delta 13C variation of C3 and C4 plants across an Asian monsoon rainfall gradient in arid northwestern China.

摘要

We have investigated carbon isotopic compositions of four plant genus/species, Bothriochloa ischaemum (C4), Stipa bungeana (C3), Lespedeza sp. (C3) and Heteropappus less (C3), along a precipitation gradient in northwest China in order to assess the impact of water availability on the carbon isotopic discrimination against 13C during carbon assimilation in this area. This information is necessary for reconstruction of paleovegetation, particularly paleo-C3/C4 plant ratios using delta 13C value of organic matter in loess and paleosols in the Chinese Loess Plateau. The delta 13C of C3 plants, as a group, exhibits a negative correlation with the annual precipitation amount with a total change and sensitivity of 5 per mil and -1.1 per mil/100 mm, respectively, for the precipitation range from 200 to 700 mm. The C4 grass, B. ischaemum responds to aridity by decreasing 1.7 per mil for over the precipitation range from 350 to 700 mm; the plant delta 13C is significantly correlated with annual precipitation with a slope -0.61 per mil/100 mm. This result implies that without considering the effect of water availability on the plant delta 13C values, reconstruction of percent C4 vegetation during the last glaciation can be overestimated by about a factor of two..