Sample collection and analysis
Plant and soil samples were collected from three fixed standard blocks (approximately 667 m2 each) in each plots in June and October each year from 2009 to 2016 and analyzed following standard procedures (GB15618-1995; GB3838-2002; GB18918-2002, People’s Republic of China). The numbers of individual plant and plant species were counted on-site. The relative standard deviations in duplicate sample analyses ranged between 8.4%-16.4% in biomass measurement and 5.1%-18.3% in metal uptake determination. The average values of the measurements were used for further calculation. The differences in related values between treatment plots were compared with LSD analysis of variance.