Use of spinach as an indicator of trace metal bioavailability: Are contamination limits valid for plants and soil?

Soil trace metal (TM) contamination is a worldwide issue affecting food safety and human health. In Europe, a regulation sets maximum levels (MLs) of TM in food to protect consumers. Similarly, European countries or regions apply soil action values (SAVs) for TM above which croplands are considered contaminated (i.e. unsafe for food production). These SAVs differ from one country/region to another, in terms of the values themselves and/or the extraction methods used to measure soil TM concentrations. We grew spinach plants on 36 substrates with previous anthropogenic TM contamination and evaluated (1) the adequacy of soil TM limits implemented across nine European countries/regions in predicting whether spinach TM concentrations exceed MLs, and (2) the predictive effectiveness of eight chemical extractants (CaCl₂, NaNO₃, NH₄NO₃, DTPA, AcEDTA, two concentrations of HNO₃ and aqua regia) in estimating soil TM concentrations relevant to spinach concentrations. Our results reveal significant shortcomings in certain European soil limits (i.e. SAVs) applied to Belgian soils in ensuring the compliance with food MLs. Specifically, we demonstrate that the extractants commonly used to establish SAVs are generally poor predictors of spinach TM concentrations. For all TM combined, we identified chelating-saline extractants (DTPA and AcEDTA) as the most reliable compromise for predicting plant TM concentrations. Based on these results, we propose a revision of SAVs for each extractant used currently in Europe, as well as new SAVs tailored to the optimal extractants identified. We believe these findings are critical for enhancing soil regulations and protecting public health.