Aged biochar changed copper availability and distribution among soil fractions and influenced corn seed germination in a copper-contaminated soil

Biochar has been recommended as a multi-beneficial amendment for the in situ remediation of heavy metals contaminated soils due to its high recalcitrance, stability, specific surface area and retention capacity, which leads to a long-lasting influence on the immobilization of soil contaminants. The influence of biochar on the availability of heavy metals such as copper is not fully understood and may be related to a change in copper association with soils fractions. Therefore, a long-time laboratory incubation study was set up as a completely randomized design to test the effect of biochar from different sources (coconut husks-CHB, orange bagasse-OBB and sewage sludge-SSB) at two rates of application (30 and 60 t ha−1) on the distribution of copper in a copper-contaminated soil after 24 months incubation. Copper distribution was evaluated through a sequential extraction procedure that fractionated copper into five fractions: F1 (soluble and exchangeable), F2 (specifically bound), F3 (organic matter bound), F4 (Fe and Mn oxide bound) and F5 (residual). Copper availability, soil pH and organic matter were also evaluated. Corn seeds were germinated in the incubated biochar soil to investigate the effect of biochar on seed germination and plantlets characteristics. All biochars increased soil pH and the concentration of oxidizable organic matter, and reduced copper availability after the 24 months incubation. CHB caused a discrete influence on copper distribution among soil fractions. OBB30 increased F1 (54.5%), F3 (24.0%), F4 (32.2%) and F5 (64.1%), and reduced F2 (39.8%); OBB60 reduced F1 (61.8%), F2 (16.5%) and F3 (16.0%) and increased F4 (18.0%) and F5 (84.4%). SSB30 strongly reduced Cu concentration in F1 (96.2%), F2 (34.0%), and F3 (22.2%), and increased F4 (54.4%); SSB60 reduced F1 (57.5%) and F3 (59.4%). Considering the high stability of biochar, the association of copper to the organic fraction leads to a long-time reduction in copper availability in the contaminated soil, which can reduce the cost and increase the efficiency of the remediation process. SSB reduced seed germination but produced vigorous and well-developed plantlets. Therefore, with proper production procedure to reduce the volatile matter content, SSB may not interfere with seed germination and has the greatest potential to be used for the remediation of copper-contaminated sites. •Biochar reduced the most available fractions of Cu in the contaminated soil.•Copper associated with orga