Agriculture Reference
In-Depth Information
than FQS (food quality standard), it is necessary to develop a simple and reliable soil tests to
predict available Cd concentration in rice grains.
Previous studies indicated that 0.01M CaCl
2
, 0.1M HCl, 0.43M HNO
3
, and 0.05M EDTA
(Na
2
-EDTA·2H
2
O) are ideal extractants to estimate soil available Cd concentration (Houba
et al., 1997; Nelson et al., 1959; Tiwari & Kumar, 1982). This study compared these methods
to assess which method is better for predicting Cd levels in rice grains. The best well-
performed regression equation to predict Cd levels in rice grain was presented here using
soil available Cd and Zn concentrations determined by 0.01M CaCl
2
:
log[Cd-grain] =
0.94 + 0.78×log[Cd-CaCl
2
] - 0.30×log[Zn-CaCl
2
], r
2
= 0.73…for Indica (1)
log[Cd-grain] =
0.60 + 0.82×log[Cd-CaCl
2
] - 0.28×log[Zn-CaCl
2
], r
2
= 0.86…for Japonica (2)
The CaCl
2
extractable Zn is also included in the equation because it is able to compete with
Cd for plant uptake and reduce toxic effects of Cd. The critical concentrations of CaCl
2
-
extractable Cd in soil under different levels of soil CaCl
2
-extractable Zn are constructed for
farmers and authorities in Taiwan to prevent the production of Cd-contaminated rice by
using above equations.
The concentration of CaCl
2
-extractable Zn in soil ranged usually from 0.1 to 50 mg kg
-1
when the total soil Zn concentration is less than 600 mg kg
-1
, the SPCS for cropping lands
enacted in Taiwan. According to the equations, less Cd will be accumulated in rice grain if
the soil CaCl
2
-extractable Zn is getting higher, therefore, only the critical concentrations of
CaCl
2
-extractable Cd in soil under the soil CaCl
2
-extractable Zn lower than 50 mg kg
-1
are
presented. If the measured soil CaCl
2
-extractable Cd is higher than the critical value, it is
possible to produce rice grain with Cd concentration exceeding the Standard for the
Tolerance of Cd in rice (0.4 mg kg
-1
) (Table 8). Further studies are required to validate the
practicability of regression equations.
Rice variety
CaCl
2
-extractable Zn in soil (mg kg
-1
)
Indica
0.007
0.019
0.035
0.046
0.060
0.071
0.079
0.086
Japonica
0.027
0.060
0.105
0.133
0.168
0.193
0.213
0.230
Table 8. Critical concentrations of CaCl
2
-extractable Cd (mg kg
-1
) in soil under different
levels of soil CaCl
2
-extractable Zn (mg kg
-1
) for the two rice varieties. Cadmium
concentration in rice grain will exceed 0.4 mg kg
-1
if the measured soil CaCl
2
-extractable Cd
is higher than the critical concentration.
To predict Cd concentration in rice grain, Simmons et al. (2008) also developed a regression
equation using soil pH (1:5) and CaCl
2
extractable Cd determined on field-moist samples
collected during the grain-filling period. The equation can predict Cd concentrations in
unpolished rice grain with an r
2
value of 0.638. If air-dried soil samples were used for Cd-
CaCl
2
and pH determination, the regression equation cannot explain the variability of Cd
levels in rice grain. Air-drying may affect soil sample conditions to an extent that CaCl
2
extractable Cd cannot represent Cd availability in soil compared to extracts collected from
