海拔高度的变化对青藏高原表土δ^(13)C_(org)的影响

EFFECT OF ALTITUDE ON THE ORGANIC CARBON-ISOTOPE COMPOSITION OF MODERN SURFACE SOILS FROM QINGHAI-XIZANG PLATEAU

通过对青藏高原 1 93个表土样品的有机碳同位素分析 ,表明碳同位素值主要分布在- 2 8 5 7‰～ - 1 5 0 8‰的范围 ,平均值为 - 2 3 5 7‰。在海拔 35 0 0m以下 ,同位素随海拔高度的增加逐渐变轻 ,从 - 2 0 5‰逐渐减少到 - 2 8 5‰左右 ;35 0 0m以上 ,同位素随海拔高度的增加呈现相反的趋势 ,从 - 2 8 5‰逐渐增加到 - 1 5 0‰左右。按照森林—灌丛—高山草甸—草原—荒漠草原—荒漠 -高寒荒漠的顺序 ,表土有机碳同位素是逐渐变重的。初步认为CO2

The stable isotope composition of carbon ( δ 13 C value) can be used to provide information on the origin of carbon in soil organic matter. Earlier investigations have shown that fully sunlit plants from high mountain sites contain relatively more δ 13 C than plants growing under comparable light and moisture regimes at low altitude . In other words,the carbon isotope discrimination decreases with altitude. It has been known that the carbon isotope composition of forest and alpine grassland soils is controlled by many factors,including species composition,temperature light levels,moisture availability,partial pressures of O 2 and CO 2,and the source of CO 2 assimilated during photosynthesis. However,the knowledge about the effect of changing altitude and vegetation types on the carbon isotope composition of modern soils in high altitude is still less. This study focused on the effect of altitude ( i.e., decreasing temperature and atmospheric pressure) on carbon isotope composition of soil organic carbon in Qinghai Xizang Plateau and surrounding areas. The aim of the study is to determine whether a predictable relationship exists between vegetation type,the δ 13 C value of surface soil organic matter,and altitude. \;A total of 193 samples used in this study was collected from forest,shrub,grassland,desert,alpine meadow,desert meadow ,and alpine desert in Qinghai Xizang Plateau and surrounding areas,between latitudes 25°～42.37°N and longitudes 77.8°～104.1°E with altitudes of 700～5?500m. The δ 13 C values are measured on an NC2500(EA/MS) mass spectrometer with PDB as reference. The average precision is less than 0.23‰. The analyzed results show that δ 13 C values range from -28.57‰ to -15.08‰, averaging -23.57‰. Below about 3?500m,the δ 13 C values decrease with increasing altitude;ranging from -20.5‰ to -28.5‰. Above this height,however,the δ 13 C values exhibit an opposite trend with increasing altitude,varying from -28.5‰ to -15.0‰. Under the vegetation types of forest,shrub,subalpine meadow,subalpine steppe,alpine meadow,alpine steppe,alpine desert steppe, and alpine desert in our study,the carbon isotope composition of surface soil organic matter shows an increase in the average δ 13 C value with increasing altitude. These results suggest that both temperature and atmospheric pressure (the partial pressure of CO 2) are possibly responsible for the altitudinal trends in δ 13 C discrimination. \ \