Improving relationship between soil characteristics and metal bioavailability by using reactive fractions of soil parameters in calcareous soils

The contribution of the nature instead of the total content of soil parameters relevant for metal bioavailability in lettuce was tested using a series of low-polluted Mediterranean agricultural calcareous soils offering natural gradients in the content and composition of carbonate, organic and oxide fractions. We compared two datasets by canonical ordination based on redundancy analysis: total concentrations (TC-dataset) of main soil parameters (constituents, phases or elements) involved in metal retention and bioavailability; and chemically-defined reactive fractions of these parameters (RF-dataset). The metal bioavailability patterns were only satisfactorily explained when using the RF-dataset and results showed that the proportion of crystalline Fe oxides, dissolved organic C, diethylene-triamine-pentaacetic-acid (DTPA)-extractable Cu and Zn, and a labile organic pool accounted for 76% of the variance. Additionally, two multipollution scenarios by metal spiking were tested showing better relationships with the RF-dataset than with the TC one (up to 17% more) and new reactive fractions involved. For Mediterranean calcareous soils, we concluded that the use of reactive pools of soil parameters rather than their total contents, improved the relationships between soil constituents and metal bioavailability. Such pool determinations should be systematically included in studies dealing with bioavailability or risk assessment