Key Variables Explaining Soil Organic Carbon Content Variations in Croplands and Non-Croplands in Chinese Provinces

Key words: 
• soil organic carbon /
• land use /
• carbon loss /
• soil property /
• climatic variable

Abstract: Soil organic carbon (SOC) plays an important role in global carbon cycles. Large spatial variations in SOC contents result in
uncertain estimates of the SOC pool and its changes. In the present study, the key variables explaining the SOC contents of croplands
(CPs) and non-croplands (NCPs) in Chinese provinces were investigated. Data on SOC and other soil properties (obtained from the
Second National Soil Survey conducted in the late 1970s to the early 1990s), climate parameters, as well as the proportion of the CP to
the total land area (Pcp) were used. SOC content variations within a province were larger than those among provinces. Soil clay and
total phosphorus content, ratio of annual precipitation to mean temperature, as well as Pcp were able to explain 75% of the SOC content
variations in whole soil samples. Soil pH, mean temperature during the growing season from May to October, and mean annual wind
velocity were able to explain 63% of the SOC content variations in NCP soils. Compared with NCP soils, CP soils had lower SOC contents,
with smaller variations within and among provinces and lower C/N ratios. Stepwise regression showed that the soil clay content was a
unique factor significantly correlated with the SOC content of CP soils. However, this factor only explained 24% of the variations. This
result suggested that variables related to human activities had greater effects on SOC content variations in CP soils than soil properties
and climate parameters. Based on SOC contents directly averaged from soil samples and estimated by regression equations, the total
SOC pool in the topsoil (0–20 cm) of China was estimated at 60.02 Pg and 57.6 Pg. Thousands of years of intensive cultivation in China
resulted in CP topsoil SOC loss of 4.34–4.98 Pg.