4 Combination of Elevated CO 2 Levels and Soil Contaminants’ Stress ... 89
contrasting results (IPCC 2007 ). Many studies have shown that elevated CO 2 levels
increase plant growth and yields (Liu et al. 2008 ; Ziska et al. 1996 ; Moya et al.
1998 ; Kim et al. 2003 ), including a study using a same FACE system that reported
that elevated CO 2 levels enhance hybrid rice grain yield by 34 % (Liu et al. 2008 ).
Recently, Duval et al. ( 2011 ) indicated that CO 2 alters the distribution of contami-
nant elements in ecosystems, with contaminant elements accumulating in plants
and declining in soil, both likely explained by the CO 2 stimulation of plant biomass.
Li et al. ( 2010 ) reported that although higher Cd concentrations and lower Cu con-
centration in rice grown on contaminated soils under elevated CO 2 were detected,
elevated CO 2 still significantly increased the total uptake of Cu and Cd owing to the
change in biomass. Similarly, the higher concentrations of Cd and lower concen-
trations of Cu in crops observed in this study were probably due to the change in
biomass under elevated CO 2 conditions (Zhang et al. 2008). As reported, elevated
CO 2 levels increased the exudation of low molecular weight organic compounds
by the roots of plants (Delucia et al. 1997 ; Andrews and Schlesinger 2001 ), but the
binding strength of Cd and Cu to organic compounds differs (Groenenberg et al.
2010 ). Cu has a relatively high binding affinity to organic matter, whereas Cd has
a relatively weak affinity. This could have an influence on the uptake of Cd and Cu
under elevated CO 2 levels, leading to differences (Hill et al. 2007 ). Cations, such as
Ca2+, Mg2+, Na+, and K+, interfere with the heavy metal bioavailability and allevi-
ate metal toxicity owing to cation competition (Voigt et al. 2006 ; Luo et al. 2008 ;
Wang et al. 2008 ; Li et al. 2009 ). Kinraide et al. ( 2004 ) reported that the addition
of Ca2+ and Mg2+ alleviates metal toxicity, but the relative ameliorative effective-
ness of Ca2+ and Mg2+ depends upon the metal. Cheng et al. ( 2010 ), who used
a similar FACE system, reported that elevated CO 2 levels significantly stimulate
Ca2+ and Mg2+ release from soil. In this study, increased Ca2+ and Mg2+ in solution
in soil could have decreased both Cu and Cd uptake owing to cation competition,
but the relative effectiveness for Cu and Cd could differ based on the plant species.
Experimental evidences supporting the above explanations are lacking. Additional
research is needed to investigate the relationship between elevated CO 2 levels and
the increased phytoavailability of heavy metals and to elucidate the different mech-
anisms of the uptake of these two metals. The data presented here were obtained
from crops grown in artificially contaminated soils in pots. More data need to be
collected from crops grown under a wide range of soil conditions and realistic field
conditions to make better predictions on the combined effects of elevated CO 2 lev-
els and multimetal-contaminated soils on the metal uptake by crops and thereby on
their contribution to food quality and safety.
Acknowledgments This work was supported by Program for New Century Excellent Talents
in University (NECT-12-0266) and the National Natural Science Foundation of China (Grant
No. 20777034 and 40110817), the Knowledge Innovation Program of Chinese Academy of Sci-
ences (Grant No. KZCX3-SW−440), the China Postdoctoral Science Foundation, and the Jiangsu
Planned Projects for Postdoctoral Research Funds, and the Fundamental Research Funds for the
Central Universities (Grant No.1085021108). The main instruments and apparatus of the FACE
system were supplied by the Japan National Institute for Agro-Environmental Sciences (NIAES)
and the Japan Agricultural Research Centre for Tohoku Region (NARCT).