Effect of biochar amendments on the growth and development of ‘Vera’ crisp lettuce in four soils contaminated with cadmium

Cadmium is an extremely toxic heavy metal that affects agricultural lands, concentrations above 0.2 mg kg-1 in leafy vegetables is restricted for human consumption. Biochar is a promising amendment for degraded soils with low fertility and high levels of heavy metals contamination, since it can reduce Cd2+ levels in vegetables. However, these reductions depend on the nature of the soil and biochar properties. This pot experiment in greenhouse conditions evaluated the effect of biochars amendments under Cd stress conditions on lettuce (Lactuca sativa L. var. crispa L.) plants grown in four soils: Ferralsol (FS), Andosol (AS), Umbrisol (US) and Technosol (TS). Six treatments were compared: Biochars doses at 3, 6, 9 and 12 t ha-1, conventional fertilization (NPK) and control absolute. Biochars were obtained by slow pyrolysis from five agricultural residues: Palm empty fruit bunches (PEFBB), palm kernel (PKB), wood waste (WWB), coffee pulp (CPB) and rose stems (RSB). The CPB amendment resulted in a fresh weight plant increase of 238% to 323% in 3 to 12 t ha-1 comparing with control treatment, while WWB induced a fresh weight decrease of 90.85%. The PKB amendment presented a 20% additional fresh weight in comparison to PEFBB in Ferralsol. These results indicate that biochar positively contributes to the water status. Application of RSB at 12 t ha-1 resulted in a decrease of 50.61% in Cd concentration and a decrease of 37.4% in Cd concentration in CPB at a 12 t ha-1, both results representing promising alternatives to remediate contaminated soils. The AS+CPB and US+RSB assays showed a significant negative correlation of leaf Cd concentration when increasing biochar doses were used indicating the mitigation of the phytotoxic effect of Cd in plants.