Rhizospheric pore-water content predicts the biochar-attenuated accumulation, translocation, and toxicity of cadmium to lettuce.

Metal bioavailability controls its behaviors in soil-plant system, especially involved in biochar amendment. This study compared a rhizospheric pore-water extraction against a BCR sequential extraction method to understand cadmium (Cd) bioavailability in two typical Chinese soils. Soils were spiked with five levels of Cd (CdCl) and remediated with 3% corn-straw derived biochar. After 60 days of lettuce growth, Cd accumulation and enzyme activities in tissues were analyzed. Results showed that biochar increased soil properties (pH, CEC and SOM) compared to un-amended soils, but decreased contents of bioavailable Cd in soil pore-water (Cd) and BCR extracted Cd (Cd). Contents of Cd were lower in yellow-brown soils than that in red soils. Pearson analysis showed that bioavailable Cd is negatively correlated with soil pH and CEC (p < 0.05). Cd accumulation in lettuce roots and leaves both were decreased by biochar addition, and the established linear equations proved that soil Cd is the best predictor for Cd accumulation in lettuce roots (r = 0.964) and in leaves (r = 0.953), followed by Cd. Transfer factor (TF) values of Cd from roots to leaves were lower than 1, and slightly better correlated with soil Cd (r = -0.674, p < 0.01) than Cd (r = -0.615, p < 0.01). Aggregated boosted tree (ABT) analyses indicated that soil properties together with Cd contribute more than 50% to root enzyme activities. Collectively, soil Cd is a promising predictor of Cd bioavailability, accumulation and toxicity in soil-plant system with biochar addition.