Land use change is one of the most important drivers of excessive carbon dioxide (CO2) emission and is partly responsible for global warming. Certain land use systems promote the sequestering of excessive carbon from the atmosphere to the soil, while other systems accelerate C loss through emissions. Herein, a study was conducted to evaluate the soil C forms and carbon stocks in the soils of three land use systems (a pasture, field crop and cocoa plantation) that were developed following the conversion of grasslands in the humid lowland landscape of Papua New Guinea. A remarkable decline (P<0.001) in the total C concentration of the grassland soils was observed due to land conversion into either field crops (44%) or a cocoa plantation (28%). Among the land use systems, organic C was the dominant pool (78.1-86.9%) compared to inorganic C, which only contributed 13.1%-21.9% to the total C stock. The soil organic C stocks were present in the following order: grassland (217.9 Mg ha -1 ) > pasture (207.6 Mg ha -1 ) > cocoa plantation (139.4 Mg ha -1 ) > field crops (131.6 Mg ha -1 ). The results of this study indicated that the conversion of grasslands to other land use systems (such as a cocoa plantation and field crops) could lead to the depletion of soil C stocks.