In two long-term field experiments the zinc (Zn)/cadmium (Cd) hyperaccumulator Sedum plumbizincicola (S. plumbizincicola) was examined to optimize the phytoextraction of metal contaminated soil by two agronomic strategies of intercropping with maize (Zea mays) and plant Downloaded by [University of Nebraska, Lincoln] at 02:30 23 October 2015 ACCEPTED MANUSCRIPT ACCEPTED MANUSCRIPT 2 densities. Soil total Zn and Cd concentrations decreased markedly after long-term phytoextraction. But shoot

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... ss and Cd and Zn concentrations showed no significant difference with increasing remediation time. In the intercropping experiment the phytoremediation efficiency in the treatment "S. plumbizincicola intercropped with maize" was higher than in S. plumbizincicola monocropping, and Cd concentrations of corn were below the maximum national limit. In the plant density experiment the phytoremediation efficiency increased with increasing plant density and 440,000 plants ha -1 gave the maximum rate. These results indicated that S. plumbizincicola at an appropriate planting density and intercropped with maize can achieve high remediation efficiency to contaminated soil without affecting the cereal crop productivity. This cropping system combines adequate agricultural production with soil heavy metal phytoextraction. Key words heavy metal, phytoremediation, long term, intercropping, plant densities Downloaded by [University of Nebraska, Lincoln] at 02:30 23 October 2015 ACCEPTED MANUSCRIPT ACCEPTED MANUSCRIPT 3 Koopmans et al. 2008). Sedum plumbizincicola (S. plumbizincicola) also has a remarkable capacity to extract zinc (Zn) and Cd from polluted soils, and pot experiments have indicated the potential of S. plumbizincicola for Zn and Cd phytoextraction (Wu et al. 2006; Jiang et al. 2010). Although S. plumbizincicola shows high ability to accumulate Zn and Cd, phytoextraction is also affected by the availability of heavy metals in the soil. Numerous factors can affect the remediation efficiency of S. plumbizincicola and systematic field experiments are required to study Downloaded by [University of Nebraska, Lincoln] at 02:30 23 October 2015 ACCEPTED MANUSCRIPT ACCEPTED MANUSCRIPT 4 agronomic measures for the optimisation of plant yield and of heavy metal accumulation for efficient extraction of metals from soils (Zhuang et al. 2009). Agronomic measures include fertilizer application, control of planting density, harvesting methods, and intercropping systems that can influence the phytoavailability of potentially toxic elements in soils (Huang et al. 2012). The planting of a hyperaccumulator species together with an agricultural crop species (co-cropping) may be a useful option (Wu et al. 2007). Maize (Zea mays) is an important agricultural crop worldwide with high yielding potential and is thus of interest in intercropping systems for soil metal phytoremediation (Wojcik and Tukiendorf 2005; Wei et al. 2011). Most of the published studies on S. plumbizincicola phytoextraction have been based on short-term remediation in-situ field phytoremediation or on pot experiments (Jiang et al. 2010; Wu et al. 2012; Li et al. 2014a; Li et al. 2014b). More long-term field experiments are thus required to support agricultural technology and extension work. In the present study two in-situ field plot experiments investigated intercropping with maize and sorghum versus monocropping and the role of planting density on heavy metal phytoextraction. Changes in soil heavy metals after repeated phytoextraction and the relationship between metals in the soil and metal uptake by the hyperaccumulator S. plumbizincicola were studied. The aim was to determine a suitable planting density of S. plumbizincicola for safe agricultural production of maize and sorghum and simultaneous phytoextraction of heavy metals from a contaminated soil. 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