高寒山区地形序列土壤有机碳和无机碳垂直分布特征及其影响因素

地形、生物气候条件具有明显差异的青藏高原约占我国陆地面积的五分之一,开展该地区土壤有机碳和无机碳分布特征的研究对于理解青藏高原土壤碳循环过程与陆地碳库的精确预测以及应对全球气候变化具有重要意义。研究选取位于祁连山中段的阴、阳坡地形序列土壤,分析了不同坡向间以及同一坡向内随海拔高度变化土壤有机碳和无机碳的垂直分布特征及其影响因素。结果表明:阴、阳坡有机碳含量均随土壤深度增加而下降,但阳坡下降的速率(66%~91%)明显高于阴坡(31%~77%);阴坡土壤中碳酸钙基本淋失,通体无机碳含量较低(5.0 g kg-1),阳坡B层土壤无机碳含量是A层的2倍,表现为明显富集。阴坡和阳坡1 m土体总碳密度相当(分别为16.1~33.9 kg m-2和11.8~32.8 kg m-2),其中,阴坡以有机碳为主(占总碳密度的82%~99%),而阳坡有机碳和无机碳密度变化均较大(分别占总碳密度的27%~81%和19%~73%)。因此,坡向是影响高寒山区土壤碳垂直分布和组成的重要因素。此外,降雨量和植被类型对地形序列土壤有机碳和无机碳含量的空间变异也具有重要影响:降雨量每增加1 mm,表层(0~20 cm)土壤有机碳含量增加0.4 g kg-1,而淀积层(40~80 cm)土壤无机碳含量下降0.2 g kg-1;植被类型在一定程度上影响了土壤有机碳的富集程度。本研究揭示了青藏高寒山区土壤碳循环及其碳库预测应充分考虑微地形对坡面尺度下土壤碳垂直分布、碳库组成和空间变异的影响。

Vertical distributions of soil organic and inorganic carbon and their controls along toposequences in an alpine region

The alpine region in the Tibetan Plateau, characterized by sharp contrasts in topographpy and bioclimate, accounts for about one-fifth of China`s total land area. Due to limited field observation and high spatial heterogeneity, distribution of soil organic and inorganic carbon in the alpine region remains unclear. A better understanding of the distributions of soil organic and inorganic carbon and their controlling factors in this region is critical for accurate assessment of terrestrial carbon storage and important in implication for dealing with global climatic change. In this study, investigations were conducted of vertical distribution of soil organic and inorganic carbon along two toposequences in the middle Qilian Mountains on the northeastern edge of the Tibetan Plateau, one on the shady or north slope, the Hulugou watershed and the other on the sunny or south slope, the Shitougou watershed. Each toposequence consists of five typical soil profiles, and soil samples were collected by soil genetic horizons. The objectives of this study were to examine changes in vertical distribution of soil organic and inorganic carbon along the two toposequences, and to identify main controlling factors for the variations of soil organic and inorganic carbon content at the slope scale in a relatively small region. Results show that organic carbon content decreased with soil depth in both toposequences, but the rate was much higher in the sunny slope (66% to 91%) than in the shady slope (31% to 77%). In the soil profiles along the shady slope, inorganic carbon was found distributed quite evenly (< 5.0 g kg-1) due to the strong leaching of carbonate, while in the soil profiles along the sunny slope, inorganic carbon in B horizons was two-fold as high as that in A horizons, which demonstrates that evident enrichment of inorganic carbon in the B horizons of the soil profiles on the sunny slope. Soil carbon in the topmost 1 meter soil layer did not vary much in density between the north and south slopes (16.1 to 33.9 kg m-2 and 11.8 to 32.8 kg m-2 respectively), but did in composition. In the north slope, the soil carbon was dominated by organic carbon accounting for 82% to 99% in density, however, the soil organic and inorganic carbon in the south slope varied sharply in density, accounting for 27% to 81% and 19% to 73% of the soil total, respectively. Therefore, it may be concluded that slope aspect plays an important role in the vertical distribution as well as composition of soil carbon in the alpine region. In addition, precipitation and vegetation are also major factors affecting spatial variability of soil carbon along the toposequences. With the mean annual precipitation increasing by 1 mm, soil organic carbon within the 0-20 cm soil layer increased by 0.4 g kg-1, while inorganic carbon within the 40-80 cm soil layer declined by 0.2 g kg-1. And vegetation type also had some effect on enrichment of soil organic carbon. All the findings in this study demonstrate that the study on soil carbon cycling and the estimation of soil carbon stocks in the alpine region should take into account the influence of micro-topography, especially slope aspect, on distribution, composition and spatial variation of soil carbon at the slope scale.