No matter whether a farmland ecosystem acts as source or sink of global greenhouse gases, it has an important impact on annual variation of the concentration of carbon dioxide in the atmosphere. In this study, four counties, namely Minhou, Pucheng, Tongan and Yongding, located in different geographical locations in Fujian Province, subtropical China, were selected as typical study zones. Based on the data of field measurements at 23 869 sampling site in 1982 and 12 521 sampling sites in 2008, scattered in these four counties, a 1:50 000 database of soil type-land use pattern was established, analysis was carried out of changes in organic carbon density in the farmlands of Fujian in the past 30 years as affected by group, sub-group, genus and land use of the soils, and estimation was done of the soil organic carbon storages in the farmlands of the province in these two historical periods, using the scaling up method. Results show that the farmlands in Fujian as a whole acted as a weak carbon sink in the past 30 years, with carbon density and carbon storage increased by C 0.24 kg m^-2 and 4.26 Tg, respectively. Contribution of the farmland ecosystems to the “carbon sink/source” varied sharply with soil type and landuse pattern. When speaking of soil types, purplish soil, acid purplish soil and calcic mud field contributed the greatest to the “carbon sink” in soil group, soil subgroup and soil genu, respectively, with organic carbon density increased by C 0.63, 0.63 and 1.25 kg m^-2, respectively; while coastal solonchaks, coastal solonchaks and yellow sandy soil were the major contributors to the “carbon source” in soil group, soil subgroup and soil genu, respectively, with organic carbon density decreased by C 0.59, 0.59 and 1.08 kg m^-2, respectively. In terms of land use, irrigated paddy fields were the major carbon sink in the province with organic carbon density increased by C 0.27 kg m^-2; while irrigated uplands the main carbon source with organic carbon density decreased by C 0.36 kg m^-2. The findings suggest that in formulating policies for management of soil organic carbon in farmlands of Fujian in the years to come, priority should be given to effective management measures for the types of soils and the patterns of land use that are large in area and cause organic carbon density to decrease.