Cadmium (Cd) is one of the major toxic heavy metals reaching the food chain. In rice soils, Cd is derived mainly from the application of Cd- containing P fertilizers and biosolids. Contamination also results from mine tailings and acid-mine drainage. Cadmium is accumulated in plants more readily than most other metals and can be translocated into edible parts before any signs of phytotoxicity. Rice is one of the most important crops grown for human consumption, and Cd accumulation in rice grains poses a potential health risk. The enrichment of Cd in paddy rice grain tends to occur during soil oxidation, which accompanies preharvest drainage of the flooded paddy.
Several studies about Cd-contaminated soils are available but the guide- lines established by individual countries worldwide to control the pollution of agricultural soils are not consistent and standardized. Scientific evidence from site-specific research, particularly long-term field trials involving all types of key conditions and factors, are necessary to understand the bio- availability of Cd in various soil types and to provide reliable parameters for health-based risk assessments. In particular, the collection of reliable databases on Cd concentrations in different conditions of soils, climates, rice cultivars, etc. must be given utmost importance. From these databases, a variety of useful information and methodologies can be developed toward achieving the ultimate goal of providing safe and high-quality rice, espe- cially in the case of lowland soils.
Unlike upland soils, paddy soils are flattened evenly for water con- trol under flooding and huge amounts of water are irrigated during rice cultivation. The irrigation of contaminated water and the utilization of contaminated soils are considered to be the most important routes of Cd accumulation in paddy soil and rice plants. The best solution for mitigating Cd contamination in paddy soils and the rice plant is to remove the sources of Cd in the environment and to prevent Cd flow into paddy soils. Hence,
further research needs to be done to determine the effectiveness in suppress- ing Cd availability when released from the sources, which include mining and industrial wastes and others. A number of soil remediation techniques for Cd-contaminated soils have been developed but some of these techniques are not efficient in terms of time, cost, and environmental compatibility. In order to select the best and most practical technique for remediation of Cd- contaminated rice paddy soils, more investigations are needed.
The biogeochemistry of Cd in rice ecosystems is complex and mostly determined by its chemical speciation resulting from chemical and biologi- cal transformations. The chemistry of soil and water (i.e. pH and Eh) plays a major role in Cd dynamics in paddy soils, which undergo changes in redox reactions during a growing season.
Risk management of Cd-contaminated paddy soils and the associated flooded water is an important issue and a great challenge. Its success is neces- sary to minimize Cd accumulation in paddy rice grains that reach the food chain. A number of physical, chemical, and biological technologies involv- ing immobilization, filtration, and phytoremediation have been developed to remediate Cd-contaminated paddy soils and the associated water. Con- ventional physical and chemical remedial measures usually are expensive but may prove highly effective. Field testing of some of these technologies has shown them to be successful in reducing Cd accumulation in rice grains.
Low Cd-accumulating rice varieties can be used to minimize Cd reach- ing the food chain. Phytoremediation, which is relatively inexpensive, has been proven effective in the remediation of metal(loid)s-contaminated sites, including those with Cd. Nonedible, Cd-hyperaccumulating crops, like ornamental and fuel crops, may be suitable for phytoremediation through which the entry of Cd into the food chain could largely be avoided.
Remediation of Cd-contaminated rice soils and Cd stripping from irri- gation waters require an integrated approach involving a combination of physical, chemical, and biological technologies for successful and effective management of Cd-contaminated rice ecosystems. Future research is, there- fore, needed along the following lines:
• Elucidation of soil and water environmental factors (e.g. pH and Eh) that govern transformations of Cd both in upland and lowland rice eco- systems.
• Examination of solid-phase and solution-phase speciation of Cd in soil and water using advanced spectroscopic-based techniques.
• Identification of biochemical mechanisms involved in the accumulation of Cd in paddy rice grains.
• Rhizosphere processes underpinning effective phytoremediation tech- nologies for Cd removal from paddy soils.
• In situ immobilization techniques in paddy soils using inexpensive con- taminant-free industrial byproducts high in FeCl3.
• Highly effective and expensive stripping methods for the removal of Cd in water supplies destined for irrigation.
ACKNOWLEDGMENTS
The senior author thanks CRC CARE for providing funding (No 2-3-09-07/08) to under- take research on landfill site remediation. Drs Makino, Ishikawa and Murakami are grateful for the grant from the Ministry of Agriculture, Forestry and Fisheries of Japan (Research project for ensuring food safety from farm to table AC-1310, -1320); part of the review was derived from the abovementioned projects. The Postdoctoral fellowship program (PJ008650042012) with Dr Won-Il Kim at National Academy of Agricultural Science, Rural Development Administration, Republic of Korea, supported Dr Kunhikrishnan’s contribution.
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