CARBON ISOTOPES OF C3 AND C4 PLANTS IN NORTH CHINA

Carbon is an important ring in ecosystems and geosystems. It exists in different forms, such as CO₂ in the atmosphere and soil, organisms in plant and animal tissues in the biosphere, HCO₃^- and CO₃^2- in the hydrosphere, organic relics and carbonates in soils and deposits in the supergene zone, and carbonate rocks and fossil fuels in the lithosphere. It plays an important role in the interaction of the Earth's system. Appearance of C4 photosynthesis in the terrestrial plants is one of the most important biological events during the Cenozoic. It is noted that the virtual absence of overlap in the ranges of the carbon isotope composition of C3 and C4 plants. It reminds us of carbon isotope might be a useful tool in understanding global biogeochemical cycles in the climate system. For that purpose, several kinds of materials were used in carbon isotopic studies, such as soil organic materials and carbonates, fossil teeth and present-day plants. It is common accepted that carbon isotopic composition of soil organic materials and soil carbonates is closely related to the relative amount of C3 and C4 plants grew on it. It should be useful to study the carbon isotopic changes during biological cycles in different environments. Our main interest focuses, therefore, mainly on the carbon isotope of modern plants and soil organic materials, their response to environment changes and the possibility to use them in quantitative paleoenvironmental studies during the Quaternary. Some measurements of carbon isotope composition of herbs taken from temperate northern China were carried out. The results show that d¹³C values shift to more positive in a westward direction. The same tendency was seen for individual representative samples of both C3 and C4 plants taken from different geographical positions indicating its humidity dependence. Negative correlation was seen between carbon isotopic value and local annual precipitation for some widespread species. There seems a gap of about 2 per mil between the carbon isotope composition of soil organic matter and that calculated from the isotopic composition of all kinds of plants and their real measured biomass. It implies that there is a fractionation when plant relics decomposed into organic matter in the soil.