中国南北过渡带土壤碳氮空间特征及暖温带与亚热带界限

秦巴山区是中国南北过渡带的主体,过渡带分界划分在学界一直存在争议,确定和改进划分指标对构建中国生态地理格局有重要作用。土壤作为过渡带的核心部分,其关键指标的空间分布及变异机制对识别过渡效应和区域特征有指示作用。本文基于土壤二普资料,采用空间模拟和地统计方法分析土壤有机碳/全氮空间特征及与主要自然地理要素的关系。结果显示,秦巴山区有机碳/全氮含量空间分布趋势一致,存在3个高值区、1个次高值区和1个低值区。高值区分布在秦岭、大巴山高海拔区域和嘉陵江以西山地,含量分别为15.03~71.04 g/kg、1782.61~7710.00 mg/kg;低值区沿秦岭北坡的渭河谷地、南五台山和伏牛山分布,含量分别为0.64~6.50 g/kg、110.00~885.96 mg/kg;次高值区主要在汉江两侧、秦巴山地之间海拔< 1000 m及嘉陵江两侧略高于1000 m的山体,含量介于以上二者之间,自西向东呈南北向宽幅逐渐增大的“喇叭状”趋势。综合考虑地形—植被—气候作用,发现秦岭南坡—大巴山北坡有机碳/全氮次高值区分布范围与1000 m等高线、暖温带落叶阔叶林带（含常绿成分）和亚热带常绿落叶阔叶混交林带上限、1月0 ℃等温线、7月24 ℃等温线较一致,区内1月、7月、季节和全年气温变化较小,各季降雨变幅大,该区是亚热带向暖温带过渡的主体,北界大致沿都江堰—茂县—平武—文县和秦岭南坡1000 m等高线分布,南以都江堰—北川—青川和大巴山北坡1000 m等高线为界。有机碳/全氮空间变化为亚热带—暖温带的划界提供一定依据,进一步识别典型区土壤过程及生态效应,将全面揭示土壤多维过渡特征及其变异机理。

Spatial patterns of SOC/TN content and their significance for identifying the boundary between warm temperate and subtropical zones in China's north-south transitional zone

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.