Co-application of sewage sludge (SS) with biochar in landscape/forestry soil is a common strategy for enhancing soil fertility and reducing the bioavailability of potential toxic elements (PTEs) derived from SS, such as Cd, Pb, Cu, Zn, and Ni. However, due to variability of biochar quality and uncertainties in responses of different plant species, whether the co-application benefits the landscape/forestry plant system remains elusive. Here, we tested the effectiveness of three types of biochar (SS-derived biochar (SB), rice straw-derived biochar (RB), and litter-derived biochar (LB)), which were added to soil amended with SS at 50% (w/w) at rates of 1.5%, 3%, and 4.5% as growth media for the landscape plant Aglaonema modestum (A. modestum). We analyzed the substrate’s physicochemical properties and assessed the alleviation of phytotoxicity by biochar application. A significant increase in the fertility index of substrate was observed in all the treatments with biochar addition. The addition of biochar reduced the potential mobility of PTEs while increasing their residual fraction in media. Nonetheless, it has been found that the addition of biochar has ineffective or even negative effects on A. modestum growth (height, biomass, root length) and nutrient absorption. Importantly, the reduction in root biomass and the increased activity of root antioxidant enzymes (SOD, POD, CAT, and MDA) indicate contamination stress of biochar on the roots of A. modestum. Toxic elements of concern—namely Cu, Cd, and Pb—were not significantly higher in tissues of A. modestum saplings planted in biochar-SS-amended soil. However, elevated levels of other elements that may pose toxicity concerns, such as Ni and Zn, increased in tissues at high biochar dosages. Based on the Entropy–Weight TOPSIS method, it was further confirmed that compared to the treatment without biochar, all treatments except for 3.0% LB application resulted in poorer A. modestum comprehensive growth. Our results emphasize the need for detailed research on the response of specific plants to biochar in specific environments, including plant adaptability and the unexplored toxicity of biochar, to understand the large variations and mechanisms behind these ineffective or negative effects before the large-scale co-utilization of SS and biochar in landscape/forestry soils.

Aims Restoring soil structure through stable aggregate formation is essential for reclaiming soils on abandoned rare earth element (REE) mine sites. Sludge amendment may improve soil aggregation, but its use is constrained by potential heavy metal (HM) risks. This study examined whether plant–sludge co-application can balance soil structure improvement with HM risk mitigation. Methods A pot experiment was conducted to evaluate the effects of sludge amendment and seven plant configurations on soil aggregation, aggregate-associated binding agents, functional groups, enzyme activities and HM availability in a REE mine soil. Results Sludge amendment and plant cultivation jointly promoted macroaggregate formation and aggregate stability. Among all treatments, sludge combined with Heptapleurum arboricola (S-H) was most effective, increasing water-stable macroaggregates by 30.72% and reducing aggregate destruction by 82.70% compared with the unamended control. High-aggregation treatments showed greater accumulation of soil organic matter, polysaccharides and glomalin-related soil protein, together with enrichment of O–H functional groups. Mantel tests and random forest analysis indicated that carbon fractions were the dominant drivers of aggregate stability, followed by organic binding agents and enzyme activities. Although sludge amendment alone increased total and DTPA-extractable Cu, Zn and Ni, plant cultivation reduced metal availability and alleviated the associated risk. Comprehensive evaluation identified S-H as the optimal treatment. Conclusion Co-applying sludge with suitable plant species can enhance soil aggregation while mitigating HM risks, providing a feasible strategy for REE mine soil restoration.