Abstract
The carbon (C) content and natural 13C abundance (δ13C) of soil organic matter were analyzed in soils collected from forests and agricultural fields in Thailand to determine the origin and turnover of soil organic carbon in the Asian tropics. The δ13C values of the soil horizons in two forests were in the range of C3 plant values. Soil δ13C values in fields with continuous cultivation of lowland rice (C3 plant) and sugarcane (C4 plant) were less negative than those in the forests. The organic carbon contents and δ13C values in 226 top-soils from agricultural fields varied in four regions of Thailand and were affected by the cultivation of preceding crops and weeds. The δ13C values of two series of top-soils from forests and sugarcane fields were investigated in the Northeastern and Central Plain Regions. Conversion of forest land to fields for sugarcane cultivation led to a rapid decrease in the content of soil organic carbon and an increase in the soil δ13C values. The calculated forest-derived carbon rapidly decreased with a half-life of 0.35 years (in the Northeastern Region) after conversion to fields for sugarcane cultivation, and sugarcane-derived carbon steadily accumulated. Rapid changes in the δ13C values of soil organic carbon after land conversion to fields with vegetation with different photosynthetic types may result from low organic carbon content and rapid turnover in the tropics.
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