Particle size primarily shifts chemical composition of organic matter under long‐term fertilization in paddy soil

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(H₂PO₄)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 in the chemical composition of SOM, with selective preservation of lignin and polysaccharides in the particulate organic matter (POM) and enrichments of lipids and N‐containing compounds in the MOM. The origin (plant‐ and microbial‐derived) and oxidation state of SOM were also regulated by particle size (especially the coarse particles) but not fertilisation treatment. Despite long‐term persistence, the MOM had a lower C oxidation state (implying higher vulnerability to microbial utilisation) than the coarse POM (cPOM) and fine POM (fPOM). In conclusion, fertilisation regime regulated the bulk SOM (SOC and TN) pool, whereas physical particle size generated a divergence trend in the chemical composition and oxidation state of SOM in the subropical paddy soil, indicating the greater role of physical arrangement in SOM chemistry than the quality of amendments. HIGHLIGHTS: Particle size, not fertilisation regime, regulated chemical composition and oxidation state of SOM. POM is enriched in lignin and polysaccharides while MOM is enriched in lipids and N‐containing compounds. Physical arrangement plays a greater role in SOM chemistry than the quality of amendments.