δ13C variation of C3 and C4 plants across an Asian monsoon rainfall gradient in arid northwestern China

We have investigated carbon isotopic compositions of four plant genus/species, Bothriochloa ischaemum (C-4), Stipa bungeana (C-3), Lespedeza sp. (C-3) and Heteropappus less (C-3), along a precipitation gradient in northwest China in order to assess the impact of water availability on the carbon isotopic discrimination against C-13 during carbon assimilation in this area. This information is necessary for reconstruction of paleovegetation, particularly paleo-C-3/C-4 plant ratios using delta(13)C value of organic matter in loess and paleosols in the Chinese Loess Plateau. The delta(13)C of C-3 plants, as a group, exhibits a negative correlation with the annual precipitation amount with a total change and sensitivity of 5 parts per thousand and -1.1 parts per thousand/100 mm, respectively, for the precipitation range from 200 to 700 mm. The C-4 grass, B. ischaemum responds to aridity by decreasing 1.7 parts per thousand for over the precipitation range from 350 to 700 mm; the plant delta(13)C is significantly correlated with annual precipitation with a slope -0.61 parts per thousand/100 mm. This result implies that without considering the effect of water availability on the plant delta(13)C values, reconstruction of percent C-4 vegetation during the last glaciation can be overestimated by about a factor of two.