Soil organic and inorganic carbon in the loess profiles of Lanzhou area: Implications of deep soils

Soil organic carbon (SOC) and inorganic carbon (SIC) are important carbon reservoirs in arid and semi-arid regions. Field data for quantifying both SOC and SIC in deep soils have been lacking. A study was carried out to evaluate soil carbon stocks in the loess of Lanzhou, a typical semiarid area of northwestern China. Soil samples were mainly collected from profiles of upper 2 meters in irrigated and non-irrigated agricultural land. Our data showed that SOC content decreased with depth, from 7.0±2.0gkg^-1 in the 0-0.2m to less than 3.0±2.0gkg^-1 below 1m. There was a large variation (3-11gkg^-1) in SOC of the topsoil (0-0.2m) but a narrow range of 1-5gkg^-1 below 1.0m. Soil inorganic carbon content showed little vertical changes with mean value of 16.0±2.5gkg^-1 in the 0-2m, but a considerable spatial variation of 10~21gkg^-1in the topsoil. Irrigation slightly enhanced SIC stock in the whole soil profile, with ~10% increase in the 0-30cm. We estimated that SOC stock was approximately 10kg C m^-2 for the 0-2m, and the SIC stock was about four times of the SOC stock. The SOC and SIC stocks in the subsoil (1-2m) were ~40% and 50% of the stocks over the 0-2m, respectively. The estimated total carbon stock was 26.8kg C m^-2 over the 0-1m, but 50.0kg C m^-2 over the 0-2m. Our study implies that neglecting deep soil may lead to underestimation of soil carbon stock by ~50% in the loess.