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The relationship between CO2 flow from soil and soil CO2 concentration was investigated at 72 permanent sampling points at two forested post-mining sites in the northwest of the Czechia. Based on the entire data set (72 points sampled monthly during the growing season), CO2 flow from the soil was positively correlated with soil CO2 concentration. CO2 concentration in deeper soil layers was positively correlated with root biomass and negatively correlated with soil microbial respiration. In individual sampling points relationship between CO2 flow and soil CO2 concentration varied from being significantly positive (30% of points) to significantly negative (7%) but mostly being non-significant (63%). The positive correlation occurred at points with high root biomass in deeper soil layers, while the negative correlation occurred at points with high soil microbial respiration per cm3 of soil. Laboratory experiments showed that the CO2 produced by microbial respiration can reduce microbial respiration but that CO2 produced by root respiration did not reduce root respiration. The results indicate that when soil ventilation is poor, microbial respiration can sufficiently increase soil CO2 concentration so as to reduce microbial respiration, which greatly increases the variability in the relationship between CO2 flow from soil and soil CO2 concentration.