Recent advances in phytoremediation of arsenic-contaminated soils - In-Situ Remediation of Arsenic-Contaminated Sites - page 79

4.4.1 Water management
Because arsenic mobility and toxicity in paddy soils is largely controlled by soil redox potential,
water management can be effective in reducing arsenic mobilization resulting from reductive
dissolution of Fe hydroxides in anaerobic conditions. Compared with the conventional flooding
cultivation, arsenic availability and uptake by rice can be remarkably decreased under aerobic
conditions (Li et al ., 2009; Xu et al ., 2008). For example, compared with control, 4-16 times
higher soluble arsenic concentrations were reported with As(III) accounting for 81-95% of the
arsenic in flooding treatment. With efficient As(III) uptake system in rice, the enhanced arsenic
solubility in flooded soils caused 10-15 fold higher grain arsenic concentrations than those under
the aerobic treatment (Xu et al ., 2008). Similarly, in a field trial with paired plots to compare
raised bed and conventional flooding, a significantly higher redox potential was observed in the
raised bed as compared with paddy conditions (0 vs . 120 mV in 0-15 cm soil), resulting in 3-6
fold lower arsenic concentrations in rice straw under aerobic condition than those in the flooded
treatment (Duxbury et al ., 2007). However, in the presence of Cd, aerobic treatment tends to
mobilize soil Cd through the oxidation of CdS into CdSO 4 (Kawasaki et al ., 2009), indicating the
potential negative effect of aerobic rice cultivation in soils contaminated with both As and Cd.
4.4.2 Silicon fertilization
Efficient As(III) uptake by rice is via Si transport system (Ma et al ., 2008), suggesting enhanced Si
availability can mitigate arsenic transfer in soil-rice system as well as improving grain yield (Ma
and Yamaji, 2006). In a pot experiment, Si fertilization (20 g SiO 2 kg − 1 soil) decreased arsenic
concentration in rice straw and grain by 78% and 16%, in spite of the 1.5-2 fold higher arsenic
concentration in soil solution [with 78-100% As(III)] (Li et al ., 2009). However, increased As
availability did not translate to higher rice arsenic uptake probably due to enhanced competition
of silicic acid for As(III) by plant uptake. Moreover, Si fertilization affected arsenic fraction by
reducing inorganic arsenic level while enhancing arsenic methylation in both rice grain and husk.
For example, 59% reduction of inorganic arsenic concentration and a concurrent 33% increase in
dimethylarsinic acid concentration was found in rice grain with Si application (Li et al ., 2009).
Therefore, Si fertilization is a promising strategy to reduce arsenic uptake and phytotoxicity in
arsenic-contaminated soil-rice system.
4.4.3 Arsenic sequestration by Fe plaque
For paddy rice as well as other aquatic species, iron plaque formed on the root surface due
to oxygenation of rhizosphere exhibits high capacity for retaining As(V), and therefore could
effectively diminish arsenic influx into rice roots. In a pot culture experiment with rice growing
under flooded condition, arsenic concentrations in the rhizosphere soil solutions were remarkably
decreased, being 2.5-fold and 16-fold lower upon the amendment of amorphous iron at 0.1 and
0.5%, resulting from enhanced arsenic sequestration by Fe plaque. In comparison with control,
arsenic content binding to Fe-plaque was 3-4 times higher in Fe treatments (0.51 vs . 1.49 and
2.41 mg pot − 1 ). As a result, arsenic concentrations in rice shoots were significantly reduced,
accounting for only 1/7-1/2 that in the control (Ultra et al ., 2009).
Recent studies have further demonstrated significant variation in Fe plaque formation and
arsenic sequestration among rice cultivars (Mei et al ., 2009) and genotypes (Liu et al ., 2006).
Based on the pot experiment with 25 rice cultivars, significant negative correlation ( p < 0.001)
was observed between grain arsenic and root porosity and the rate of radial O 2 loss (Mei et al .,
2009). Rice cultivars with higher root porosity and rate of radial O 2 loss exhibit higher oxidizing
ability by releasing more O 2 to the rhizosphere, and hence possess higher capacity in limiting
arsenic influx into rice roots via effective arsenic fixation by Fe plaque. Taken together, Fe
amendment and breeding provide potential strategy to minimize arsenic transport into rice by
enhancing arsenic binding to the rhizosphere Fe plaque.