Carbon dioxide concentration can limit the identification of C4 plants by stable isotope composition

Abstract

The global expansion of C4 plants during the Late Cenozoic is an important event in the Earth’s environmental-ecosystem evolution. Taking into account the nonglobal expansion of C4 plants, the CO2 concentration is not the only prerequisite for C4 expansion. The direct driving factors for C4 expansion may be associated with regional arid and/or warm seasonal precipitation and temperature variations. However, a large change in the CO2 concentration occurred, varying from 100∼300 to >2000 ppm during geological history; thus, it is necessary to understand the impact of such changes in the atmospheric CO2 concentration on the δ13C values of C3 and C4 plants during different geological periods. In this article, we analyzed the δ13C values for seven C4 and C3 plants grown under different CO2 concentrations for 10, 20, and 30 days. According to our study, the δ13C records for all the C3 and C4 plants show a rapid decrease followed by a slowly stable decreased trend with increasing CO2 concentration when the plants were grown for 10, 20, and 30 days. Comparing with the value averaging about −26 to −27 ‰ for C3 plants and about −12 ‰ for C4 plants, when the CO2 levels reach 20,000 ppm, all of the δ13C values of C4 plants become more negative, varying from −27.5 to −36.1 ‰, which fall into the range of δ13C values for C3 plants. Namely, the increase of carbon dioxide can limit the identification of C4 plants by carbon isotope composition. Thus, the expansion of C4 plants as reconstructed from the geological records of carbon isotopes possibly does not reflect the origin and expansion of C4 plants when atmospheric CO2 concentration was reduced to a threshold value, as previously thought. However, another possible explanation is that the atmospheric CO2 concentration was reduced to a threshold value from which the carbon isotope values of C3 and C4 plants can be distinguished. Therefore, we suggest that it is necessary to understand the δ13C range for C3 and C4 plants, to accurately evaluate the origin and expansion of C4 plants, for geological periods when a high CO2 concentration occurred.