【Objective】Carbon (C) and nitrogen (N) cycles in terrestrial ecosystems are closely coupled and reflect the flow of energy and nutrients, respectively. Elevated atmospheric N deposition increases, decreases, or does not affect vegetation and soil C storages across ecosystems and studies. Moreover, the efficiency of C sequestration caused by N amendment varies widely. There is a great uncertainty in the response direction and the magnitude of C sequestration. Soil organic carbon (SOC) transformation and C storage dynamics in forest ecosystems are specific to the increase in atmospheric N deposition, depending on soil initial N status, type, rate, and duration of N fertilization. In the past decades, related research mainly focused on the effect of inorganic N deposition. It is still unclear how organic N deposition affects the SOC and its components in the temperate needle-broadleaved mixed forest. There is few research exploring microbiological mechanism responsible for the change of SOC under N enrichment. The objectives of this research were: (1) to investigate whether multiple levels of N addition significantly changes the C concentrations of bulk soil and different physical fractions, microbial biomass and community structure, as well as general soil properties; (2) to estimate the N critical loads for the alteration of soil C and N concentrations in the temperate needle-broadleaved forest; and (3) to explore the potential linkages between changes in SOC concentration and microbial community abundance. 【Method】In this paper, the in-situ urea fertilization experiment with four levels (0, 40, 80, and 120 kg•hm^-2•a^-1，N) was set up in the Changbaishan temperate coniferous and broad-leaved mixed forest. After fertilization for three years, soil samples in 0—10 cm mineral layer were collected to determine the soil C and N concentrations, percentage of soil aggregates and SOC content of different particle sizes. The relative abundance and community structure of soil microorganisms were determined using phospholipid fatty acid (PLFA) technique. The relationships between changes in SOC content and changes in microbial community were explored. 【Result】The results showed that three years of N fertilization significantly increased the contents of NO₃^--N, DON, and TN in soils, and soil acidification was significant. Although N fertilization did not significantly increase SOC content in the surface soil, it significantly increased the contents of labile SOC fractions (particulate organic C and aggregate associated organic C) by 27.5% to 96.3%, resulting in the accumulation of SOC fractions. The critical load of N deposition is estimated as 80 kg•hm^-2•a^-1. The change in SOC content (ΔSOC) was positively correlated with the change in aggregate associated organic C and particulate organic C. Except for aerobic bacterial abundance, N fertilization did not change the abundance of microbial groups, but significantly changed the structure of microbial community, G+/G- ratio increased while aerobic/anaerobic ratio decreased. There was a significant correlation between labile SOC content, soil aggregates and microbial PLFA abundance, suggesting a close relationship between microbial community structure and SOC accumulation and stability. 【Conclusion】The above results indicate that organic N enrichment tends to promote the formation of soil aggregates in the temperate needle-broadleaved mixed forests, produce anaerobic microenvironment, change microbial community structure, and thereby lead to soil carbon accumulation. Our results suggest that atmospheric N deposition rate below the threshold could lead to a slow accumulation of SOC in the temperate mixed forest over the short term.