72 H. Guo et al.
contaminated with heavy metals (Huang et al. 2009 ). Heavy metals are toxic to
plants, animals, and humans at different concentrations, and are known to cause
significant environmental damage and human health problems (Huang et al. 2009 ;
Mulligan et al. 2001 ; Nahmani et al. 2005 ; Maksymiec 2007 ). Of the heavy metals,
copper (Cu) is an essential micronutrient for plants and animals. However, when in
excess, Cu can interfere with numerous physiological processes, resulting in cell
toxicity. Cadmium (Cd) is a toxic trace element in the environment that can be eas-
ily taken up by plants and subsequently transferred to humans through food chains.
In the future, further increases in global CO 2 levels and contamination with
heavy metals are likely. More research is needed to investigate the response of
crops grown in soils contaminated with metals under elevated CO 2 levels. In the
few reports available, Duval et al. (Duval et al. 2011 ) indicated that CO 2 alters the
distribution of contaminant elements in ecosystems; Wu et al. ( 2009 ) showed that
elevated CO 2 level increases cesium (Cs) concentrations in rice shoots and roots;
Li et al. ( 2010 ) reported that elevated CO 2 levels decrease or barely affect Cu con-
centrations in six rice varieties grown in contaminated soils, but increase Cd levels
in three rice varieties. These studies highlight the need for a better understanding of
the mechanisms by which CO 2 and heavy metals jointly affect crop growth and up-
take of metals, especially from the viewpoint of food safety. One also must consider
that these studies were conducted in open-top chambers (OTCs). Long et al. ( 2006 )
reported that the effects on plants grown in OTCs are often greater than on plants
grown under open air. Therefore, the results of such studies cannot be extrapolated
to address the effect of long-term, more realistic CO 2 fumigation on plants. One
way to approach such a study is to use free-air CO 2 enrichment (FACE). FACE
experiments are conducted in open fields, allow the best simulation of elevated CO 2
environments (Long et al. 2006 ), and have been carried out in many countries (Lief-
fering et al. 2004 ; Andrews and Schlesinger 2001 ; Hoosbeek et al. 2007 ).
With the aim of predicting future food safety and the combined stress of CO 2 and
soil pollution, we used a full-size (14 m diameter) FACE system in farm fields in
Jiangsu Province, China, to investigate the effect of elevated atmospheric CO 2 on
Cd and Cu levels in two important crops worldwide, rice and wheat.
4.2 Materials and Methods
4.2.1 Experimental Site
The FACE system was established in the town of Xiaoji, Jiangdu County, Jiangsu
Province, China (119°42’E, 32°35’N). Here, rice–wheat rotation system is prac-
ticed. This region lies within the northern subtropical monsoon climate. The annual
mean temperature is 14–16 °C, and the mean annual precipitation is 980 mm. The
annual length of the nonfrost period is approximately 220 days. The soil is Shaji-
ang-Aquic Cambosols with a sandy–loamy texture.