Elevated CO2 increases Cs uptake and alters microbial communities and biomass in the rhizosphere of Phytolacca americana Linn (pokeweed) and Amaranthus cruentus L. (purple amaranth) grown on soils spiked with various levels of Cs

General concern about increasing global atmospheric CO2 levels owing to the ongoing fossil fuel combustion and elevated levels of radionuclides in the environment, has led to growing interest in the responses of plants to interactive effects of elevated CO2 and radionuclides in terms of phytoremediation and food safety. To assess the combined effects of elevated CO2 and cesium contamination on plant biomass, microbial activities in the rhizosphere soil and Cs uptake, Phytolacca americana Linn (pokeweed, C3 specie) and Amaranthus cruentus L. (purple amaranth, C4 specie) were grown in pots of soils containing five levels of cesium (0, 100, 300, 500 and 1000 mg Cs kg−1) under two levels of CO2 (360 and 860 μL L−1, respectively). Shoot and root biomass of P. americana and Amaranthus crentus was generally higher under elevated CO2 than under ambient CO2 for all treatments. Both plant species exhibited higher Cs concentration in the shoots and roots under elevated CO2 than ambient CO2. For P. americana grown at 0, 100, 300, 500 and 1000 mg Cs kg−1, the increase magnitude of Cs concentration due to elevated CO2 was 140, 18, 11, 34 and 15% in the shoots, and 150, 20, 14, 15 and 19% in the roots, respectively. For A. cruentus, the corresponding value was 118, 28, 21, 14 and 17% in the shoots, and 126, 6, 11, 17 and 22% in the roots, respectively. Higher bioaccumulation factors were noted for both species grown under elevated CO2 than ambient CO2. The populations of bacteria, actinomycetes and fungi, and the microbial C and N in the rhizosphere soils of both species were higher at elevated CO2 than at ambient CO2 with the same concentration of Cs. The results suggested that elevated CO2 significantly affected plant biomass, Cs uptake, soil C and N concentrations, and community composition of soil microbes associated with P. americana and A. cruentus roots. The knowledge gained from this investigation constitutes an important advancement in promoting utilization of CO2 fertilization for improvement of phytoextraction of soils contaminated with radionuclides.

► Elevated CO2 significantly promoted growth of two plants.
► Shoot/root metal concentrations were altered under elevated CO2.
► Bioaccumulation factors were enhanced under elevated CO2.
► Microbial population, microbial C and N increased at elevated CO2 for same Cs level.
► Elevated CO2 can be used to improve phytoextration of radionuclide contaminated soil.