Path Analysis on Environmental Factors Controlling Soil Organic Carbon and Total Nitrogen Contents in the Upstream Watershed of Miyun Reservoir, North China
Soil comprises a major pool and plays an important role in the global C and N cycles. Miyun Reservoir is the largest reservoir in North China and also the Chinese capitals most important drinking water source. There is high variability in the environmental factors that influence the regional soil C and N pools in the Upstream Watershed of Miyun Reservoir with complex terrain, climate change, diverse soil types and vegetation types. Effects of climate (temperature and precipitation), topography (elevation and slope) and soil properties (soil bulk density, pH, water and clay contents) on soil organic carbon (SOC) and total nitrogen (TN) contents were investigated in the study to identify dominant environmental factors controlling SOC and TN contents in the Upstream Watershed of Miyun Reservoir of North China by soil sampling, laboratory analysis, stepwise regression analysis and path analysis. And the results showed as follows. Temperature, soil bulk density, pH, water and clay contents were the significant factors controlling SOC content, and the direct path coefficient of each factor was ranked in order of soil water content (0.439)>soil bulk density (-0.324)>soil pH (-0.238)>temperature (-0.209)>soil clay content (0.092), while the indirect path coefficient was ranked as soil bulk density (-0.425)>soil clay content (0.305)>soil water content (0.287)>soil pH (-0.179)>temperature (-0.043). Therefore, temperature, soil water content and pH mainly had direct effects on SOC content, while soil bulk density and clay content mainly had indirect effects through their contributions to other factors. Elevation, soil bulk density, pH, water and clay contents were the significant factors influencing TN content, and the direct path coefficient of each factor was ranked in order of soil water content (0.456) > elevation (0.234) > soil bulk density (-0.228) > soil pH (-0.190) > soil clay content (0.094),while the indirect path coefficient was ranked as soil bulk density (-0.484)> soil clay content (0.301)>elevation (0.247)>soil water content (0.257)>soil pH (-0.202). Therefore, soil water content had the greatest direct effect on TN content, while soil bulk density and clay content had considerable indirect effects on TN content through their influences on soil water content, and additionally the direct and indirect effects of both elevation and soil pH on TN content were equally important. All the selected environmental factors jointly explained 75.0%and 71.2%of SOC and TN contents variability, respectively. However, there were large values of remaining path coefficients of other factors influencing SOC and TN contents (i.e. 0.50 and 0.54), which indicated that some important factors (such as soil parent material, land use and management practices, etc) are not included in this study and should be taken into account in further researches.