Characteristics and Influencing Factors of Soil Organic Carbon in Upstream Watershed of Miyun Reservoir in North China

Soil organic carbon(SOC) pool constitutes an important portion of the global carbon pool and has significant impacts on land productivity and global climate change.The study on soil organic carbon has been one of the hot issues of the world.However,related research of the filed in the Upstream Watershed of Miyun Reservoir,the largest reservoir in North China,is relatively rare.The distribution characteristics and influencing factors(such as climate,topography and soil characteristics) of soil organic carbon in seven kinds of typical land use types of the Upstream Watershed of Miyun Reservoir were analyzed in the study,and the results showed as follows.1) In the target domain,soil organic carbon contents in natural secondary forests and grasslands were much higher than those in shrubs and artificial forests,while that in croplands was the lowest.In the whole soil profile(0–40 cm),the average soil organic carbon content was in order of natural secondary Populus davidiana Dode-Betula platyphylla Suk.mixed forestgrasslandnatural secondary Quercus wutaishanica Blume forestshrubartificial Larix principis-rupprechtii Mayr.forestartificial Pinus tabulaeformis Carr.forestcropland.2) Soil organic carbon contents in the top soil layer(0–10 cm) were the highest and decreased rapidly with the increase depth in the six kinds of land use types except grassland,which showed a slight increase from 0 to 20 cm and a small drop from 20 to 40 cm in depth.3) Soil organic carbon content in each layer was significantly positively correlated with altitude,soil water content and nitrogen content(p0.01),but remarkably negatively correlated with annual mean temperature,annual precipitation,soil bulk density and pH(p0.01).In addition,soil organic carbon content showed a little positive correlation with slope(p0.05).Further partial correlation analysis indicated that the most principal factors influencing soil organic carbon content varied with soil depth,and they were soil nitrogen content,bulk density and pH in the 0–10 cm layer,soil nitrogen content,bulk density and slop in the 10–20 cm layer,soil nitrogen content and annual precipitation in the 20–40 cm layer,respectively.4) Furthermore,the most significant factors influencing soil organic carbon content also varied with land use types,and they were soil nitrogen content and annual mean temperature in grassland and cropland,soil nitrogen content and water content in artificial Larix principis-rupprechtii Mayr.forest,soil bulk density,nitrogen content and pH in artificial Pinus tabulaeformis Carr.forest,soil nitrogen content and bulk density in natural secondary Populus davidiana Dode-Betula platyphylla Suk.mixed forest and natural secondary Quercus wutaishanica Blume forest,and only pH in shrub,respectively.