The chemistry of soil organic matter (SOM) is critical for soil functions. However, the chemical composition and oxidation state of physical SOM fractions under long-term mineral and organic amendments in paddy soils are still unclear. Here, we investigated this issue using three particle size classes of SOM sampled from five long-term (37-years) fertilisation regimes in a subtropical paddy soil which were analysed using pyrolysis-gas chromatography-mass spectrometry (Py-GC/ MS). Coarse particulate organic matter (cPOM, >250 μm), fine particulate organic matter (fPOM, 53-250 μm) and mineral-associated organic matter (MOM, <53 μm) were isolated. The five fertilisation regimes were (1) unfertilised (control),(2) nitrogen (N, urea), (3) nitrogen + phosphorus + potassium (NPK, urea + Ca (H 2 PO 4 ) 2 + KCl), ( 4) NPK + straw (NPKS) and ( 5) NPK + Astragalus sinicus L. + pig manure (NPKM). After 37-years, the SOM content of the control, N, NPK, NPKS and NPKM treatments had increased by 16.3%, 12.3%, 31.6%, 41.7% and 59.8% compared with the initial level, respectively. The pools of soil organic carbon (SOC) and total N (TN), as well as the C/N ratio in the soil matrix, were localised by particle size, with the majority in the mineral-associated organic matter (MOM). Particle size, not fertilisation regime, primarily associated with shifts