Lotus roots accumulate heavy metals independently from soil in main production regions of China

The global issue of heavy metal pollution causes concern on lotus root, a popular aquatic vegetable grown in sludge supposedly heavily contaminated. The relevance of heavy metals in soil and lotus roots in production remains unclear. In this work, four heavy metals, lead (Pb), cadmium (Cd), mercury (Hg) and arsenic (As), in lotus roots and ambient soil from main production areas of China were determined. The bioconcentration factor (BCF) of each heavy metal and correlation coefficient among heavy metals in soil and lotus roots were analyzed. Red soil (pH 6-7) in one location exceeded the maximum allowable limit of Cd by 91%, while soils in seven locations met the environmental requirements for producing safe lotus roots, including three red soils (pH 4-5), two moisture soils (pH 4-7), one cinnamon soil (pH 6-7), and one paddy soil (pH 6-7). Lotus roots were individually contaminated with Pb in two locations (red soil and paddy soil), As in two locations (red soil and cinnamon soil), and Hg in one location (red soil), exceeding the maximum allowable limit by 3-16%. Heavy metal levels of lotus roots produced in all investigated areas still met the safety standard as indicated by integrative pollution indexes (IPI). However, the close-to-limit IPI will certainly raise the safety concern for lotus root production. BCF of heavy metals decreased in an order of Cd (5.03-46.67%), Hg (2.42-7.20%), As (1.93-6.68%) and Pb (0.02-1.82%), showing lotus root's ability to accumulate metals varied with metal types. BCF and correlation coefficient showed that the lotus roots tended to accumulate heavy metals less efficiently from soil with higher levels of heavy metals, and vice versa. Heavy metal levels were positively correlated in soil but largely irrelevant in lotus roots. Thus, heavy metals in lotus roots were accumulated primarily independently from or partially negatively correlated with soil, complicating the enrichment mechanism of heavy metals in lotus roots. The possible causes of soil independent accumulation of heavy metals in lotus roots are discussed.