganic fertilizers increased the contents of aromatic carbon and carboxyl carbon in soil organic carbon within aggregates, promoted the transformation of organic phosphorus in the soil, reduced phosphorus fixation, thereby enhancing the availability of soil phosphorus and being more conducive to the growth of oilseed rape.