During the annual rice cultivation period in subtropical regions of China, the climatic conditions and above-ground vegetation during the winter fallow period differ significantly from those during the rice-growing season. However, the differences in soil organic matter content and its sources remain unclear. This study employed a combined characterization method of lignin phenols and amino sugars to investigate the effects of plant-derived carbon and microbial-derived carbon during the rice cultivation period and fallow period on the accumulation of soil organic carbon (SOC) in red soil rice fields. Compared to the cropping period, SOC content significantly increased during the fallow period, with an increase of up to 68.8% (P < 0.001). Regarding plant-derived carbon inputs, the content of cinnamoyl monomers (C) increased by 80.1% during the fallow period (P < 0.05). Both the C/V ratio (cinnamoyl monomers/vanillyl monomers) and the (Ac/Al)s ratio (acyl/aliphatic monomers) significantly rose during fallow, indicating a shift in plant community structure. enhanced microbial conversion of lignin, and accelerated oxidation of side chains. However, the total amount of plant-derived carbon showed no significant difference between fallow and cropping periods (P > 0.05). Regarding microbial-derived carbon accumulation, the fallow period significantly increased glucosamine (GlcN), muramic acid (MurA), and total amino sugar content (P < 0.05), elevating microbial residue carbon from 4.92 g·kg?1 to 10.64 g·kg?1(P < 0.01). Regarding relative contributions, compared to the cropping period, the relative contribution rate of microbial-derived carbon increased by 8.2% during fallow, while that of plant-derived carbon decreased by 6.1%. This study elucidates the crucial role of microbial-derived carbon during fallow periods in enhancing soil organic carbon (SOC). The findings provide scientific support for the precise management of carbon pools in paddy fields during fallow periods.