Objective 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 evaluates the comparative impacts of five fertilization regimes on soil aggregate dynamics, nutrient availability, and crop productivity in a rice-rape rotation system.

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 (macroaggregates: >2 mm; microaggregates: 2−0.25 mm; silt-clay fraction: <0.25 mm). The chemical structure of SOC was characterized using solid-state cross-polarization magic-angle spinning (CPMAS) ¹³C nuclear magnetic resonance (NMR) spectroscopy.

Results Compared with NPK, NPKM treatment significantly increased soil organic matter, total and quick-acting nitrogen and potassium content, increased effective phosphorus content in soil and aggregates of all grain levels, effective phosphorus content in 2-0.53 mm aggregates, and had no significant effect on organic phosphorus content in aggregates of all grain levels; alkaline phosphatase and urease activities were significantly increased in large, small and microaggregates of NPKM treatment 13.78%−88.06% (P<0.05).NPKM treatment obtained the highest rapeseed grain and straw yield, which was about 2−4 times higher than that of CK and NPK treatments. Compared with CK and NPK treatments, NPKM treatment decreased the proportion of alkyl carbon and increased the proportion of aromatic and carbonyl carbon in the agglomerates at each grain level, but the proportion of alkoxy carbon increased only in the powder sticky grain level agglomerates. The results of correlation analysis showed that the proportions of aromatic and carbonyl carbon in large and small grain-level aggregates showed positive correlations with soil AP and inorganic phosphorus content, and positive correlations with UE and S-ALP activities.

Conclusions Compared with long-term CK or NPK fertilizer treatments, organic and inorganic fertilizer dosing increased the aromatic and carbonyl carbon content of soil organic carbon in the agglomerates, promoted the transformation of organic phosphorus in the soil, reduced phosphorus fixation, and therefore improved the effectiveness of soil phosphorus, which was more conducive to the growth of oilseed rape.