Abstract:

The Qinling-Daba Mountains form the main body of China's north-south transitional zone. However, because there is a controversy about the specific location of the geographical boundary in academic community, it is important to determine and improve the existing classification indices to construct the ecological geographical pattern in China. Soil is the core section of the transitional zone, the spatial distribution and variations in key soil indexes in the Qinling-Daba Mountains are important indicators for identifying the transition effect and regional characteristics of China's north-south transitional zone. This paper analyzes the spatial characteristics of soil organic carbon (SOC) and total nitrogen (TN), and their relationships with major geographical factors by means of spatial analysis and geostatistics, using data from the second national soil survey, as well as terrain, climate and vegetation data. Results show that spatial trends of SOC and TN contents are similar, and that there are three areas with high contents, one secondary area with high content and one area with low content. The high contents are found in the high-altitude regions of the Qinling-Daba Mountains and in the mountainous areas to the west of the Jialing River. Here, SOC and TN contents range from 15.03-71.04 g/kg and 1782.61-7710.00 mg/kg, respectively. The low-content areas spread from west to east across the Weihe Valley, to the southern Wutai and Funiu mountains, and along the north slope of the Qinling Mountains. Here, SOC and TN contents range from 0.64-6.50 g/kg and 110.00-885.96 mg/kg, respectively. The secondary high-content area is mainly located on the both sides of the Hanjiang River, in the Qinling-Daba Mountains where altitudes are less than 1000 m, in the Funiu Mountains at altitudes less than 1000 m and on both sides of Jialing River at altitudes slightly higher than 1000 m. Here, SOC and TN contents are between the above two ranges, with a gradual increase in content forming a "horn-shaped" pattern from west to east. In terms of the spatial characteristics and functions of vegetation, topography and climatic factors, it is found that the SOC/TN range in the secondary high-content area is consistent along the 1000 m contour line, at the upper limit of the warm temperate deciduous broadleaved forest belt (containing evergreen forest) and of the subtropical/warm temperate mixed evergreen-deciduous broadleaved forest belts, as well as along the 0 °C isotherm line in January and the 24 °C isotherm line in July. The temperature changes are stable in January, July, annually and during the four seasons, but the rainfall varies greatly in each season. This region is the main body area of the transtional zone between subtropical zone and warm temperate zone: the northern boundary is roughly distributed along the line of Dujiangyan-Maoxian-Pingwu-Wenxian to the west of the Jialing River and the 1000 m contour line on the southern slope of the Qinling Mountains. The southern boundary lies along the line of Dujiangyan-Beichuan-Qingchuan to the west of the Jialing River and the 1000 m contour line of the northern slope of the Daba Mountains. The results show that the spatial variation of SOC/TN content provides a reference for the demarcation of the subtropical-warm temperate zone. A further understanding of the soil processes and ecological effects in typical regions and typical mountains will help reveal the multi-dimensional transition characteristics and variation mechanism in the region.

Key words: China's north-south transitional zone, SOC, TN, terrains, vegetation, climatic boundary