Soil organic carbon dynamics of different land use in Southeast Asia

Soil respiration, CO₂ efflux from the soil surface, is an important process of the carbon (C) cycle in terrestrial ecosystems. Soil respiration includes many processes involving biotic factors, such as respiration from roots and microorganisms, along with abiotic factors and various temporal and spatial factors. Many researchers have examined soil respiration of various ecosystems. Recently, tropical forests have been converted into secondary forests or agricultural forests. This land use change might strongly affect the global C cycle; nevertheless, few data are available to reflect land use effects on dynamics of soil organic carbon (SOC) in Southeast Asia. Therefore, we established study sites at four different ecosystems (primary forest, secondary forest, oil palm plantation, and rubber plantation) in the Pasoh area of Malaysia in Southeast Asia. This study was designed to determine spatial and temporal variations (diurnal and seasonal change) of the soil respiration rate and to estimate the annual C efflux from soil and dynamics of SOC in different ecosystems. Seasonal data suggest that the soil respiration rate is negatively correlated with soil water contents in the primary forest, secondary forest, and rubber plantation. The soil water content shows a negative correlation with the gaseous phase content. The gaseous phase content shows a positive correlation with soil respiration rate at all sites. The annual C efflux from soil was estimated as 16.9-19.2 t C ha^-1 in the primary forest, 17.5-18.5 t C ha^-1 in the secondary forest, 14.3-14.5 t C ha^-1 in the oil palm plantation, and 9.0-11.2 t C ha^-1 in the rubber plantation. Moreover, we estimated the annual SOC budgets using the three-box model. Results suggest that the biomass of dead roots, turnover time, and contribution of heterotrophic respiration are important factors for accurate evaluation of soil C dynamics and budgets.