88 H. Guo et al.
CO 2 levels increased the concentration of oxalate in the soil, and that this low mo-
lecular weight organic acid solubilized inorganic phosphorus, making it available
for uptake by the plant. Andrews and Schlesinger ( 2001 ) observed an increase in
cation concentration in the deep soil (200 cm) in the third year of CO 2 fumigation,
and proposed that the observed increase in cation availability was caused by the
increased organic acid content. Wu et al. ( 2009 ) showed that elevated CO 2 levels
lowered the pH by 0.2–0.4 units compared to ambient CO 2 levels, which implies
that the lower pH in the rhizosphere zone could help the plants take up more Cs. Li
et al. ( 2010 ) reported that the decrease in pH of 0.04–0.15 in the rhizosphere soil
of rice was due to elevated CO 2 levels, and considered that this slightly decreas-
ing trend might be linked to higher Cd concentrations in rice. Cheng et al. ( 2010 )
reported that elevated CO 2 levels significantly increased the concentration of Ca2+
and Mg2+ in soil solution and reduced the solution pH, and total cations in plant
biomass were also significantly higher under elevated CO 2 levels. In this study
after the second rice harvest, especially for heavy-metal-contaminated soils, the
pH of the soil also exhibited a decreasing trend and the acid-extractable fraction
of metals in soils exhibited an increasing trend at elevated CO 2 levels. It is known
that the mobility and bioavailability of heavy metals in the acid-extractable form
are greater than that of other fractions (Mulligan et al. 2001 ). These changes can
link elevated CO 2 levels to the increasing phytoavailability of heavy metals and
are probably sufficient to explain the higher Cd concentrations in rice and wheat
in this study. Thus, we propose that at elevated CO 2 levels, the exudation of low
molecular weight organic compounds by the roots of plants lowers the pH of rhi-
zosphere soils, facilitates metal solubility and bioavailability, and increases the up-
take of metal by plants. But if the soils are contaminated with little or no Cd under
elevated CO 2 conditions, the slight decrease in the pH of the soil will not lead to a
significant uptake of Cd by crops.
Since in this study the bioavailability of both Cu and Cd increased under el-
evated CO 2 levels, we were surprised that Cd concentrations in the crops increased,
but Cu concentrations decreased. There are a few possible explanations for these
Table 4.2 pH of soil after the second harvest of rice (November 2007) from FACE and ambient
plots to which Cu or Cd was added
Heavy metal pH
Ambient plots FACE plots
Cu (0 mg kg−1) (control) 7.06 ± 0.02 6.85 ± 0.03
Cu (50 mg kg−1) 7.04 ± 0.03 7.11 ± 0.03
Cu (200 mg kg−1) 6.93 ± 0.06 6.80 ± 0.01
Cd (0 mg kg−1) (control) 7.48 ± 0.11 7.38 ± 0.03
Cd (0.5 mg kg−1) 7.36 ± 0.01 7.31 ± 0.02
Cd (2 mg kg−1) 7.35 ± 0.01 7.06 ± 0.07*
Values represent means ± SD. An asterisk indicates a significant difference in pH between FACE
and ambient conditions ( p < 0.05)
FACE free-air CO 2 enrichment