In the humid subtropics, conversion of cropland to forest has been recognized to influence carbon cycling (e.g., soil CO2 emissions) and the associated ecosystem carbon balance. A three-year field study was conducted in situ to quantitatively evaluate effects of land use change on carbon budget in a cropland (under winter wheat-summer maize rotation) comparison with the adjacent forest ecosystem. During the three-year experimental period, on average, soil heterotrophic respirations were 35.19 mg C.m(-2).h(-1) for the cropland and 40.02 mg C.m(-2).h(-1) for the adjacent forest ecosystem. The quantified net primary production (NPP) were 8724.78 kg C.ha(-1).year(-1) for the cropland (3218.14 kg C.ha(-1) for winter wheat season and 5506.64 kg C.ha(-1) for summer maize season) and 6478.99 kg C.ha(-1).year(-1) for the adjacent forest ecosystem. Thus, the average positive net ecosystem production (NEP) of 5139.33 kg C.ha(-1).year(-1) and 2790.43 kg C.ha(-1).year(-1) were gained in the cropland and the adjacent forest ecosystem, respectively. Nonetheless, if take into consideration of crop grain harvest (i.e., removal), the mean NEP was only 976.69 kg C.ha(-1).year(-1) for cropland which were over three-fold lower than for the adjacent forest ecosystem. The practice of conversion of cropland (maize-wheat rotation system) to forest consequently resulted in an average annual net carbon sequestration of 1813.74 kg C.ha(-1).year(-1) in the study. Therefore, our findings highlight that practices of conversion of subtropical cropland to forest commonly conducted in the last decades act as sinks of atmospheric CO2 in southwest China.