Enhancing the effect of biochar ageing on reducing cadmium accumulation in Medicago sativa L

Biochar (BC) application to farmland soil can reduce the mobility and bioavailability of Cd. Nevertheless, BC is prone to natural ageing in soil, which alters its structure, physicochemical properties, thereby affecting the immobilisation of Cd. We used dry-wet and freeze-thaw cycles to mimic the natural ageing of BC, and used adsorption experiments to explore the changes of Cd adsorption capacity of BC and aged BC (ABC). We conducted a pot experiment to investigate the effects of BC and ABC on soil biotic and abiotic factors, alfalfa growth, and Cd accumulation in agricultural soils with high and low Cd concentrations. The increase of specific surface area, pore size, oxygen containing functional groups and mineral composition leads to better adsorption capacity of ABC. The adsorption of Cd(II) by BC and ABC is mainly by monolayer adsorption and chemical adsorption. Applying BC and ABC to Cd-contaminated soil significantly increased the aboveground biomass and decreased the Cd accumulation by reducing the Cd bioconcentration factor in alfalfa. At high Cd levels, adding BC and ABC reduced the Cd content in alfalfa shoots by 32.8 % and 35.1 %, respectively; the fixing effect of ABC was better than that of BC. Adding BC and ABC significantly increased the microbial biomass and geometric mean of enzymes. BC addition increased soil pH by 0.32-0.36 units and cation exchange capacity (CEC) by 15.5 %. Adding BC and ABC significantly increased soil organic matter (SOM) by 5.7 % and 6.2 %, respectively. Random forest analysis showed that SOM, total organic carbon, and fluorescein diacetate hydrolase were important variables for Cd content in alfalfa shoots. Structural equation modelling showed that BC indirectly affected the Cd content in alfalfa shoots by affecting soil pH, CEC, SOM, microbial biomass, and microbial metabolic activity. BC has a long-term effect on alleviating Cd pollution in farmland.