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Forest soil carbon (C) sequestration has an important effect on global C dynamics and is regulated by various environmental factors. Mixed and pure plantations are common afforestation choices in north China, but how forest type and environmental factors interact to affect soil C stock remains unclear. We hypothesize that forest type changes soil physicochemical properties and surface biological factors, and further contributes to soil active C components, which together affect soil C sequestration capacity and C dynamic processes. Three 46-year-old 25 m × 25 m pure Pinus tabulaeformis forests (PF) and three 47-year-old 25 m × 25 m mixed coniferous-broadleaf (Pinus tabulaeformis-Quercus liaotungensis) forests (MF) were selected as the two treatments and sampled in August 2016. In 2017, soil temperature (ST) at 10 cm were measured every 30 min for the entire vegetation season. Across 0–50 cm (five soil layers, 10 cm per layer), we also measured C components and environmental factors which may affect soil C sequestration, including soil organic carbon (SOC), soil total nitrogen (STN), dissolved organic carbon (DOC), microbial biomass carbon (MBC), soil moisture (SM) and soil pH. We then incubated samples for 56 days at 25 °C to monitor the C loss through CO2 release, characterized as cumulative mineralization carbon (CMC) and mineralized carbon (MC). Our results indicate that ST, pH, SM and litter thickness were affected by forest type. Average SOC stock in MF was 20% higher than in PF (MF: 11.29 kg m−2; PF: 13.52 kg m−2). Higher CMC under PF caused more soil C lost, and CMC increased 14.5% in PF (4.67 g kg−1 soil) compared to MF (4.04 g kg−1 soil) plots over the two-month incubation period. SOC stock was significantly positively correlated with SM (p < 0.001, R2 = 0.43), DOC (p < 0.001, R2 = 0.47) and CMC (p < 0.001, R2 = 0.33), and significantly negatively correlated with pH (p < 0.001, R2 = −0.37) and MC (p < 0.001, R2 = −0.32). SOC stock and litter thickness may have contributed to more DOC leaching in MF, which may also provide more C source for microbial decomposition. Conversely, lower SM and pH in MF may inhibit microbial activity, which ultimately makes higher MC and lower CMC under MF and promotes C accumulation. Soil mineralized C drives more C stock in coniferous-broadleaf mixed plantations compared to pure plantations, and CMC and MC should be considered when soil C balance is assessed.