Carbonate, organic and clay fractions determine metal bioavailability in periurban calcareous agricultural soils in the Mediterranean area

- Soil inorganic constituents govern Cd and Zn bioavailability for lettuce.
- Organic matter plays a dual role in metal bioavailability patterns.
- Labile organic fraction favours Cu and Pb translocation to lettuce leaves.
- Copper and Pb absorption by lettuce roots was limited by labile organic fraction.
- Neutral salts and complexing agents were suitable for simulating metal uptake.

A set of periurban calcareous agricultural Mediterranean soils, with a natural gradient in the content and composition of organic matter (OM), was spiked with a mixture of Cd, Cu, Pb and Zn at two levels - designated Tt1 and Tt2 - within the limit values proposed by the current European legislation and incubated for ≤ 12 months. We conducted bioassay experiments with romaine and iceberg varieties of lettuce (Lactuca sativa L.). Metal bioavailability patterns were evaluated as a function of soil constituents and extractable metals.Metal contamination produced a decrease in plant biomass (up to 50%) and nutrient imbalance. The redundancy analysis showed that the carbonate, OM and fine mineral fractions account for up to 85% of the variance in metal bioavailability patterns in lettuce plants. Carbonate and fine mineral fractions were negatively related to Cd and Zn bioavailability in plants respectively. Organic matter explained Cu and Pb bioavailability, although patterns varied between plant tissues according to their organic composition. The labile fraction limited metal absorption of Pb by roots at the Tt2 level and Cu at both levels, but Cu and Pb translocation was enhanced at both levels. There were significant correlations between metals in leaves and metals extracted with both neutral salts and complexing agents. The high rate of metal translocation and the considerable influence of soil properties may indicate that the limiting values of metals in soils proposed by the current European legislation should be revised in the case of multi-component systems, and/or be established according to the soil characteristics.