Spatial and temporal changes of organic carbon in agricultural soils of Songnen Plain maize belt

In this paper, a database of soil organic carbon in topsoil (0-20 cm) of Songnen Plain maize belt was obtained from the second national soil survey in Jilin Province and its four counties in 1980, and from field measurements in 2003－2006. The density and storage of organic carbon in agricultural topsoil was estimated from different soil types, and then the spatial and temporal variations of soil organic carbon in the past 25 years were analyzed. The results showed that the area-weighted average organic carbon density and storage in agricultural soils had an increasing trend. The significant carbon gains were 33% (4.16 kg/m2), 23.05% (3.79 kg/m2), 16.51% (3.74 kg/m2), and 12.20% (3.77 kg/m2) for Solonetzs, Dark-brown earths, Black soils and Meadow soils, respectively. On the contrary, the soil organic carbon density decreased by 30.79% (2.18 kg/m2) in the chernozems. The spatial distributions of soil organic carbon density were the same for two periods, which was higher in centre and lower in the edge region. There was a significant negative relationship (r=-0.615**, P<0.01) for the carbon changes in the past 25 years to its original contents in 1980, and the turning point of soil organic carbon was at 4.04 kg/m2. Under the current land use patterns and traditional cultivation and tillage, the soil organic carbon sequestration potential would be 0.33Tg/a in Songnen Plain maize belt.