【英文摘要】The relative contributions of C3 and C4 plants to vegetation at a given locality may be estimated by means of δ13C ratios of soil organic matter. This approach holds great potential for paleoenvironmental reconstruction. However, one major uncertainty that carbon isotope ratios vary during organic matters decomposing exists, which limit this application.In this study we measured stable carbon isotope ratios of soil organic matter and standing bulk vegetation under varied vegetation types in China. Stable carbon isotope ratios of soil organic matter are found to be systematically higher than that of original bulk vegetation. Decomposing of organic matter will leads to at least 3.2‰ enrichment of carbon isotope in soil organic matters relative to original bulk vegetation, suggesting that at least 3.2‰ variation of carbon isotope would be added when recontructing C3/C4 change by carbon isotopes of soil organic matters. Uncertainties exist in the effect of water availabity on carbon isotope of C4 plants and the control of soil water on occurrences of C4 species. In this study we observed that carbon isotopes of C4 plants incresed with water availability, opposite to C3 plants, furthermore, the varied extent of carbon isotope for C4 plants is less than C3 species under same condition. In addition, C4 species occurrences within different microhabitats were investigated and C4 plants were observed to be absent and/or scarce within relatively lower soil moisture microhabitats, whereas occurred and/or even had a high abundance within relatively higher soil moisture microhabitats, suggesting limited moisture available was a key factor of limiting C4 distribution in arid region of Northwest China.