Accumulation, transfer, and potential sources of mercury in the soil-wheat system under field conditions over the Loess Plateau, northwest China

Abstract

There is limited information on accumulation, transfer, and source of mercury in wheats under field conditions over the Loess Plateau, northwest China. The present study collected 26 pairs of topsoil and whole wheat samples (roots, stems, leaves, shells, and grains) from Dongdagou stream watershed and upper Xidagou stream watershed, Baiyin City, northwest China. Hg concentrations from these samples were used to identify their relationships with soil properties, interactions with other metals, localization of Hg in the different wheat tissues, bio-concentration and transfer of Hg, and major sources of Hg in wheat. Results show that Hg levels in 11 out of 26 sampled soils (42.3% of soil samples) exceeded Hg limit of grade II soil environmental quality standards in China (1.0mg·kg−1). Likewise, it was also found that Hg in over 50% of wheat grain samples reached or exceeded the maximum permissible food safety levels (0.02mg·kg−1) according to the General Standard of Contaminants in Food in China (GB 2762-2012). The spatial distribution pattern of Hg in wheats grains was different from that in the sampled soils. Hg concentrations in different wheat tissues were highest in roots, followed by leaves, stalks, shells, and grains, respectively. Bio-concentration factors (BCF) of Hg in almost all grains samples were one or two orders of magnitude lower than that in roots, except for two wheat samples. The translocation factors (TF) of Hg in wheat tissues on average were leaves>stems>shells>grains. The spatial distribution of Hg and its correlation with other heavy metal detected simultaneously in the soil samples suggested that the Hg soil contamination was probably caused by past sewage irrigation practices and atmospheric deposition. Correlation analysis revealed that the principle source of Hg in wheat roots was very likely from Hg contaminated soils.