Temperate and boreal forests in the Northern Hemisphere cover about 2 × 10⁷ km² and act as a major carbon sink, sequestering 0.6–0.7 Pg C per year. While forest expansion after agricultural abandonment contributes to this sink, other factors like increased CO₂, temperature, management practices, and nitrogen (N) deposition also influence the carbon balance. However, the effects of these factors are difficult to disentangle due to the complexity of forest ecosystems. The study shows that stand-replacing disturbances significantly affect forest carbon sequestration, but after accounting for these disturbances, nitrogen deposition is the main driver of carbon sequestration, largely due to human activities. The results suggest that nitrogen saturation is unlikely under natural conditions, as the positive relationship between nitrogen deposition and carbon sequestration is consistent across the range of available data. The study also highlights that forest ecosystems are affected by disturbances, such as fire or logging, which initially make them net carbon sources before they become net carbon sinks. Age-related dynamics account for 92% of the variability in net ecosystem production (NEP) in five chronosequences analyzed as part of the CARBOEUROPE project. The average NEP over the entire rotation is only 56% of peak NEP, indicating that disturbance effects must be considered when estimating net carbon sequestration. The study further shows that ecosystem respiration and gross primary production are positively correlated with mean annual temperature, while NEP is only weakly correlated. Nitrogen deposition is strongly correlated with NEP, suggesting that human activities, particularly nitrogen deposition, are a major factor in controlling forest carbon balance. The study also questions the ecological plausibility of the strong correlation between NEP and nitrogen deposition, arguing that the relationship is influenced by site productivity and environmental factors, leading to a much smaller NEP response to nitrogen deposition than previously thought. The study concludes that nitrogen saturation is unlikely under natural conditions, but the risk of widespread nitrogen saturation in densely populated and industrialized regions remains.Temperate and boreal forests in the Northern Hemisphere cover about 2 × 10⁷ km² and act as a major carbon sink, sequestering 0.6–0.7 Pg C per year. While forest expansion after agricultural abandonment contributes to this sink, other factors like increased CO₂, temperature, management practices, and nitrogen (N) deposition also influence the carbon balance. However, the effects of these factors are difficult to disentangle due to the complexity of forest ecosystems. The study shows that stand-replacing disturbances significantly affect forest carbon sequestration, but after accounting for these disturbances, nitrogen deposition is the main driver of carbon sequestration, largely due to human activities. The results suggest that nitrogen saturation is unlikely under natural conditions, as the positive relationship between nitrogen deposition and carbon sequestration is consistent across the range of available data. The study also highlights that forest ecosystems are affected by disturbances, such as fire or logging, which initially make them net carbon sources before they become net carbon sinks. Age-related dynamics account for 92% of the variability in net ecosystem production (NEP) in five chronosequences analyzed as part of the CARBOEUROPE project. The average NEP over the entire rotation is only 56% of peak NEP, indicating that disturbance effects must be considered when estimating net carbon sequestration. The study further shows that ecosystem respiration and gross primary production are positively correlated with mean annual temperature, while NEP is only weakly correlated. Nitrogen deposition is strongly correlated with NEP, suggesting that human activities, particularly nitrogen deposition, are a major factor in controlling forest carbon balance. The study also questions the ecological plausibility of the strong correlation between NEP and nitrogen deposition, arguing that the relationship is influenced by site productivity and environmental factors, leading to a much smaller NEP response to nitrogen deposition than previously thought. The study concludes that nitrogen saturation is unlikely under natural conditions, but the risk of widespread nitrogen saturation in densely populated and industrialized regions remains.