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 ± 2.0 g kg− 1 in the 0-0.2 m to less than 3.0 ± 2.0 g kg− 1 below 1 m. There was a large variation (3-11 g kg− 1) in SOC of the topsoil (0-0.2 m) but a narrow range of 1-5 g kg− 1 below 1.0 m. Soil inorganic carbon content showed little vertical changes with mean value of 16.0 ± 2.5 g kg− 1 in the 0-2 m, but a considerable spatial variation of 10 ~ 21 g kg− 1in the topsoil. Irrigation slightly enhanced SIC stock in the whole soil profile, with ~ 10% increase in the 0-30 cm. We estimated that SOC stock was approximately 10 kg C m− 2 for the 0-2 m, and the SIC stock was about four times of the SOC stock. The SOC and SIC stocks in the subsoil (1-2 m) were ~ 40% and 50% of the stocks over the 0-2 m, respectively. The estimated total carbon stock was 26.8 kg C m− 2 over the 0-1 m, but 50.0 kg C m− 2 over the 0-2 m. Our study implies that neglecting deep soil may lead to underestimation of soil carbon stock by ~ 50% in the loess.