Stable Carbon Isotopes Of Plants And Soils In Chinese Loess Plateau

Loess-paleosol sequences on Chinese Loess Plateau have recorded abundant information of climate and environment change. Stable carbon isotope ratio of soil organic matter is used as the indicator of changing vegetation and environment. In this paper, ?13C value of organic matter and magnetic susceptibility of plants and soils are tested. The main conclusions are as follows:1. Analyzed the relationship between C3/C4 biomass and their growing environment in Loess Plateau table, the mean geographic unit on the Loess Plateau, in order to get a better understanding of factors influencing C3/C4 biomass. We calculated C4 biomass of 67 sampling sites, mainly in Shaanxi, Shanxi and Gansu province along the rainfall gradient from 700 mm to 300 mm. We estimated the ratio of C4 plants in each investigated site by choosing 1~2 quadrats covering 4~8 m2. The investigations show that the Bothriochloa ischaemum(Plains bluestem or Yellow bluestem), which is favorable in warm season and more rainfall, is the most abundant of C4 plant on the loess table. Other C4 plant, Cleistogenes squarrosa, setaria viridis and Themeda triandra, etc., are less abundant. Natural C3 plants are composed ofgrasses and shrubs such as Stipa bungeana(Gramineae), Lespedeza davurica(Leguminosae) and Kobresia graminifolia(Labiatae), etc., and they are widely distributed in the shady slope and valley in the research area. The mean stable carbon isotope values of C3 and C4 plants are –27.04‰ and –12.99‰, respectively. Our results confirmed that stable carbon isotope of surface soil organic matter is a reliable proxy in reconstructing palaeovegetation. Our results also show that growing season precipitation(from May to October) results in C4 biomass increase and C4 biomass is responding to changes of seasonal precipitation of Loess Plateau today. Theseresultshelp us understand how C3/C4 biomass changes under the control of Asian monsoon climate.2. Soil magnetic susceptibility is also an important proxy in palaeoclimate study using loess-paleosol sequences. We have investigated soil magnetic susceptibility, organic carbon content, organic carbon isotope composition and C/N ratio of modern soils from the arid Loess Plateau in China. Soil samples were collected from loess-desert area, loess platforms and forest area, which represent modern soil types in the Loess Plateau. The results show that magnetic susceptibility for loess platforms, forest area, and loess-desert area vary from 26.6~61.4, 68.6~107.5, 8.5~44.4, respectively. Organic carbon content for loess platforms and forest area varies from 0.05~0.62%, 1.19~3.35%, respectively. Similarly, C/N ratio for loess platform and forest ranges from 0.6~6.1 and 6.2~11.83, respectively. Moreover, soil magnetic susceptibility, which increases from loess-desert area to forest areas, is positively related to organic C content and C/N ratio. We conclude that organic matter content contributes to the increase of soil magnetic susceptibility. Organic matter indicates the amount of vegetation, which is also positively related to soil magnetic susceptibility. Burning organic matter helps nonmagnetic minerals turn to magnetic ones. Magnetic bacteria might also be a possible cause of the increase in soil magnetic susceptibility.3. Studied an 80 m loess-paleosol sequence in Luochuan and tested organic carbon isotope composition of 201 samples from that sequence. The organic carbon isotope record shows that the ?13C value of soils ranges from-19.37‰ to-26.38‰. The loess-paleosol ?13C records have a similar changing pattern with magnetic susceptibility records in the glacial-interglacial scales. The results demonstrate that the vegetation coverage in Luochuan was a mixture of C3 and C4 plants, with more C3 plants and fewerC4 plants. The organic carbon isotope and magnetic susceptibility of Chinese Loess Plateau loess-paleosol sequence have recorded the variation of Asian summer monsoon intensity.