To assess contribution of multiple environmental factors to carbon exchanges between the atmosphere and forest soils, four old-growth forests referred to as boreal coniferous forest, temperate needle-broadleaved mixed forest, subtropical evergreen broadleaved forest and tropical monsoon rain forest were selected along eastern China. In each old-growth forest, soil CO<sub>2</sub> and CH<sub>4</sub> fluxes were measured from 2003 to 2005 applying the static opaque chamber and gas chromatography technique. Soil temperature and moisture at the 10 cm depth were simultaneously measured with the greenhouse gas measurements. Inorganic N (NH<sub>4</sub><sup>+</sup>-N and NO<sub>3</sub><sup>−</sup>-N) in the 0–10 cm was determined monthly. From north to south, annual mean CO<sub>2</sub> emission ranged from 18.09 ± 0.22 to 35.40 ± 2.24 Mg CO<sub>2</sub> ha<sup>−1</sup> yr<sup>−1</sup> and annual mean CH<sub>4</sub> uptake ranged from 0.04 ± 0.11 to 5.15 ± 0.96 kg CH<sub>4</sub> ha<sup>−1</sup> yr<sup>−1</sup> in the four old-growth forests. Soil CO<sub>2</sub> flux in the old-growth forests was mainly driven by soil temperature, followed by soil moisture and NO<sub>3</sub><sup>−</sup>-N. Temperature sensitivity (<i>Q</i><sub>10</sub>) of soil CO<sub>2</sub> flux was lower at lower latitudes with high temperature and more precipitation, probably because of less soil organic carbon (SOC). Soil NO<sub>3</sub><sup>−</sup> accumulation caused by environmental change was often accompanied by an increase in soil CO<sub>2</sub> emission. In addition, soil CH<sub>4</sub> uptake decreased with an increase in soil moisture. The response of soil CH<sub>4</sub> flux to temperature was dependent upon the optimal value of soil temperature in each forest. Soil NH<sub>4</sub><sup>+</sup>-N consumption tended to promote soil CH<sub>4</sub> uptake in the old-growth forests, whereas soil NO<sub>3</sub><sup>−</sup>-N accumulation was not conducive to CH<sub>4</sub> oxidation in anaerobic condition. These results indicate that soil mineral N dynamics largely affects the soil gas fluxes of CO<sub>2</sub> and CH<sub>4</sub> in the old-growth forests, along with climate conditions.