Heavy metals and metalloids concentrations across UK urban horticultural soils and the factors influencing their bioavailability to food crops

Urban horticulture (UH) has been proposed as a solution to increase urban sustainability, but the potential risks to human health due to potentially elevated soil heavy metals and metalloids (HM) concentrations represent a major constraint for UH expansion. Here we provide the first UK-wide assessment of soil HM concentrations (total and bioavailable) in UH soils and the factors influencing their bioavailability to crops. Soils from 200 allotments across ten cities in the UK were collected and analysed for HM concentrations, black carbon (BC) and organic carbon (OC) concentrations, pH and texture. We found that although HM are widespread across UK UH soils, most concentrations fell below the respective UK soil screening values (C4SLs): 99 % Cr; 98 % As, Cd, Ni; 95 % Cu; 52 % Zn. However, 83 % of Pb concentrations exceeded C4SL, but only 3.5 % were above Pb national background concentration of 820 mg kg⁻¹. The bioavailable HM concentrations represent a small fraction (0.01–1.8 %) of the total concentrations even for those soils that exceeded C4SLs. There was a significant positive relationship between both total and bioavailable HM and soil BC and OC concentrations. This suggest that while contributing to the accumulation of HM concentrations in UH soils, BC and OC may also provide a biding surface for the bioavailable HM concentrations contributing to their immobilisation. These findings have implications for both management of the risk to human health associated with UH growing in urban soils and with management of UH soil. There is a clear need to understand the mechanisms driving soil-to-crop HM transfer in UH to improve potentially restrictive C4SL (e.g. Pb) especially as public demand for UH land is growing. In addition, the UH community would benefit from education programs promoting soil management practices that reduce the risk of HM exposure - particularly in those plots where C4SLs were exceeded.