Use of a Rhizosphere-Based Method for the Assessment of Heavy-Metal Bioavailability in Soils Amended with Polluted Sewage Sludge

A rhizosphere-based method (a low-molecular-weight organic acid solution) was evaluated for the assessment of cadmium (Cd), chromium (Cr), and lead (Pb) bioavailability to barley (Hordeum vulgare) roots from rhizosphere soils (n = 36) following a 15.71 g dry weight kg(-1) application of a metal-spiked sewage sludge under greenhouse conditions. Statistically significant correlation coefficients were found: r = 0.758, P < 0.001 for Cd, r = 0.762, P < 0.001 for Cr, and r = 0.723, P < 0.001 for Pb. The correlations were greater in acidic soils (n = 6; r = 0.983, P < 0.001 for Cd, r = 0.888, P < 0.01 for Cr, and r = 0.898, P < 0.01 for Pb). This extraction failed to assess heavy-metal bioavailability in basic soils (n = 30; r = 0.111, P > 0.05 for Cd, r = 0.002, P > 0.05 for Cr, and r = 0.037, P > 0.05 for Pb). The overall predictability was greatly improved when soil properties were considered (n = 36; r(2) = 0.730, P < 0.001 for Cd, r(2) = 0.800, P < 0.001 for Cr, and r(2) = 0.719, P < 0.001 for Pb), and texture was observed in all the prediction models.