[Objective] Soil microbes are an important factor driving turnover of soil organic matter (SOM), through participating in SOM formation, linking aboveground and underground ecosystems in evolution process, and influencing the structure and function of ecosystems. However, it is still a hard nut to evaluate the contribution of soil microbes (especially microbial residues) to SOM, and a hotspot of research in this aspect, due to limitation in technology. In this study, efforts were dedicated to examining (1) how forest secondary succession influences the content of microbial residues and its contribution to soil organic carbon (SOC); (2) how soil depth affects content of microbial residues and its contribution to SOC; and (3) how fungal and bacterial residues regulate the contribution of microbial residues to SOC.[Method] To explore changes in the content of microbial residues and contribution of microbial residues to SOC, soil samples were collected from the topsoil (0-5 cm) and subsoil (5-15 cm) in five plots of secondary forests with different age or succession (i.e. 20 years, 80 years, 120 years, 200 years and ≥ 300 years) in the Changbai Mountain National Nature Reserve, Jilin, Northeast China for analysis of amino sugars, including glucosamine, galactoamine, and muramic acid, as biomarkers for microbial residues. Based on the analysis, contents of fungal and bacterial residues were worked out. Besides, soil organic matter in the sample was analyzed for composition (i.e. aromatic C, aliphatic C and polysaccharides) with fourier transform mid-IR spectroscopy.[Result] Our study showed significant increases in content of microbial residues and contribution of the residues to SOC in both topsoil and subsoil during the period of 80-200 years of the succession, whereas a reverse trend was found after 300 years. Path analysis showed that content of microbial residues was positively related to microbial biomass carbon (MBC) in the two soil layers during the succession, indicating that changes in MBC influence the accumulation of microbial residues. In the soils under secondary forests 80-200 years old, labile SOM (low in aromatic C/polysaccharides ratio) was relatively higher, which was beneficial to utilization of microbial carbon (high in MBC/SOC ratio) and accumulation of microbial residues, and promoted contribution of the residues to SOC, while in the soils under secondary forests >300 years old, recalcitrant SOM (high in aromatic C/polysaccharides ratio) was relatively higher, which inhibited utilization of microbial C, thus leading to decline in content of microbial residues and contribution of the residues to SOC. Content of microbial residues varied with soil depth, being higher in the topsoil than in the subsoil as SOC did. Higher content of SOC in the topsoil induced generation of more microbial biomass, thus leading to higher accumulation of microbial residues, whereas the existence of higher contents of recalcitrant fractions of SOM in the topsoil caused decrease in contribution of the residues to SOC. In addition, the variation of SOM utilization rate from low in the topsoil to high in the subsoil caused decline of fungal residue contribution to SOC, but a reverse trend with bacterial residues.[Conclusion] In summary, changes in availability of carbon resources (i.e. SOC concentration and SOM components) trigger variation of content and accumulation of microbial residues in SOC. All the findings in this study may provide certain theoretical support for us in exploring effects of microbial metabolites on SOM formation from the perspective of microbial ecology. Therefore, this study suggests that the anabolic pathways of soil microbes be integrated into the current terrestrial ecosystem carbon models, which will sure facilitate better prediction and evaluation of SOC response to ecosystem managements.