Conversion of deforested areas into oil palm plantation has been suggested as a means to improve soil quality and carbon sequestration capacity in forest margin areas. Soil organic carbon fractions, such as particulate organic carbon is the most sensitive indicator in improving soil organic matter and soil quality. We determined particulate organic matter - carbon (POM-C) and mineral associated carbon (MAC) in Bengkulu and West Java Provinces Indonesia. The study areas were grain crop fields, secondary forest, 5 yr-oil palm plantation, 10 yr-oil palm plantation, 15 yr-oil palm plantation and 25 yr-oil palm plantation. Soil organic carbon fraction magnitudes varied in the surface of 0 to 30 cm of grain crop fields, secondary forest and oil palm plantations. Twenty five year- oil palm plantation provided the largest total organic carbon, while 5 yr-oil palm plantation and grain crop fields contained similar amount of total organic C. Both POM-C and MAC fractions were increased in the plantation sites with oil palm trees and secondary forest compared to the grain crop fields. The plantation sites had a larger POM-C content than grain crop fields, but the increase of POM-C was limited to the surface of 0 to 10 cm of soil. Carbon stock in 0-30 cm under oil palm plantations were reduced up to 20% compared to secondary forests and 28% compared to rubber plantations in Bengkulu and West Java Provinces. On average, converting forest to plantations led to a loss of 10 Mg C/ha after about 10 years of conversion. However, the C stock in the subsoil was similar under the forest and the plantations in Bengkulu. Further, limited C input from litter would eventually cause more losses of SOC in oil palm plantations compared to rubber plantations. In conclusion, investigating the deeper profiles and soil erosion may be an important tool to unfold the trends of soil organic carbon fraction dynamics and magnitudes after the conversion of natural ecosystems to intensive plantations
