Dose-dependent Effect of Biochar as Soil Amendment on Reducing Copper Phytotoxicity and Mobility

Using biochar to remediate metal-contaminated agricultural soil has attracted increasing attention due to it is relatively low cost and high efficiency. In this study chemical speciation, greenhouse experiments, and column leaching study were conducted to understand the mechanisms of copper (Cu) immobilization by pine needle biochar (PNB) and sweet potato residue biochar (SPRB) to evaluate the effect of biochar amendment rate for remediation Cu contaminated soils. The results showed that both PNB and SPRB amendment effectively raised soil pH and converted Cu into more stable chemical forms. 1% (W/W) SPRB was adequate for reducing exchangeable Cu fractions from 43.3 to 9.9 mg kg−1 in the contaminated soils. 1% (W/W) SPRB and 5% (W/W) PNB treatments show the best result to promote sweet potato growth. Over liming caused by 5% (W/W) SPRB resulted in inhibition of plant growth. 5% (W/W) PNB treatment also shows the best result for reducing Cu mobility: the cumulative amount of Cu in the leachate after 9 leaching events was reduced from 7.0 to 1.0 mg kg−1. However, 5% (W/W) SPRB amendment is a little effective for reducing Cu mobility. Although the biochar amendment can provide additional sorption sites for metal pollution remediation, soil pH after an amendment is also a critical factor to estimate optimal biochar application dosage for promoting plant growth and reduce metal leachability.