Soil Carbon Dioxide Fluxes from Three Forest Types of the Tropical Montane Rainforest on Hainan Island, China

Tropical forests play an important role in carbon cycle. However, the temporal and spatial variation in soil carbon dioxide (CO₂) emission of tropical forest remains uncertain, especially near the Tropic of Cancer. In this research, we studied the annual soil CO₂ fluxes from three tropical montane rainforests on the Hainan Island of China (pristine montane rainforest, PF; secondary montane rainforest, SF; and Podocarpus imbricatus plantation, PP). The results showed a lower annual average soil CO₂ flux as 6.85 ± 0.52 Mg C-CO₂ ha⁻¹ (9.17 Mg C-CO₂ ha⁻¹ in the wet season and 4.50 Mg C-CO₂ ha⁻¹ in the dry season). The CO₂ fluxes exhibited obviously seasonal variation during the study period. Among the three forest types, PF had the highest average CO₂ flux rate of 317.77 ± 147.71 mg CO₂ m⁻² h⁻¹ (433.08 mg CO₂ m⁻² h⁻¹ in the wet season and 202.47 mg CO₂ m⁻² h⁻¹ in the dry season), followed by PP of 286.84 ± 137.48 mg CO₂ m⁻² h⁻¹ (367.12 mg CO₂ m⁻² h⁻¹ in the wet season and 206.56 mg CO₂ m⁻² h⁻¹ in the dry season) and SF of 255.09 ± 155.26 mg CO₂ m⁻² h⁻¹ (351.48 mg CO₂ m⁻² h⁻¹ in the wet season and 155.71 mg CO₂ m⁻² h⁻¹ in the dry season). We found between CO₂ fluxes and soil temperature a highly significant linear relation (P < 0.01) at 5 cm depth and a highly significant exponential correlation (P < 0.01) at 10 cm depth for all three forest types; a significant linear relation (P < 0.05) between CO₂ fluxes and soil moisture content was found for SF and PF, but not for PP (P > 0.05). The CO₂ flux was significantly correlated (P < 0.05) with water-filled pore space only for PF. In conclusion, our results suggested soil CO₂ fluxes in the three forest types that exhibit obviously spatial and temporal variation, and the temperature is the major factor affecting soil CO₂ fluxes from this region.