Distribution Of Organic Carbon Isotope Composition For Modern Soils From The Eastern Margin Of The Tibetan Plateau And Its Main Controlling Factors

Organic carbon isotope composition(?¹³C)of surface soil is related to the plant community influenced by modern environment factors.Revealing the relationships among the surface soil ?¹³C values,the vegetation types and the enviroment factors are helpful to study the paleoclimate and provide reference for the quantitative estimation of plateau uplift.In this study,surface soils from 588-4846m in the eastern margin of the Tibet Plateau were collected to get their ?¹³C values.Research on the distribution of plant community and meteorological factors was carried out to explain the ?¹³C variations.Surface soils were grouped according to the plant vertical zonality.Seen as a whole,increased ?¹³C values are found with increased elevation.Both 2000m and 3500m are shift heights for the soil ?¹³C values changing with altitude.With increasing altitude the ?¹³C values decrease below these heights and exhibit an opposite trend above these heights.The ?¹³C values of different plant soils are quite different from each other.The ?¹³C values get heavier in the sequence of evergreen broad-leaved and deciduous broad-leaved mixed forests?-27.28‰ on average?,evergreen broad-leaved forests?-27.25‰ on average?,subalpine shrub and meadow?-25.81‰ on average?,subalpine coniferous forests?-25.81‰ on average?,alpine shrub and meadow?-25.16‰ on average?,dry valley shrubs?-24.07‰ on average?.Along the elevation gradient,the ?¹³C values of soils from evergreen broad-leaved and deciduous broad-leaved mixed forests and alpine shrub-meadow distribution belt show an upward trend;from evergreen broad-leaved forests,subalpine coniferous forests and subalpine shrub-meadow distribution belt decrease at first and then increase.Although samples from different dry valley shrubs belts show irregular changes along the altitude,the ?¹³C values of soils from the same valley show a fluctuated downward trend.Soil organic carbon isotope composition is mainly controlled by plant composition,especially the proportion changes of different photosynthetic pathway plants.Plant physic-morphological adaptation can also lead to the increased ?¹³C values only when the proportion of different photosynthetic pathway plants is stable.The dropping proportion of C4 and CAM plant is the main reason for the decline and physic-morphological adaptation in C3 plants causes the increase in carbon isotope.The temperature and moisture are crucial factors for controlling the growth of C4?C3?CAM plant.Solar radiation intensity also plays an important role in the growth of C4 plants,but the effective height is limited.In the absence of favorable moisture and temperature conditions,low pCO2 alone is not sufficient to cause the distinct changes in the ?¹³C values of soils.