The forest ecosystem has the dual functions of carbon source and carbon sink, and regulating the carbon input mode in forest is of great significance for realizing the goal of “carbon neutrality” in China. The effects of changes in carbon (C) inputs (such as above-ground litter and under-ground roots) on soil nitrogen (N) cycling have long been a research focus in forest ecosystems. Based on previous studies worldwide, this paper reviews the research progress on the effects of different C inputs on the responses of the forest soil active N pool and N transformation processes, including soil active N pools, soil N mineralization, nitrification processes, and nitrous oxide (N₂O) emissions. We found: (1) Above-ground C removal reduced soil available nitrogen contents (mainly $NH_{4}^{+}$-N and $NO_{3}^{-}$-N). However, under-ground C removal increased the soil $NH_{4}^{+}$-N content. Changes in different C inputs showed inconsistent effects on soil microbial biomass nitrogen (MBN), which may be related to the ecosystem type, tree species composition, timescale, and other factors. In addition, the influence of under-ground C removal on soil soluble nitrogen (DON) was greater than that of above-ground C removal, and the under-ground root system may be the main contributor to soil DON. (2) The effects of above-ground C inputs on soil N mineralization and nitrification rates were greater in the short but less in the long term. Soil N mineralization and nitrification processes were affected indirectly by changes in soil microbial activity. The under-ground C removal increased soil N mineralization rates, and the effect was more obvious with an increase in a time scale. (3) Above-ground C inputs indirectly affected N₂O emissions by changing the available C sources of nitrifying and denitrifying microorganisms and was significantly affected by tree species. However, the effect of the under-ground C inputs on N₂O varied with the root quality. Current research mainly focuses on the impacts of a certain type of above-and under-ground C inputs and its removal on soil N pool components and N transformation. In conclusion, soil active N pools and N cycling differed in the response mechanisms to different C inputs in forests and were greatly affected by the ecosystem type, species, time, and other factors. The dynamic model and accurate budget for optimizing the above- and under-ground carbon inputs of forest ecosystems are insufficient, and a complete ecological compensation mechanism for carbon emission reduction has not been established. Future research needs to quantitatively understand the mechanisms of soil N affected by different C inputs and interactions between two or more species in different forest ecosystems, and more consideration should be given to the long-term soil N dynamics. Therefore, it is necessary to improve the ability of accounting and forecasting forest carbon neutralization above-and under-ground, accelerate the development of C neutralization technology in forest ecosystems in future research, which provides a scientific and technological support for achieving the “C neutrality” strategic goal in advance.