4 Combination of Elevated CO 2 Levels and Soil Contaminants’ Stress ... 87
on FACE and ambient plots with 0 and 0.5 mg kg−1 Cd did not significantly differ
(Fig. 4.8f). Similar results were observed for wheat (Fig. 4.9).
In this study, the Cd concentration in wheat grains of all samples far exceeded
the legal limits (wheat flour: 0.1 mg kg−1; Ministry of Health 2005 ). The Cd con-
centration in rice seeds of the first and second seasons from plants grown on either
FACE or ambient plots was below the legal limits (rice: 0.2 mg kg−1; Ministry of
Health 2005 ). But after exposure to elevated CO 2 level, the Cd concentration in
rice seeds of second season is more close to the legal limits than that of the first
season. Such increasing trends of Cd concentrations in rice seeds under elevated
CO 2 suggest that the levels of these toxic metals could exceed the legal limit in the
future. Cadmium can accumulate in the human body and damage kidneys, bones,
and reproductive system (Jarup and Akesson 2009 ). To keep the Cd levels in creati-
nine in urine below 1 μg Cd g−1 in 95 % of the population by age 50, the European
Food Safety Authority (EFSA 2009 ) has suggested that the average daily dietary Cd
intake should not exceed 0.36 μgCd/kg body weight, which corresponds to a weekly
dietary intake of 2.52 μg Cd/kg body weight (EFSA 2009 ). For an average adult of
60 kg with a daily intake of 261.1 g rice or wheat (Pan et al. 2007 ), this estimated
weekly dietary intake the levels of Cd far exceeds the levels suggested by EFSA in
all of the wheat samples from this study grown in control and contaminated soils
and in rice samples grown in highly contaminated soil and elevated CO 2 levels. Li
et al. ( 2010 )also found significantly higher Cd concentrations in three rice variet-
ies grown on contaminated soils under elevated CO 2 levels. In China, farmland
polluted by Cd has reached 20 × 104 ha and produces 14.6 × 108 kg of agricultural
products annually (Li et al. 2003 ). Since almost the entire population in China cur-
rently depends on rice and wheat as staple foods, the high, toxic concentrations of
Cd accumulated in crops threaten food quality and safety. This threat will increase
as the CO 2 levels increase in the future.
4.3.4 Variations in Soil pH, and Cu and Cd Fractionation in Soil
After the second rice harvest (October 2007), the pH of the soil was slightly but
significantly lower in the FACE plots than in the ambient plots (Table 4.2). Elevated
CO 2 levels also led to the changes in the available Cu and Cd in the soil. Compared
to soil from ambient plots, the acid-extractable fraction of Cu in soil from FACE
plots with 50 and 400 mg kg−1 Cu was 10.5 and 16.4 % higher ( p < 0.05), and the
reducible fraction of Cu in soil from FACE plots was 3.9 and 7.9 % lower ( p < 0.05),
respectively. Compared to the soil from ambient plots, the acid-extractable and re-
ducible fractions of Cd in soil from FACE plots with 2 mg kg−1 Cd were 4.7 and
6.9 % higher ( p < 0.05), and the oxidizable and residual fractions of Cd were 45.9
and 7.1 % lower ( p < 0.05), respectively.
Several studies have indicated that elevated CO 2 levels lower the pH of rhizo-
sphere soils, favor the release of elements into soil solution, and as a result, help
the plant to take up more elements. DeLucia et al. ( 1997 ) reported that elevated