To investigate the effects of returning farmland to forest on soil organic carbon (SOC) content, its components, and their interconversion, this study utilized the natural abundance of stable carbon isotopes (δ_¹³C) to analyze particulate organic carbon (POC), and mineral-associated organic carbon (MAOC) content, as well as δ_¹³C values, in eucalyptus forests with different restoration durations (1, 3, 4, and 10 years), using farmland (0 years) as a control. The carbon flow pathways between carbon components were also quantified. The results showed that returning farmland to forest significantly increased SOC content. After short-term restoration (1 year), SOC content increased by 104.2%, with POC and MAOC increasing by 56.2% and 189.0%, respectively. During long-term restoration (10 years), SOC accumulation reached 124.7%, with POC increasing further by 110.5%, while MAOC remained similar to the 1-year level. All three components exhibited a synchronous growth trend (p < 0.05). The δ_¹³C values of POC and MAOC gradually decreased, indicating that returning farmland to forest slowed down SOC decomposition. The carbon flow pathways shifted from MAOC to POC in farmland and the first year of restoration, and from POC to MAOC during 3–10 years of restoration. The intensity of carbon flow initially increased and then decreased, peaking in the fourth year. Correlation analysis revealed that SOC and its components were significantly positively correlated with pH and total nitrogen (TN), while δ_¹³C_POC was significantly positively correlated with soil water content (SWC). In contrast, δ_¹³C_MAOC was significantly negatively correlated with SWC, SOC, and TN, highlighting the close relationship between SOC dynamics and soil physicochemical properties under the context of returning farmland to forest. The study demonstrates that returning farmland to forest significantly increases SOC content, reduces δ_¹³C values, and slows down SOC turnover rates, thereby enhancing soil carbon storage.