Potential of Carbon Sequestration After Reforestation and Grass Establishment on Tropical Degraded Soils

Increased attention is being given to the accumulation of CO2, a potential ‘greenhouse gas’, in the earth’s atmosphere, and to the potential for global warming caused by the consequent ’greenhouse effect.’ Carbon dioxide is increasing in the atmosphere, partly from the combustion of fossil fuels and also from changes in land use from forest and range ecosystems to agriculture. Recent concerns about global warming have led to attempts at estimating the effects of land management on carbon sequestration in the soil. The research described herein was initiated to help assess the potential of C sequestration in forest soils, agriculture soils, and grasslands in tropical regions to ameliorate the buildup of atmospheric CO2. The objectives of this study were to determine the amount of soil organic C (SOC), the rate of CO2 emission, and the potential rate of C-sequestration in soils after restoration of forests and grasslands at a certain time (year), and to predict the economic valuation of soil C-storage assets. The C pool storage of previously cultivated clay soils (Typic Palehumult and Typic Paleudult) in Bengkulu, Sumatra returned to forest and to grasslands of Imperata cylindrica and Chromolaena odorata were compared with those of cultivated soils in three areas. Both field and laboratory studies measured carbon dioxide emissions from soils. Estimation of C sequestration utilized the method of Rosenzweig & Hillel (2000). Potential carbon dioxide emissions were reduced by about 31-40% when the soils were returned to forest cover, and about 14-36% when the soils were under grass. Soil carbon storage increased from 39-54% and 16-35% when cultivated fields were re-established to forest and grassland, respectively. Returning the previously cultivated soils resulted in an increase in soil organic carbon in the surface 60 cm at a mean rate 0.59-1.22 g C kg-1 yr-1 in the forest and 0.27- 1.04 g C kg-1 yr-1 in grassland. In general, the relative rate of carbon sequestration during reforestation was two times higher compared to grass establishment following cultivation. This implies that an aggrading forest ecosystem has the highest capability to sequester C in the soil, thereby helping to offset carbon emissions. The SOC mass in the surface 60 cm of the agricultural soils was 25% to 32% less than that of forest soils and 5% to 11% than that of grassland sites. In conclusion, the forest and grass sites can be said to be providing a global service function worth US$ 95 billion and $47 billion on the unit of the global cropland area (1.5 x 10 7 km2), respectively. However, this study also suggests that the values of C accumulation/sequestered vary from site-to-site according to the extent and value of agricultural activity associated with each