Distribution and impact factor of soil organic carbon on topsoil in karst mountain:A case study of Beibei District of Chongqing

Beibei district lies in the northeast of Chongqing and covers an area of 753 km2 with subtropical humid monsoon climate，parallel low hill landform and calcareous soil and yellow soil. Besides Devonian and Tertiary strata，all strata would be found in this district. Carbonate rock layers are mainly composed of the Jialingjiang group and the Feixianguan group of Triassic and the Maokou group of Permian in the area. In order to provide theoretical basis for soil improvement and land use，the distribution and impact factor of soil organic carbon (SOC) on topsoil are analyzed by means of geo-statistics on the basis of field investigation and lab analysis in the paper. The results show that，spatially，the content of SOC decreases remarkably from the surface layer to the bottom layer with the decreasing range being 35.02% and 47.12% respectively; temporarily，compared with the second soil survey conducted in 1984，the SOC content of surface，central and bottom layerall has an increasing tendency as well，with increasing range being 4.36%，31.92% and 14.74% respectively. The results of influence factor analysis show that the SOC content increases with the increase of the total nitrogen，alkaline hydrolysis nitrogen and clay particle in the soil. The SOC content in the soil that formed by carbonate rocks with clastic rocks is higher than that formed by pure carbonate rocks. The SOC increases with altitude as follows，mountain top (21.94 g/kg) > flat dam (19.53 g/kg) > trough valley (15.60 g/kg) >mountainside (13.40 g/kg). With respect to land use，the SOC content displays as woodland (26.16 g/kg) > grassy slope (21.95 g/kg)> vegetable plot (16.75 g/kg) > orchard (15.31 g/kg) > cultivated land (12.85 g/kg).Traditional tillage method is easy to lead loss of SOC. It is suggest that to increase SOC by return farmland to forests or grassland，terracing and applying more organic fertilizer in karst mountain; to strengthen carbon sequestration capacity of the farmland by no-tillage，straw returning and so on.