Integrated microbiome-metabolome analysis reveals phytotoxicity of biodegradable and non-biodegradable microplastics in tobacco and biochar-mediated mitigation via particle size modulation

Microplastic (MP) contamination in agricultural soils poses a growing threat to crop health, yet its impacts on industrial crops—particularly under the influence of emerging biodegradable plastics—and potential mitigation strategies remain poorly understood. Using tobacco (Nicotiana tabacum L. cv. Yunyan 87) as a model industrial crop, we investigated the physiological, rhizospheric, and metabolic responses of seedlings exposed to polyethylene (PE) and poly(butylene adipate-co-terephthalate) (PBAT) MPs (1% w/w), with or without amendment of wheat straw-derived biochar at two particle sizes (30–50 μm: SBC; 840–900 μm: LBC). Both PE and PBAT induced oxidative stress, evidenced by elevated malondialdehyde (MDA) and hydrogen peroxide (H₂O₂) levels, coupled with disrupted antioxidant enzyme activities (SOD, POD, CAT, APX). High-concentration PBAT exhibited greater phytotoxicity than PE, likely due to additive leaching or surface reactivity. MP exposure significantly altered soil physicochemical properties and reshaped rhizosphere microbial communities, notably reducing beneficial genera such as Streptomyces and Paraglomus. Untargeted root metabolomics revealed profound perturbations in central carbon metabolism, including suppression of the TCA cycle and accumulation of phenylpropanoid pathway intermediates (e.g., cinnamic acid), suggesting a defense-oriented metabolic reprogramming. Critically, biochar, especially SBC, effectively alleviated MP-induced stress by restoring redox homeostasis, improving soil structure, enriching plant-beneficial microbes, and normalizing metabolic profiles. Our findings demonstrate that both conventional and “biodegradable” MPs threaten early-stage development of an economically important industrial crop, while highlighting wheat straw biochar as a sustainable, waste-derived strategy for enhancing crop resilience in MP-contaminated agroecosystems. • High-concentration PBAT MPs were more toxic than PE; low-concentration slightly boosted growth. • MPs impaired antioxidants and altered soil properties and microbial communities. • Biochar, especially SBC eased stress by tuning microbiota and restoring key metabolisms.