Current status of agricultural soil pollution by heavy metals in China: A meta-analysis

In the last decades, agricultural soil pollution by heavy metals has been extensively investigated in China. However, nearly all studies were field monitoring in small regions and/or with limited samples, which may not represent soil pollution situation at the national scale. In this paper, attempt was made to provide a comprehensive report about heavy metal pollution in China based on meta-analysis of reviewed data. Given the characteristics of field monitoring studies, the weighted mean values based on "sampling number", "study area", and "standard deviation" were calculated to represent national mean values. In addition, subgroup analysis and cumulative meta-analysis were applied to explore the spatial and temporal variations as well as the influence of cropping systems. 336 articles published from 2005 to 2017 were reviewed in the analysis. Eight heavy metals (cadmium (Cd), chromium (Cr), mercury (Hg), lead (Pb), arsenic (As), copper (Cu), zinc (Zn) and nickel (Ni)) were analyzed. The contents of Cd and Hg were increased compared to background values, while, other six elements showed no significant accumulation. Little pollution was found in normal farmland, which was far from obvious anthropogenic emissions, but Cd and Hg in mining & smelting areas and industrial areas continued to accumulate significantly. Moreover, the accumulation had slowed down or decreased since 2012, which might be due to reduced use of coals, non-ferrous metals and agro-chemicals. Heavy metal contents were generally higher in southwest and south coastal areas but lower in northwest regions, whereas vegetable and paddy fields had higher concentrations than upland and other land use. This study provides information on soil pollution caused by heavy metals and its affected regions and cropping systems on a national scale. It can be useful for developing heavy metal pollution control and management strategies in China.