Carbon Dynamics of Surface Soil after Land Use Change in a Seasonal Tropical Forest in North-eastern Thailand: Application of a Sable Carbon Isotope Mixing Model

Abstract:

Globally, soil is vital to the mitigation of climate change. In tropical forests, the soil contains an estimated 216 Gt of carbon, equivalent to half of the total carbon in the tropical forest ecosystem. Little is known regarding changes in soil carbon following land use changes in tropical regions. We examined the differences in carbon dynamics in a chronosequence of Acacia mangium plantations established on grasslands following the deforestation of natural forest in north-eastern Thailand. The study site was located at the Sakaerat Silvicultural Research Station (14º28′06.1″N, 101º54′15.0″E; altitude 420 m asl), Nakhon Rachasima Province, north-eastern Thailand. Mean annual air temperature was 26ºC, and annual precipitation was 1,100 mm, with a dry (November–April) and wet (May–October) season. Soil carbon and the stable carbon isotope ratio (d¹³C) in the surface soil (0–5 and 5–10 cm deep) were determined at 12 and 24 years following establishment of A. mangium plantations, as well as for secondary forest and grassland. Using the stable carbon isotope mixing model based on differences in the natural abundance of d¹³C in plants with C₃ (i.e., trees) and C₄ (i.e., grasses) pathways for CO₂ fixation, the amount of soil carbon derived from the plantations, forest, and grassland was calculated. Soil carbon at a depth of 10 cm was higher in the secondary forest (1,929 gCm^-2) and grassland (2,508 gCm^-2) than in the plantations (1,703 gCm^-2 at 12 years, 1,673gCm^-2 at 24 years). Soil carbon derived from A. mangium was 67% (0–5 cm deep) and 62% (5–10 cm deep) of total soil carbon at 12 years, and was 100% (0–5 cm deep) and 90% (5–10 cm deep) at 24 years in the plantations. We found that most of the soil carbon at a depth of 0–5 cm in the young plantation changed from grass-derived to tree-derived carbon within a relatively short period of 24 years. Because of changes in soil carbon, exotic, fast growing plantations like those of A. mangium are needed to quickly convert back to an indigenous forest.