Recent advances in phytoremediation of arsenic-contaminated soils - In-Situ Remediation of Arsenic-Contaminated Sites - page 78

planted lysimeters during the five-month field trial. Furthermore, an interactive effect of plant
species and type of P amendment on arsenic mobilization has been observed (Mains et al ., 2006).
For example, organic bioboost amendment (dehydrated sewage sludge) induced higher leached
arsenic than superphosphate fertilizer from barley (2.4 vs . 1.0 mg L − 1 ), but not for rye, corn or
blue lupin. The fact that soil arsenic mobilization may occur upon plant establishment with P
amendment highlights the necessity of pre-assessment to choose proper amendment strategy and
plant candidate to minimize arsenic leaching and soil-plant transfer during phytostabilization.

4.3.5 Organic matter
As a complex mixture of varying components, organic matter (OM) has inconsistent effects on
arsenic mobility, mainly due to the type of compost applied, degree of humification and pH
variation (Juwarkar et al ., 2008; Lagerkvist et al ., 2008; Shiralipour, 2002). For example, a
significant increase in soil soluble arsenic from 5.7 mg L − 1 to 7.1 mg L − 1 was reported upon
application of municipal solid waste and biosolids compost in a greenhouse experiment (soil pH
6.87) (Cao and Ma, 2004). Increased OM under neutral soil pH may improve microbial activity,
and hence facilitate As(V) reduction to more mobile As(III). At the end of the experiment, As(III)
concentration in soil solution was 20-24% in OM treatment compared to < 10% in the control. In

contrast, a reduction of water-soluble arsenic from 32 mg L − 1 to 25 mg L − 1 upon OM application
was obtained in an arsenic-spiked soil with pH of 5.45 in the same experiment (Cao and Ma,
2004). This is probably due to arsenic adsorption onto OM in acidic conditions. Therefore, it is
critical to monitor arsenic availability using OM to reduce the potential environmental risks of
arsenic mobilization while improving substrate conditions.

∼

4.3.6 Mycorrhiza
Proper inoculates of As-tolerant mycorrhiza can serve as a potential strategy to confer host-
enhanced tolerance and facilitate phytostabilization by selectively accumulating P over As(V)
(Sharples et al ., 2000). For example, short-term As(V) and P uptake kinetics of mycorrhizal
fungus Hymenoscyphus ericae from the roots of Calluna vulgaris were identical for the mine-
impacted and control population. However, enhanced efflux of As(III) from mycelia preloaded
with 0.1 mM As(V) for 1 h was observed in the resistant H. ericae from the mine population
(14.4% h − 1 vs . 6.6% h − 1 ), indicating the role of fungus as biofilter to maintain low plant arsenic
via efficient As(III) efflux (Sharples et al ., 2000). White clover ( Trifolium repens Linn.) and
ryegrass ( Lolium perenne L.), which represent legumes and grasses that are commonly used
in revegetation, exhibit high dependence on the mycorrhizal associations with G. mosseae for
surviving in soils with severe arsenic contamination (Dong et al ., 2008). Using a compartmented
cultivation system, mycorrhizal inoculation of G. mosseae increased plant P concentration by
50-200%, with shoot arsenic being reduced by 9%-30% in the presence of 1 and 205 mg kg − 1
arsenic. The data indicate the beneficial effect and potential use of AMF in phytostablization via
selectively accumulating P over As(V).

4.4 PHYTOEXCLUSION

For agriculture soils contaminated by arsenic, it is impractical to employ non-food crops for either
phytoextraction or phytostabilization. Rice, which feeds half of the world's population, is more
efficient in arsenic accumulation than other cereals through efficient silicon transport pathway (Ma
et al ., 2008), particularly under flooding conditions. As a result, elevated arsenic has been widely
reported in paddy rice with arsenic TF often approaching unity (Abedin et al ., 2002; Stroud et al .,
2010; Voegelin et al ., 2010; Williams et al ., 2007; 2009). To improve agriculture sustainability
and food safety, a range of agronomic strategies and biotechnologies have been developed (Zhao
et al ., 2010), which may provide effective solutions to remediate arsenic-contaminated agriculture
soils and reduce arsenic uptake by rice.