Rice straw- and rapeseed residue-derived biochars affect the geochemical fractions and phytoavailability of Cu and Pb to maize in a contaminated soil under different moisture content

Management of toxic elements contaminated upland and wetland soils using biochar is of great concern from both agricultural and environmental points of view. The impact of rice straw- and rapeseed residue-derived biochars produced under 300 °C and 550 °C (added to the soil at 2% and 5%; w/w) on the geochemical fractions, phytoavailability, and uptake of Cu and Pb in a contaminated mining soil under different moisture contents (80%, 60%, and 40% of soil field capacity) was investigated in a greenhouse pot experiment using maize. The higher rate of rice straw-derived biochar pyrolyzed at 550 °C caused a significant reduction in the mobile (soluble + exchangeable) fraction of Cu (59.42%) and Pb (75.4%) and increased the residual fractions of Cu (37.8%) and Pb (54.7%) in the treated soil under the highest moisture content (80%) as compared to the untreated soil. Therefore, this biochar significantly decreased the phytoavailability (CaCl₂-extractable form) of Cu by 59.5% and Pb by 67.6% under the highest moisture content. Also, at the same moisture level (80%), the higher rate of rapeseed residue-derived biochar pyrolyzed at 550 °C decreased significantly the phytoavailability of Cu by 46.5% and Pb by 60.52% as compared to the untreated soil. The 5% rate of the higher temperature pyrolyzed rice straw and rapeseed biochars decreased the uptake of Cu and Pb by the roots and shoots of maize up to 51% for Cu and 45% for Pb. Immobilization of Cu and Pb in the biochar-treated soil at 80% moisture content may possibly due to the associated increase of soil pH and poorly-crystalline Fe oxides content, and/or the metals precipitation with sulfides. These results indicated that application of high temperature pyrolyzed rice straw- and rapeseed residue-derived biochars at 5% could immobilize Cu and Pb and decrease their uptake by maize under high levels of moisture content; consequently, they can be used for phyto-management of Cu and Pb contaminated wetland soils.