Soil Organic Carbon Sequestration During Restoration of Upland Evergreen Forest in Northern Thailand

This study evaluated sequestration of soil organic carbon (SOC) during restoration of upland evergreen forest by the framework species method (FSM) in northern Thailand. Soil samples were collected from four plots: 12-year restoration forest, 24-year restoration forest, reference forest (natural forest), and control (abandoned agricultural field). By using a customized split-core sampler at three different depths (0-5 cm, 5-10 cm, and 10-15 cm), soil samples were collected from eight sampling points per plot. SOC was analyzed at each depth, along with nitrogen (N), phosphorus (P), potassium (K) and pH. Natural forest had the highest SOC at 56.94 tC/ha, followed by the 12-year-old restoration (R12) plot, at 48.74 tC/ha. The control plot had a lower SOC stock at 38.20 tC/ha, while the 24-year-old restoration (R24) plot showed the lowest SOC stock among the studied sites with 33.10 tC/ha. The restored plots showed higher N and K and slightly lower pH than the control plot, indicating enhanced nutrient accumulation and organic matter cycling during forest recovery. Phosphorus levels were elevated in the control plot (probably due to fertilizer application) and lower in both the restored and reference plots. The results demonstrated that the FSM can increase SOC levels and soil fertility in the first decade, but land-use legacy can influence such variables long after restoration has been initiated. The study raises the possibility of generating carbon-credit income to pay restoration costs and incentivize stakeholders through enhanced soil-carbon sequestration in restored upland evergreen forests.

Keywords: Soil organic matter, Soil organic carbon, Climate change, Forest restoration, FORRU, Framework species method.