To investigate the effects of the Grain for Green Program (converting farmland to forests) on soil organic carbon (SOC) dynamics, including the content and interconversion of its fractions, a study was conducted based on natural abundance of stable carbon isotopes (δ¹³C). Eucalyptus plantations with different restoration durations (1, 3, 4, and 10 years) were studied, with a cropland (0 year) serving as the control. The changes in the content and δ¹³C values of particulate organic carbon (POC) and mineral-associated organic carbon (MAOC) were analyzed and the carbon flow pathways between these fractions were quantified. The results showed that forest restoration significantly increased SOC content. After short-term restoration (1 year), the SOC content increased by 104.2%, with POC and MAOC increasing by 56.2% and 189.0%, respectively. In the long term (10 years), the SOC increase reached 124.7%, and the POC increase further expanded to 110.5%. The MAOC content was similar to that at the 1-year stage, with all three pools showing a synchronous increasing trend (P<0.05). The decreases in δ¹³C_POC and δ¹³C_MAOC indicated that forest restoration slowed SOC decomposition. The carbon flow pathway shifted from MAOC to POC in 1-year restored soil, and from POC to MAOC in soils restored for 3-10 years. The carbon flow intensity increased initially and then decreased, peaking at the 4-year mark. Correlation analysis revealed significant positive correlations between SOC (and its fractions) with soil pH and total nitrogen (TN). δ¹³C_POC was significantly positively correlated with soil water content (SWC), whereas δ¹³C_MAOC was significantly negatively correlated with SWC, SOC, and TN. These correlations highlight the close relationships between SOC dynamics and soil physicochemical properties following farmland-to-forest conversion. In conclusion, the conversion of farmland to forest significantly enhances soil carbon storage by increasing SOC content, reducing δ¹³C values, and slowing the SOC turnover rate.