Effect of cultivation on dynamics of organic and inorganic carbon stocks in Songnen Plain

The accurate estimation of soil C stock is important for understanding the global C cycles. Particularly in arid regions, quantification of soil inorganic carbon (SIC) is required in addition to the more frequently reported soil organic carbon (SOC). We analyzed SOC, SIC, soil bulk density (BD), electrical conductivity (EC) and pH of alkali'saline soils at a 0- to 100-cm depth in a cultivation chronosequence that consisted of a meadow steppe site and four cropped sites with maize (Zea mays L.) for 1, 7, 17, and 24 yr in Songnen Plain, Northeast China to quantify SOC and SIC dynamics. Results showed that SOC stock in 0 to 100 cm soil was 92.3, 83.7, 80.9, 78.2, and 76.9 Mg C ha'1 for Steppe, Crop-1Y, Crop-7Y, Crop-17Y, and Crop-24Y, respectively, decreased at a rate of 3.63 Mg C ha'1 yr'1. In contrast, SIC stock, ranging from 159.0 to 179.4 Mg C ha'1 increased at a rate of 0.53 Mg C ha'1 yr'1 in 0- to 100-cm depth. The soil total carbon (STC) stock increased at a rate of 0.46 Mg C ha'1 yr'1 following land use conversion, peaked in the seventh year of cultivation and then declined at a rate of 0.89 Mg C ha'1 yr'1. The SOC concentrations were negatively correlated with BD, EC, and pH, but SIC concentrations were positively correlated with soil BD, EC, and pH. The findings underline the importance of including SIC in the soil C estimates while considering deep soil profile and longer cultivation period.