Human-induced Changes of Organic and Inorganic Carbon Storage in Soils of China

Land use has been a significant source of atmospheric CO2 through conversion of natural vegetation to farming. In terrestrial ecosystems, soil carbon is the largest terrestrial carbon pool. The huge carbon pool of soils and the significant changes related to land use by human activity suggest a considerable potential to enhance the rate of carbon sequestration in soils through suitable management, and thereby to decrease the atmospheric CO2 level. Using the data compiled from China's second national soil survey, measured data from individual studies from 1980 to 2000, and an improved method of soil carbon bulk density, we have estimated the changes of soil organic (SOC) and inorganic (SIC) carbon due to land use, and compared the spatial distribution and storage of SOC and SIC in cultivated soils and non-cultivated soils in China. For SOC, results reveal that the most significant SOC 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 C and ~8.8 kg C m-2, respectively, compared to a SOC storage of ~70.3 Pg C 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 C and a decrease of ~0.8 kg C m-2 SOC density due to increasing human activities. 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. For SIC, results show that the total SIC storage in China is ~55.3 Pg C with a current average content of ~6.3 kg C m-2, representing 5.8% of the global SIC pool. Land use has significantly affected SIC levels in cultivated soils. ~51% of total cultivated soil surfaces in China have experienced carbon loss where the most significant loss has been observed in the eastern part of northern China in dry farmlands as well as irrigated soils and paddy soils. In contrast, SIC has increased (~10%) in irrigated soils in northwestern China. No significant change (~39%) has been observed in soils in southern and the eastern part of northeast China. The total loss of SIC in China is ~1.6 Pg C due to extensive human activity. Results of this study indicate that human activity may have had a great impact on SIC as well as SOC pools. 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 the development of irrigated and paddy land uses, 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 C from the atmosphere.