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Abstract : Soil characteristics and soil management affecting the metal bioavailability and food safety of heavy metal-contaminated soils in Taiwan
( Zueng Sang Chen )
UCI I410-ECN-0102-2015-500-000630384

The heavy metal regulation (digested by aqua regia) of agricultural soils in Taiwan are listed as (mg/kg): As 60, Cd 5, Cr 250, Cu 200, Hg 5, Ni 200, Pb 500, and Zn 600. The upper levels of background total concentration of heavy metals in surface soil (0-15 cm) in Taiwan are listed as (mg/kg): As 18, Cd 2, Cr 50, Cu 35, Hg 0.4, Pb 50, Ni 50, and Zn 120 (Taiwan EPA, 2012) http:// sgw.epa.gov.tw/public/En/Default.aspx. The heavy metal regulations in vegetables in Taiwan are listed as (mg/kg, fresh weight basis): Cd 0.2 (leaf vegetables), Cd 0.1 (root and stem vegetables), Cd 0.05 (fruit vegetables), Pb 0.3 (leaf vegetables), Pb 0.3 (root and stem vegetables) and Pb 0.1 (fruit vegetables). The mean water contents in rice is 13% and in vegetables is 90%. http:// dohlaw.doh.gov.tw/Chi/FLAW/FLAWDAT0202.asp About 472 ha of rural soils in Taiwan were contaminated by the discharged wastewater from industrial parks, especially in the western regions of Taiwan, and 126 ha of rural soils are still contaminated now. The metals pollutants, major on Cd, Cu, Zn, Pb, Cr, Ni, were absorbed in the surface soils of 0-30 cm depth of rural soils. The rice was contaminated by the rural soils and affected the food safety through food chain process. The main factors affecting the metals uptake by crop are the concentrations and speciation of the metal in the soil solution released from different soil characteristics, and its translocation from root to the shoot and grain. Soil key factors controlling metal soil bioavailability are soil pH, sorption of metals on the particle surfaces (clay or oxides minerals) and soil redox potential depending on the water management by flooding or drainage. Applying lime materials to increase soil pH and to reduce the metal solubility into soil solution can significantly reduce the metals uptake by rice or vegetables in the metals-contaminated soils. The occurrence of Cd-polluted rice usually depends on soil properties, especially on soil pH, the contents of clay and iron oxides, and water management. There is no clear relationship between Cd in soil and Cd in brown rice in Japan, Korea and Taiwan, however, the Cd concentration of brown rice grown in contaminated soils is much higher than that in the uncontaminated soil. For different rice species, the Indica rice varieties significantly absorbed more Cd than Japonica rice varieties grown in slight or serious Cd-contaminated sites in Taiwan. The relations between copper (or zinc) concentration of brown rice and soil bioavailability of copper (or zinc) extracted by 0.1M HCl in contaminated soils meet the “plateau theory”. There is no significant increase on Cu (or Zn) in the brown rice grown on the contaminated sites, but the rice productivity is significantly reduced at least 20-50%. The arsenic bioavailability of Gang-Du Plain soils near Taipei, total As content in 63 ha of rural soils ranged 60-545 mg/ kg, are significantly controlled by the amounts of amorphous Fe and Al oxides of volcanic soil characteristics. The As concentration of brown rice ranged 0.15-0.35 mg/kg and there are no any toxic symptom and productivity problem at this site. Arsenic concentration in vegetable can be predicted by extraction method with 0.5 M sodium bicarbonate. In conclusion, the amounts of the metals of the crops are affected by soil properties, metal adsorption ability, and rice or vegetables species. Soil amendments significantly reduce the Cd uptake of brown rice, especially applied by lime materials. The concentrations of As, Cd and Zn in the brown rice are relative higher than those of other countries. For different rice species, the Indica rice varieties significantly absorbed more Cd than Japonica rice varieties in slight or serious Cd-contaminated sites in Taiwan. The metal concentration (As, Cd, Cu and Zn) of brown rice and vegetables can be mostly predicted by soil bioavailability extracted with different extraction solution.

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