【Objective】Soil organic carbon (SOC) is the core of soil fertility and is particularly crucial for improving arable land quality. Straw returning is a key measure for cropland carbon sequestration in the North China Plain, yet the specific mechanisms by which the stoichiometric C/N ratio of returned organic materials regulates the transformation of soil carbon fractions and carbon sequestration efficiency in Fluvo-aquic Cinnamon soil remain inadequately understood. Therefore, this study aimed to evaluate the effects of returning organic materials with distinct C/N ratios on soil carbon fraction transformation and carbon sequestration in Fluvo-aquic Cinnamon soil. 【Method】Based on the principle of equivalent carbon input, an outdoor pot experiment was conducted using Fluvo-aquic Cinnamon soil from the North China Plain. Six organic materials with different C/N ratios were incorporated into the soil: corn straw (CS, C/N 45), wheat straw (WS, C/N 82), rapeseed straw (RS, C/N 30), soybean straw (SS, C/N 21), soybean meal (SM, C/N 7), and cattle manure (CM, C/N 12). 【Result】The results showed that the addition of exogenous organic materials significantly affected the differentiation of soil carbon fractions, exhibiting a distinct temporal lag as well as a mismatch between microbial growth and carbon sequestration function across the C/N gradient. Particulate organic carbon (POC) showed pulse-like growth in the early stage (3–9 d), whereas mineral-associated organic carbon (MAOC) accumulated slowly in the mid-to-late stage (69 d), displaying an evident lag relative to POC. Meanwhile, microbial biomass, respiratory metabolism, and nutrient supply–demand status closely associated with carbon fraction transformation also shifted accordingly, with pronounced differentiation among C/N treatments. Specifically, although the low C/N material (SM) alleviated nitrogen limitation and promoted early POC and MAOC formation, its high net inorganic nitrogen accumulation in the early stage, together with low microbial biomass (MBC) and a high metabolic quotient (qCO2) during the mid-to-late stages, resulted in a low net MAOC accumulation by the end of the experiment. In contrast, the medium-to-high C/N treatment (CS) exhibited nitrogen immobilization in the early stage, maintained relatively high MBC levels in the mid-to-late stages, and effectively averted a surge in qCO2. By the end of the experiment, the CS treatment achieved the highest net MAOC accumulation among all treatments, with the highest soil carbon sequestration efficiency (SCSE, defined as the percentage of net MAOC accumulation relative to the total exogenous organic carbon input over the incubation period, 23.96%). 【Conclusion】Based on the carbon sequestration performance of the six organic materials with different C/N ratios, this study preliminarily revealed the pathway by which the material C/N ratio regulates carbon sequestration efficiency through inorganic nitrogen supply–demand dynamics and microbial metabolic status. This suggests that maintaining the C/N ratio of returned organic materials at approximately 45 optimizes the carbon sequestration capacity of Fluvo-aquic Cinnamon soil in the North China Plain, maximally converting exogenous organic materials into stable MAOC for long-term preservation in soil, thereby enhancing soil carbon sink reserves and improving arable land quality. These findings provide a theoretical basis and scientific guidance for the resource utilization of agricultural organic wastes and the development of green agriculture.