This study aimed to evaluate the seasonal variations of Greenhouse Gas fluxes (CH₄, CO₂, and N₂O), Greenhouse Gas (GHG) emissions, and Global Warming Potential (GWP) over the extent of the regions and understand the controlling factors. CH₄, CO₂, and N₂O fluxes were measured along with their environmental variables from the over-mature forest, Korean pine plantation, and five 60-year-old natural secondary forests in mountainous regions in Northeast China from May 2015 to April 2016. The results revealed that secondary forests, except for Betula platyphylla forest, significantly increased CH₄ absorption by 19.6% to 51.0% and 32.6% to 67.0% compared with over-mature forest (OMF) and Korean pine plantation (KPP). Five secondary forests significantly increased CO₂ flux by 32.9% to 78.6% and 14.1% to 53.4% compared with OMF and KPP, respectively. According to the annual statistics, the N₂O fluxes had significant differences among seven forest types and decreased in the following order: mixed deciduous forest (MDF) > OMF > KPP > Populous davidiana forest (PDF) > hardwood forest (HWF) > Mongolian oak forest (MOF) > Betula platyphylla forest (BPF). The CH₄ absorption and CO₂ emission peaks occurred in summer, while the peak N₂O fluxes occurred in spring. Stepwise multiple linear regression showed that CH₄ and CO₂ fluxes from soils were strongly influenced by air and soil temperature, soil volumetric water content (SVWC), nitrate nitrogen (NO₃⁻-N), ammonium nitrogen (NH₄⁺-N), and soil organic carbon (SOC) across the whole year. Air temperature, SVWC, pH, NO₃⁻-N, and NH₄⁺-N were the dominant factors controlling N₂O fluxes from OMF and five secondary forests (except for BPF). No significant relationships were observed between these environmental factors and N₂O fluxes from KPP and BPF. Additionally, the total cumulative CH₄, CO₂, and N₂O fluxes were –13.37 t CH₄ year⁻¹, 41,608.96 t CO₂ year⁻¹, and 3.24 t N₂O year⁻¹, and the total cumulative GWP were 42,151.87 t CO₂ eq year⁻¹ through the whole year in seven forest types at the Maoershan Ecosystem Research Station in Northeast China. For the annual GWP per hectare, secondary forests and KPP averaged a higher GWP by 33.7%–80.1% and 17.9% compared with OMF. This indicates that the effects of early human activities have not been completely eliminated in the middle stage of KPP and secondary forests.

The effects of replacing over-mature forest with secondary forests and plantations are significant for terrestrial ecosystem carbon (C) dynamics. However, the carbon balance and recovery time of this replacement process remain unclear. This study measured the fluxes of CH₄ and CO₂ in soils and the annual net C sequestration (ANCS) from seven ecosystems with different vegetation types (over-mature forest (OMF), Korean pine plantation (KPP), hardwood forest (HWF), Betula platyphylla forest (BPF), Populous davidiana forest (PDF), mixed deciduous forest (MDF), and Mongolian oak forest (MOF)) using the static chamber-gas chromatography method and the relative growth equation method. We examined the effects of environmental factors (e.g., air and soil temperature, soil volumetric water content (SVWC), soil pH, nitrate nitrogen (NO₃⁻-N), ammonium nitrogen (NH₄⁺-N), and soil organic carbon (SOC)) on CH₄ and CO₂ fluxes at the Maoershan Ecosystem Research Station in Northeast China. The carbon source or sink of OMF, KPP, and five secondary forests (HWF, BPF, PDF, MDF, and MOF) were then evaluated based on net ecosystem C balance. The results revealed that the mean annual CH₄ fluxes varied between −0.046 and −0.077 mg m⁻² h⁻¹. The mean annual absorption of CH₄ in the secondary forests and OMF were respectively 1.09–1.67 times and 1.11 times higher than that of KPP (0.046 mg m⁻² h⁻¹, p < 0.05). The mean annual CO₂ fluxes varied between 140.425 and 250.023 mg m⁻² h⁻¹. The CO₂ fluxes in the secondary forests and KPP soils were respectively 1.33–1.78 times and 1.16 times higher than that of OMF (140.425 mg m⁻² h⁻¹, p < 0.05). The CH₄ and CO₂ fluxes were mainly influenced by air and soil temperature, SVWC, soil pH, NO₃⁻-N, NH₄⁺-N, and SOC in Northeast China. The ANCS of vegetation (3.41 ± 0.27 − 6.26 ± 0.75 t C ha⁻¹ y⁻¹) varied widely among different forest types: KPP had the largest ANCS (6.26 ± 0.75 t C ha⁻¹ y⁻¹, which was higher than secondary forests and OMF by 1.20–1.84 times and 1.46 times, respectively, p > 0.05). Carbon sources and sinks were significantly different among the seven types of vegetation: OMF and KPP were observed to be the greatest C sinks, and secondary forests were shown to be the weakest carbon sinks or net C sources in the study region.