Abstract: Grain for Green Program is the largest ecological project in China, playing a crucial role in enhancing terrestrial ecosystem functions, such as water and soil conservation. Although extensive research has been conducted to investigate how restoring farmland to forest affects soil organic carbon (SOC) and its chemical components, our understanding of the effectiveness and mechanisms underlying SOC accumulation largely remains unclear. We reviewed the impacts of restoring farmland to forest on SOC content, its components, and chemical structure by analyzing 76 studies regarding SOC dynamics following the project over the past 15 years. The results showed that SOC exhibited inconsistent trends during the early phase of the poject. After 20 years of the implementation of the project, SOC generally exhibited an increasing trend, with the rate of increase tending to stabilize at an average of 52.76%. Regarding composition and structure of SOC, the conversion of cropland to forest significantly increased soil labile organic carbon fractions and accelerated the SOC turnover rate. Concurrently, there was a concomitant increase in the proportion of stable components, such as aromatic and alkyl carbon. This indicates that after undergoing rapid turnover, SOC is gradually transforming into more stable chemical forms, thereby enhancing both the total SOC stock and stability. We propose that future studies should focus on the following three aspects: litter decomposition and its microbial mechanisms, establishing long-term networked monitoring experiments, and developing evaluation systems for soil carbon sink functions. These efforts aim to gain in-depth understanding of the dynamics and mechanisms of SOC following this project, providing a scientific basis for developing ecosystem management technologies that enhance carbon sequestration and sink functions in Grain for Green ecosystems.

Key words: restoring farmland to forest, soil organic carbon, carbon sink