Biochar has been recognised as an efficacious amendment for the remediation of compound heavy metal contamination in soil. However, the molecular mechanism of biochar-mediated tolerance to compound heavy metal toxicity in cotton is unknown. The objective of this research was to investigate the positive impact of biochar (10 g.kg(-1)) on reducing damage caused by compound heavy metals (Cd, Pb, and As) in cotton ( Gos- sypium hirsutum L.). The results revealed that biochar reduced Cd concentrations by 24.9 % (roots), and decreased Pb concentrations by 37.1 % (roots) and 59.53 % (stems). Biochar maintained ionic homoeostasis by regulating the expression of metal transporter proteins such as ABC, HIPP, NRAMP3, PCR, and ZIP, and genes related to the carbon skeleton and plasma membrane. Biochar also downregulated genes related to photosynthesis, thereby increasing photosynthesis. Biochar re-established redox homoeostasis in cotton by activating signal transduction, which regulated the activity of the enzymes POD, SOD, and CAT activity; and the expression of related genes. This research revealed the molecular mechanism by which biochar confers resistance to the harmful effects of compound heavy metal toxicity in cotton. The application of biochar as a soil amendment to neutralise the toxicity of compound heavy metals is recommended for cash crop production.

Biochar contains biotoxic aromatic compounds, and their influence on nitrogen-fixing cyanobacteria, the critical nitrogen fixer in paddy soil, has never been tested. Here, the physiological, metabolomic, and transcriptomic analyses of Nostoc sp. PCC7120 in response to biochar leachate were performed. The results suggested that biochar leachate inhibited the efficiency of photosynthesis, nitrogen fixation, and nitrate assimilation activities of nitrogen-fixing cyanobacteria. Biochar leachate containing aromatic compounds and odd- and long-chain saturated fatty acids impaired the membrane structure and antenna pigments, damaged the D1 protein of the oxygen evolution complex, and eventually decreased the electron transfer chain activity of photosystem II. Moreover, the nitrogen fixation and nitrate assimilation abilities of nitrogen-fixing cyanobacteria were inhibited Contents lists available at ScienceDirect Science of the Total Environment