Effects of multiple environmental factors on CO₂ emission and CH₄ uptake from old-growth forest soils

Abstract. 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 CO₂ and CH₄ 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 (NH₄⁺-N and NO₃⁻-N) in the 0–10 cm was determined monthly. From north to south, annual mean CO₂ emission ranged from 18.09 ± 0.22 to 35.40 ± 2.24 Mg CO₂ ha⁻¹ yr⁻¹ and annual mean CH₄ uptake ranged from 0.04 ± 0.11 to 5.15 ± 0.96 kg CH₄ ha⁻¹ yr⁻¹ in the four old-growth forests. Soil CO₂ flux in the old-growth forests was mainly driven by soil temperature, followed by soil moisture and NO₃⁻-N. Temperature sensitivity (Q₁₀) of soil CO₂ flux was lower at lower latitudes with high temperature and more precipitation, probably because of less soil organic carbon (SOC). Soil NO₃⁻ accumulation caused by environmental change was often accompanied by an increase in soil CO₂ emission. In addition, soil CH₄ uptake decreased with an increase in soil moisture. The response of soil CH₄ flux to temperature was dependent upon the optimal value of soil temperature in each forest. Soil NH₄⁺-N consumption tended to promote soil CH₄ uptake in the old-growth forests, whereas soil NO₃⁻-N accumulation was not conducive to CH₄ oxidation in anaerobic condition. These results indicate that soil mineral N dynamics largely affects the soil gas fluxes of CO₂ and CH₄ in the old-growth forests, along with climate conditions.