The real emission mitigation by the ecological restoration projects depends upon the integrated effect of all greenhouse gas (GHG) budgets rather than the carbon sequestration alone. However, a comprehensive and robust methodology for estimating the relevant GHG budgets and net mitigation of China's ecological restoration projects is still urgently to await development. Based on the methods from IPCC and statistical data of the management practices under the projects, we constructed a methodology for carbon accounting and determining net mitigation for ecological restoration projects in China (CANM-EP). GHG emissions generated from different processes and practices of the projects were included in the CANM-EP, and by this methodology, carbon sequestration, GHG balance changes induced by ecological response, on-site and off-site GHG emissions could be estimated. Therefore, the CANM-EP provides comprehensive methods to estimate the whole GHG budgets as well as the net mitigation of China's ecological restoration projects. •The CANM-EP provides accounting methods for comprehensive processes and management practices under respective ecological restoration projects in China.•The CANM-EP could simultaneously estimate carbon sequestration and GHG emissions of the projects.•The CANM-EP indicates net carbon sequestration and net contribution of China's ecological restoration projects to climate change mitigation.

Replanting is a widely used method for improving the health and carbon sequestration capacity of degraded forests. However, its impact on soil carbon pools remains controversial. This study investigated the effects of replanting broadleaf Phoebe bournei (Hemsl.) Yang in a typical degraded fir forest. Soil carbon content, nutrient levels, and microbial community structure and function were measured at 0, 5, 8, and 12 years after replanting. The degraded fir forests were originally limited in nitrogen and phosphorus. Phoebe bournei replanting significantly increased soil total carbon but reduced total nitrogen and phosphorus levels, resulting in increased soil carbon:nitrogen, carbon:phosphorus, and nitrogen:phosphorus ratios. Microbial biomass carbon, nitrogen, and phosphorus were all significantly reduced, whereas microbial carbon:phosphorus and nitrogen:phosphorus ratios were enhanced. Enzyme activities related to nutrient cycling and carbon decomposition (acidic invertase, polyphenol oxidase, peroxidase, urase, nitrate reductase, and acidic phosphatase activities) were significantly lowered by replanting. Microbial richness and diversity significantly increased, and microbial community composition changed significantly due to replanting. Structural equation modeling revealed the significant role of total phosphorus in microbial biomass, microbial community composition, and enzyme activity, highlighting it as the main factor accelerating soil carbon accumulation. Network analysis identified Leifsonia, Bradyrhizobium, and Mycolicibacterium members as key microbial players in the soil carbon cycle. In summary, P. bournei replanting exacerbated soil phosphorus deficiency, leading to a decrease in soil microbial biomass and changes in community structure, reduced nutrient cycling and carbon-decomposition-related enzyme activities, less litter decomposition, and increased organic carbon accumulation. These findings demonstrate the importance of nutrient limitation in promoting soil carbon accumulation and offer new insights for soil carbon regulation strategies in forestry.