Effects of Invasive Solidago canadensis and Biochar on the Remediation of Soil Cd Contamination and Greenhouse Gas Emissions.

Cadmium (Cd) contamination in agricultural soils threatens food security and exacerbates climate change through its impact on greenhouse gas (GHG) (CO2, N2O and CH4) emissions, in which N2O and CO2 are the dominant fluxes of the terrestrial carbon-nitrogen cycle whose magnitude is directly amplified by Cd stress. Key remediation approaches for this dual challenge are phytoremediation and biochar amendment. This study aims to investigate the effects of Solidago canadensis (CGR) and biochar (BC) on soil remediation and GHG emissions under different levels of Cd contamination. A pot experiment with four Cd concentration gradients (0, 5, 10, and 30 mg kg-1, i.e., Cd-0, Cd-5, Cd-10, and Cd-30, respectively) and three remediation measures (control, BC addition, and CGR cultivation) was set up to measure available soil Cd (ACd), soil physicochemical properties, GHG emissions, and plant Cd accumulations. The results demonstrated that ACd was significantly reduced by BC via adsorption through surface complexation and by CGR via immobilization through root uptake and sequestration. CGR decreased ACd by 46.2% and 41.7% under mild and moderate Cd contamination, respectively, while BC reduced ACd by 8.9% under severe contamination. In terms of GHG emissions, CGR increased cumulative CO2 by 83.4% in Cd-10 soil and 53.8% in Cd-30 soil, whereas BC significantly lowered N2O emissions by 22.1% in Cd-5 soil. Mantel analysis revealed strong correlations between ACd and key carbon and nitrogen indicators, which mediate the bioavailability of Cd. Therefore, CGR cultivation is better suited to mild-to-moderate contamination given its high removal efficiency, while BC amendment is targeted at severe contamination by stabilizing Cd and mitigating N2O. This provides a scientific basis for the remediation of Cd-contaminated soils.