supported by the National Natural Science Foundation of China(Grants Nos.40273006,40072059,49925307 and 90102001).
Carbon isotope ratios (δ^(13)C) of 89 C_4 plant samples were determined fromthe loess area in North China. δ^(13)C values vary between -10.5per thousand and -14.6per thousandwith a mean of -12.6per thousand. Along a...