Environmental Factors Influence the Effects of Biochar on the Bioavailability of Cd and Pb in Soil Under Flooding Condition

Biochar, as a sustainable amendment, effectively remediates soils contaminated with potentially toxic metals. However, the immobilization efficiencies of biochar can vary according to the soil properties. To investigate the critical impact factor of soil properties on the immobilization of cadmium (Cd) and lead (Pb) in soil by biochar, this study was conducted with an incubation batch experiment and a pot experiment by using an acid soil sample (soil A), a weakly acidic soil sample (soil B), and an alkaline soil sample (soil C). The results showed that the CaCl2-extracted Cd in the three soil samples was reduced by 15.2% (soil A), 44.3% (soil B), and 22.0% (soil C) with biochar application, and the decrease rate of the available Cd concentration is significantly negatively (P < 0.05) related with soil pH, cation exchange capacity, and the proportion of illite and illite-smectite mixed layers of clay minerals. Biochar significantly reduced the soil available Pb concentration of soil A, soil B, and soil C by 57.2%, 52.3%, and 40.3%, respectively. And the decreased rate of available Pb concentration is significantly negatively related to soil pH and soil organic materials (P < 0.05). The Cd concentration of rice shoots is positively related to the amount of iron plaque. Biochar application decreased the formation of iron plaque on rice roots grown in soils A and B due to the biochar slowing down the decreasing trend of the redox potential during flooding. However, biochar increased the amount of iron plaque on rice roots grown in soil C, which had a higher pH. As a result, biochar reduced the accumulation of Cd and Pb in rice shoots and promoted the biomass of rice grown in soils A and B (acid soils) but had the opposite effect on soil C (an alkaline soil).

Automated summary

Biochar is an effective amendment for remediating soils contaminated with potentially toxic metals, although its immobilization efficiencies may vary depending on soil properties. This study investigated the impact of soil properties on the immobilization of cadmium (Cd) and lead (Pb) by biochar. The results showed that biochar reduced the CaCl2-extracted Cd concentration in all soil samples, with the decrease rate significantly related to soil pH, cation exchange capacity, and clay mineral composition. Similarly, biochar significantly decreased the soil available Pb concentration, and the decrease rate was related to soil pH and organic materials. Additionally, the formation of iron plaque on rice roots was influenced by biochar, reducing it in acid soils but increasing it in alkaline soil. This had a corresponding effect on Cd and Pb accumulation in rice shoots and biomass.