Litter input plays an important role in the soil organic carbon (SOC) balance between new C formation and old C loss in forests. However, the relative contributions of aboveground litter vs. belowground litter inputs to the SOC balance in various forests along a climatic gradient remain highly uncertain. In this study, we experimentally transplanted sugarcane soil with a heavy δ¹³C value to forest soil with a light δ¹³C value to evaluate the relative effects of aboveground and belowground litter inputs on new SOC formation and old SOC retention in natural secondary forests (boreal, temperate, subtropical and tropical forests) located across four different climatic regions in China. Our results showed that belowground root litter inputs can potentially lead to higher new SOC formation and lower old SOC retention compared to aboveground litter inputs. Moreover, the magnitudes of new SOC and old SOC change after litter inputs varied among natural forests, with SOC in tropical forests being more sensitive to belowground root inputs than SOC in temperate and subtropical forests. While the total and new SOC contents were positively correlated with mean annual temperature (MAT) and precipitation (MAP), the old SOC contents were negatively correlated with MAT and MAP. Our findings indicated that belowground root litter inputs have more profound influences on new SOC formation and old SOC loss than aboveground litter inputs, and climatic factors play a fundamental role in determining litter input-induced changes in SOC balance across different forest ecosystems.