【Objective】Soil organic carbon (SOC) sequestration in agricultural ecosystems is critical for mitigating climate change and maintaining soil fertility, with mineral-associated organic carbon (MAOC) playing a central role in long-term C stabilization. Paddy soils with higher SOC density exhibit distinct biogeochemical cycles due to periodic flooding and anaerobic conditions, making their SOC dynamics particularly complex. While biochar amendment has emerged as a promising strategy to enhance SOC storage, the specific mechanisms by which biochar interacts with soil mineral fractions and modulates native SOC stability remain poorly understood. Previous studies have primarily focused on total SOC changes, overlooking the differential responses of mineral-bound C pools to biochar input. This knowledge gap hinders accurate assessments of biochar"s long-term C sequestration potential in paddy systems. The present study aimed to address this gap by investigating how biochar amendment affects SOC distribution across density-based mineral fractions and alters native SOC dynamics through advanced spectroscopic and isotopic tracing techniques.【Method】In this study, a field experiment was established in a typical paddy soil in southern China, with two treatments: biochar application at 15 t·ha?1 (C15) and no biochar (C0). After two years of rice cultivation, soil samples were collected from the 0-15 cm depth and subjected to sequential density fractionation using sodium polytungstate solutions with gradient densities (1.65, 1.85, 2.05, 2.25, 2.45, 2.65 g·cm?3). Each fraction was characterized for SOC content, stable isotope composition (δ13C), and chemical functional group via Fourier-transform infrared spectroscopy (FTIR). Scanning electron microscopy (SEM) coupled with energy-dispersive X-ray spectroscopy (EDS) was used to visualize particle morphology and elemental composition, while X-ray diffraction (XRD) identified dominant mineral phases in each fraction. Isotopic mixing models were applied to quantify biochar-derived C versus native SOC contributions across density gradients.【Result】The results showed that (1) Based on SOC content and soil minerals categories, density fractionation successfully separated soil into three functionally distinct pools: particulate organic carbon (POC, <1.85 g·cm?3), clay mineral-associated C (1.85~2.45 g·cm?3), and primary mineral-bound C (>2.45 g·cm?3). XRD analysis confirmed that the 1.85~2.45 g·cm?3 fraction was enriched in 2:1 phyllosilicate (e.g., montmorillonite, illite) and Fe/Al oxides, whereas the >2.45 g·cm?3 fraction contained quartz and feldspars. Fourier-transform infrared spectroscopy (FTIR) demonstrated that the intensities of O-H stretch (2923 cm–1) for aliphatic structures and C=C stretch (1610 cm–1) for aromatic compounds gradually decrease in both biochar application (C15) and non-application of biochar (C0) treatments with increasing density, while SOC stability progressively increased. (2) SOC content of density-specific changes varied under biochar amendment: Contribution of SOC in the <1.65 g·cm?3 fraction increased by 150.1%, driven by biochar particles, while the 1.65~1.85 g·cm?3 fraction showed a 60.9% increase, due to biochar-derived C adsorption onto clay minerals. Conversely, the 1.85~2.05 g·cm?3 clay fraction exhibited a 37.4% reduction in SOC contribution. δ13C analysis confirmed biochar-C presence across all fractions, with the highest incorporation (64.5%) in the <1.65 g·cm?3 fraction. Native SOC depletion was observed in five density intervals, with the most severe loss (-41.2%) in the <1.65 g·cm?3 fraction, indicating strong positive priming. Notably, priming extended to the 1.85~2.25 g·cm?3 clay fraction (-14.6%), suggesting biochar-induced microbial activity stimulated decomposition of relatively stable mineral-protected C.【Conclusion】This study demonstrates that biochar amendment effectively enhances total SOC content in paddy soil within two years, but its C sequestration efficiency is offset by priming-induced native SOC losses across labile and mineral-protected pools. The findings highlight the need to account for biochar-microbe-mineral interactions when evaluating long-term C sequestration. By linking density fractionation with spectroscopic and isotopic tools, this research advances understanding of mineral-mediated C stabilization in biochar-amended soils, providing a basis for optimizing biochar application strategies (e.g., feedstock selection, application rate) to maximize C sink capacity in rice-based systems. Future work should focus on long-term monitoring of priming effects and microbial community shifts to refine sustainable soil C management practices.