Contrasting Effects of Long‐Term Fertilization on Pore Structure and POM in Upland and Paddy Soils Revealed by X‐Ray CT

ABSTRACT Soil pore structure plays a fundamental role in particulate organic matter (POM) dynamics by regulating water and air exchange within the soil matrix. However, the effects of long‐term fertilization on POM distribution, pore characteristics, and their interrelationships may differ between upland and paddy soils. In this study, we examined two long‐term fertilization field experiments conducted in contrasting agroecosystems in Jiangxi, China: a 38‐year upland field and a 43‐year paddy field. Four fertilization regimes were compared: no fertilizer (control), inorganic fertilizer (NPK), double‐rate inorganic fertilizer (2NPK), and inorganic fertilizer combined with pig manure (NPKM). X‐ray computed tomography (CT) was used to quantify soil pore structure and POM, and a random forest model was trained on a manually classified dataset of 1322 POM fragments to operationalize the decomposition degree, distinguishing fresh (e.g., exhibiting low blobness, plateness) from decomposed (e.g., exhibiting high sphericity, compactness) morphologies based on four morphological features. The NPKM treatment significantly increased fresh and decomposed POM volume density by 2.05% (7.51 mm 3 cm −3 ) and 47.7% (2.01 mm 3 cm −3 ) in upland soil, and 253% (15.2 mm 3 cm −3 ) and 103% (1.47 mm 3 cm −3 ) in paddy soil, respectively. NPKM also enhanced air permeability ( K a ), image‐based porosity, connected porosity, > 300 μm porosity, and surface area density in both soils. POM in upland soil is rapidly aerobically decomposed and physically fragmented within dynamic pore networks, while in paddy soil, it is preserved by anaerobic conditions that suppress microbial activity. Overall, our findings demonstrate that combined organic–inorganic fertilization promotes both POM accumulation and the development of pore structure in upland and paddy soils, with distinct mechanisms operating under different land‐use systems.