Apparent reduction in recoverable cadmium in agricultural soils exposed to Moringa oleifera Lam. during controlled exposure periods

Abstract

Cadmium contamination of agricultural soils calls for remediation strategies that reduce the recoverable metal fraction without inhibiting plant growth. In this study, we evaluated the variation in recoverable soil Cd during exposure to Moringa oleifera Lam. seedlings in accordance with three field-derived initial concentration classes, 1.6, 1.8 and 2.0 mg kg^-1, and under three exposure periods, 8, 17 and 39 days. A 3 × 3 factorial arrangement was implemented in a randomized complete block design with five blocks and 45 independent pots. Recoverable Cd was quantified via EPA Method 3050B and digested by ICP-OES using EPA Method 6010D, and seedling morphology was characterized at each destructive harvest. Final Cd was found to have decreased with time in all the soil classes. The 1.8 mg kg^-1 class exposed for 39 days produced the lowest residual concentration, 0.30 ± 0.16 mg kg^-1, and the greatest apparent reduction, 83.33 ± 8.78%. Initial concentration class, exposure time and their interaction affected final Cd concentration, with p < 0.001, p < 0.001 and p = 0.002, respectively. The full block model explained 89.21% of the variation. The proportion of units at or below the Peruvian agricultural-soil reference value of 1.4 mg kg^-1 increased from 26.67% at 8 days to 100% at 39 days. The 1.8 mg kg^-1 and 39-day treatment also recorded the highest descriptive plant-growth means. Because unplanted controls, tissue Cd concentrations and a total mass balance were unavailable, the decrease can be interpreted as an apparent reduction in recoverable soil Cd, rather than as direct evidence of phytoextraction.