Rice is one of the most important staple foods worldwide. Soil contamination with heavy metals and food safety problems occur in many countries as a result of numerous human activities, particularly wastewater and solid waste disposal. This review paper provides a schematic summary of heavy metals in identification processes, transport in soil to different rice varieties, and soil remediation strategies and techniques surrounding the agroenvironmental impact in paddy soils based on a description of Taiwan's experiences and database. In terms of the soil control standard, heavy metals including As, Cd, Cr, Cu, Hg, Ni, Pb and Zn are regulated by the Soil and Ground Water Pollution Remediation Act of Taiwan. Owing to the heavy metal source from wastewater along irrigation systems, heavy metals not only accumulate in the surface soil (0-30 cm), but are also highly distributed at the main entrance of irrigation water into individual paddy fields. Moreover, sediments in the irrigation canal have to be dredged and the irrigation system needs to be isolated from the discharge system of wastewater to maintain soil quality. Cadmium in rice grains accumulates more significantly in Indica varieties than in Japonica varieties, and this accumulation exceeds the food quality standard. The best well-performing metal uptake models have been developed to predict Cd levels in rice grains for Indica and Japonica varieties using soil bioavailable Cd and Zn concentrations extracted by 0.01 mol L )1 CaCl 2 . Soil remediation techniques, including turnover and dilution, in situ stabilization by chemical amendments and phytoremediation, have been tested and recommended in Taiwan. Although the high background levels of As, Cr and Ni, which were higher the soil control standard in some paddy soils, are derived from andesite and serpentinites in Taiwan, rice quality and yield were not adversely affected by these metals when labile concentrations were very low. Overall, it is necessary to identify the bioavailability of heavy metals in different soil types from specific case studies to provide reliable parameters for health-based risk assessments and to further achieve the goal of food safety and sustainable agriculture.

TaiwanChelate-induced phytoextraction is an innovative technique for cleaning metalcontaminated soil. The present study evaluates the degree of metal mobilization in soil and enhancement of phytoextraction of cadmium (Cd), lead (Pb), and zinc (Zn) by Sesbania sesban (L.) Merr. from artificially contaminated soil by application of ethylenediaminetetraacetic acid (EDTA). After 30 days of plant growth, the pots were divided into three sets (0.0, 2.5, and 5.0 mmol EDTA per kg soil). Experimental results indicated that levels of diethylenetriaminepentaacetic acid (DTPA)-extractable metals and metals in the leachate decreased as the EDTA dose increased. Plant growth parameters and total chlorophyll contents in the plants with EDTA applied were less than those of control. However, EDTA application significantly reduced metal accumulation in root and increased metal accumulation in the shoot of plants; similar results were obtained for the bioconcentration factor and translocation factor. The application of 5 mmol EDTA kg -1 to metal-spiked soil may be an efficient alternative for the chemically enhanced phytoextraction by S. sesban.

Arsenic contamination in a large area of agricultural fields on the Guandu Plain of northern Taiwan was confirmed in a survey conducted in 2006, but research concerning the relationship between bioavailable As concentrations in contaminated soils and crop production in Taiwan is not available. Pot experiments were conducted to examine the growth and accumulation of As in four vegetable crops grown in As-contaminated soils and to assess As intake through consumption. The phytotoxic effects of As in soils were not shown in the pot experiments in which vegetable crops were grown in soils contaminated with different As levels in situ collected from Guandu Plain (120–460 mg/kg) or artificially spiked As-contaminated soils (50–170 mg/kg). Experimental results showed that the bioavailable As extracted with 5 M NaHCO3 from soils can be used to estimate As concentrations in vegetables. The As concentrations in the vegetables were compared with data shown in the literature and As limits calculated from drinking water standards and the provisional tolerance weekly intake (PTWI) of inorganic As established by the Food and Agriculture Organization of the United Nations/World Health Organization (FAO/WHO). Although the As levels in the vegetables were not high and the bioavailability of As in the soils was quite low, long-term consumption may result in higher As intake in the human body.

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Abstract. Cu and Zn are frequently added to livestock diets as additives to increase feed efficiency and production. This practice resulted in the higher contents of Cu and Zn in excrement of livestock. The aim of this study is to evaluate the effect of Cu and Zn concentration of manure compost and its application rates on the production and quality of pakchoi and rice. The pot experiments were conducted and the six manure compost were applied at 3 rates (20, 40, and 80 ton/ha), including the control and chemical fertilizer treatments. Results showed that the yield of the crops was enhanced by the compost application, and the Cu and Zn concentration in the edible part of crops were in normal range (pakchoi: Cu 1.8-10.4 mg/kg, Zn 39-160 mg/kg; rice grain: Cu 0.6-4.0 mg/kg, Zn 58-79 mg/kg). The potential risk of long-term manure compost application on soil quality was also evaluated. The total Zn concentration in soils may reach the regulation standard after 22 years of manure compost application at the rate of 40 ton/ha/year.