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The broad-leaved and Korean pine mixed forests in the Liangshui National Natural Reserve, China, are important components of boreal forests in areas that are sensitive to global climate change. Respiration rates of excised roots were measured using an oxygen electrode (Hansatech Oxy-Lab 2, UK) to examine the variation in and identify the controlling factors of fine root respiration (Rr) during the growing period (May-October) in 2016. The relationships between Rr and biotic/abiotic factors [e.g., soil temperature, live fine root biomass (LFRB), and the soil carbon index] were subsequently analyzed. The results showed that Rr was significantly higher in the secondary broad-leaved forest (SF) than that in the primary Korean pine forest (PK) (p < 0.001); whereas soil carbon sequestration showed the opposite trend (p < 0.05). Rr tended to exponentially increase with soil temperature (p < 0.001) in both forest types. The Q(10) value of the PK was significantly higher than that of the SF (p < 0.01). Pearson correlation and regression analyses revealed that Rr was significantly linearly correlated with LFRB, total soil organic carbon, soil soluble organic carbon, and soil microbial biomass carbon (p < 0.01). Stepwise regression was used to further explain that LFRB and soil microbial biomass carbon were the major contributors to changes in Rr (F = 53.97, p < 0.001, R-2 = 0.75). The findings of the present study offer insights into the variability of the soil carbon sink function in the degraded and successional primary Korean pine forests.