In the last two centuries, land use has been a significant source of atmospheric through conversion of natural vegetation to farming. China has CO₂ 137.5 million hectares of cropland. The long history of agricultural exploitation and the changes of land use suggest that the terrestrial ecosystem of China would have played an important role in the global carbon cycle

Using the data compiled from China¢s second national soil survey, we have estimated the changes of soil organic carbon due to land use, and compared the spatial distribution and storage of soil organic carbon (SOC) in cultivated soils and non-cultivated soils in China. The results reveal that 57% of the cultivated soil subgroups (31% of the total soil surface) have experienced a significant carbon loss, ranging from 40% to 10% relative to their non-cultivated counterparts. The most significant carbon loss is observed for the non-irrigated soils (dry farmland) within a semi-arid/semi-humid belt from northeastern to southwestern China, with the maximum loss occurring in northeast China. On the contrary, SOC has increased in the paddy and irrigated soils in northwest China. No significant change is observed for forest soils in southern China, grassland and desert soils in northwest China, as well as irrigated soils in eastern China.

The SOC storage and density under non-cultivated conditions in China are estimated to 77.4 Pg and 8.8 kg C m^-2, respectively, compared to a SOC storage of 70.3 Pg and an average SOC density of 8.0 kg C m^-2 under the present-day conditions. This suggests a loss of 7.1 Pg SOC and a decrease of 0.8 kg C m^-2 SOC density due to increasing human activities, in which the loss in organic horizons has contributed to 77%. This total loss of SOC in China induced by land use represents 9.5% of the world¢s SOC decrease. This amount is equivalent to 3.5 ppmv of the atmospheric CO₂ increase.

Since 78% of the currently cultivated soils in China have been degraded to a low/medium productivities and are responsible for most of the SOC loss, an improved land management, such as soil erosion prevention and controlling the conservation tillage, would have a considerable potential in restoring the SOC storage. Assuming a restoration of 50% of the lost SOC during the next 20-50 years, the soils in China would absorb 3.5 Pg of carbon from the atmosphere.