Agriculture Reference
In-Depth Information
field-moist soil. However, the soil samples used for developing regression equations in the
study of Taiwan as mentioned above were air-dried and collected during rice harvest
period, an easier pretreatment for soil samples and more suitable for routine monitoring.
Brus et al. (2009) recently developed a multiple regression model using 0.43M HNO
3
extractable Cd, pH, clay, and SOM as predictors to predict Cd levels in rice grain harvested
form the paddy fields in Fuyang, Zhejiang province, China. The model performed much
better (r
2
adj
= 0.661) than the linear model using only 0.01M CaCl
2
extractable Cd as a
predictor (r
2
adj
= 0.281). The field study in Taiwan as mentioned above also developed a
multiple regression model using 0.43M HNO
3
extractable Cd, pH, and CEC to predict Cd
levels in rice grain. Although the model using more predictors to reflect the effects of pH
and CEC on the availability of Cd, it did not perform much better (r
2
= 0.81 and 0.74 for
Japonica and Indica, respectively) than the model using 0.01M CaCl
2
extractable Cd and Zn
as predictors (r
2
= 0.86 and 0.73 for Japonica and Indica, respectively). Therefore, the latter
simpler model is preferred to be validated and used in Taiwan. Since different
environmental and soil factors affect the accumulation of Cd in rice grain in different ways
and extents, the predicting models developed by using local data will be more reliable to be
used for the specific area.
7. As-contaminated soils in Guandu plain
Arsenic is a contaminant of public concern since it is highly toxic and carcinogenic. It may
be accumulated in plants and eventually be transferred to humans through the food chain.
A regular monitoring for HM concentrations in soil conducted by Taipei government found
that some soil samples in Guandu Plain were contaminated by As. Further comprehensive
survey conducted in 2006 showed that more than 60 ha of rice-growing soils located in that
area were contaminated by As. The maximum As concentration in topsoil (0-15 cm) reached
535 mg kg
-1
in this area, which was almost 9 times of the SPCS (60 mg kg
-1
) enacted in
Taiwan. The contamination source of As in this area may come from the hot spring water of
Thermal Valley. The hot spring water flowed out and mixed with the stream water which
was used as irrigation water for the As-contaminated area of the Guandu Plain (Chang et
al., 2007). Some studies indicated that the soil parent materials may also contribute to the
high levels of As in soils of Guandu Plain (Su & Chen, 2008; Wu, 2007).
Arsenic in soils occurs mainly as inorganic species (Huang, 1994). In well-aerated soils,
arsenate (As(V)) is the predominate form, whereas in reduced environment such as paddy
soils, arsenite (As(III)) species prevails. Previous studies showed that As(V) in aerated soils
will be reduced to more mobile and toxic As(III) in paddy soils and transferred to rice
(Huang, 1994; Masscheleyn et al., 1991). Since As(III) is much more toxic, more soluble, and
more mobile than As(V), it is a big chance that arsenic in rice-growing soils in Guandu Plain
may transfer to rice and reduce rice yield or even impairs food safety. Meharg & Rahman
(2003) indicated that As levels of paddy soils in Bangladesh irrigated with As-contaminated
groundwater reached only 46 mg kg
-1
but the As concentration in rice grains were as high as
1.7 mg kg
-1
DW. Liao et al
.
(2005) also reported high levels of As in rice (0.5-7.5 mg kg
-1
DW)
grown on As-contaminated soils in China. Whether the rice produced in highly As-
contaminated soil in Guandu Plain is safe for human consumption or not is an emergent and
important issue of local residents and government agency.
