Carbon (C) sequestration in terrestrial ecosystems can play a role in counteracting the rapid increase in CO2 concentration in the atmosphere. In this study, soils were sampled from some 2700 sites under contrasting land uses in four selected landscape units as representatives of the lowland (LL), low hill (LH), high hill (HH) and karst mountain (KM) areas in subtropical China. The aim was to quantify the potential for C sequestration in paddy ecosystems in the region. The mean values of organic C stocks in paddy soils (0–20 cm) in the landscape units varied from 37.8 to 60.4 t ha−1, and were considerably larger than those for soils under arable cropping and orchard managements, and even under woodland except in the KM landscape unit. In the LH unit, the mean organic C concentration in paddy soils increased by 1.67 times (P < 0.05) during the period between 1979 and 2003. This increase was consistent with the prolonged increase (since the 1950s) in rice productivity in the region. It is concluded that paddy ecosystems in subtropical China have the ability to accumulate organic C faster than other ecosystems. Because these landscape units represent the real situation in paddy ecosystems under farmers' practices for rice production, the data confirm that prolonged organic C accumulation in paddy soils has occurred in subtropical China.