Land-Use Changes Influencing C Sequestration and Quality in Topsoil and Subsoil

Soil capacity as a major carbon (C) sink is influenced by land use. Estimates of soil organic carbon (SOC) sequestration have mostly focused on topsoils [0-30 cm official Intergovernmental Panel on Climate Change (IPCC) soil depth]. We investigated SOC stocks and their quality as influenced by land-use changes. Soil samples were collected from five soil depths down to 100 cm of three adjacent fields each representing a different land use-forest, cassava, and rice paddy-in Northeast Thailand. Sequestration of SOC in topsoils under all land uses was higher, as indicated by SOC stocks (59.0-82.0 Mg ha(-1)) than subsoils (30-100 cm) (27.0-33.0 Mg ha(-1)). The soil profile (0-100 cm) of the forest had higher stocks of SOC and humic acid (115.0 and 6.8 Mg ha(-1), respectively) than those of cultivated land uses [paddy (100.0 and 4.8 Mg ha(-1), respectively) and cassava (87.0 and 2.3 Mg ha(-1), respectively)], which accounted for an average 30% increase in SOC sequestration over those with only topsoil. Topsoils of the forest had higher humic acid content but narrower E4:E6 ratio [the ratio of absorbances at 465 nm (E4) and at 665 nm (E6)] of humic acids (2.8), indicating a higher degree of humification and stabilization than the cultivated soils (3.2-3.6). Subsoil C was higher quality, as indicated by the lower E4:E6 ratio of humic acids than topsoils in all land uses.