Nano-modified biochar mitigates toxicity from co-contamination with lead and microplastics in alfalfa by modulating photosynthesis, nitrogen assimilation, and antioxidant systems

Microplastics (MPs) and heavy metals are released into the environment and pose serious challenges to crop productivity and human health. The role of biochar and calcium oxide nanoparticle (CaO-NPs) modified (NBC) in mitigating MPs and lead (Pb 2+ ) toxicity has not yet been explored. Therefore, we tested whether the BC-CaO-NPs composite can mitigate the toxic impacts of individual and combined MPs and Pb 2+ on alfalfa plants. The study comprised different treatments: control (CK), Pb stress (100 mg kg −1 ), MPs (1%), Pb (100 mg kg −1 ) + MPs (1%), Pb (100 mg kg − 1 ) + NBC (2%), MPs (1%) + NBC (2%), and Pb (100 mg kg −1 ) + MPs (1%) + NBC (2%). Lead and MPs interaction decreased alfalfa growth and biomass yield by reducing chlorophyll (Chl) content, photosynthetic rate (Pn), transpiration rate (Tr), stomatal conductance (gs), intercellular CO 2 (Ci), and soil nutrient availability, while increasing malondialdehyde (MDA), electrolyte leakage (EL), and hydrogen peroxide (H 2 O 2 ), and decreasing enzyme activities. The decrease in growth was also linked to poor nitrogen assimilation owing to a decrease in the activity of enzymes such as glutamate dehydrogenase (GH), glutamine synthetase (GS), nitrate reductase (NR), and glutamate synthase (GOGAT) involved in N assimilation. Biochar enhanced chlorophyll synthesis (66.50%), Pn (18.69%), Tr (36.31%), gs (87.50%), Ci (38%), antioxidant activities (APX, CAT, POD, SOD, GSH, and MDHAR), osmolyte synthesis (proline and soluble proteins), and hormone production (indole acetic acid: IAA, gibberellic acid: GA), and decreased EL (37.72%), MDA (54.18%), and H 2 O 2 (39.24%), contributing to better growth and biomass yield. Biochar also enhanced nitrogen assimilation in response to stress by increasing NR (43.68%), GS (42.86%), GOGAT (48.13%), and GH activity (34.60%), which, in turn, ensured better growth. Furthermore, the NBC amendment reduced soil Pb 2+ availability, increased soil nutrient and carbon availability, and increased soil enzyme activities, contributing to better growth and biomass yield. Therefore, these findings suggest that NBC application is an effective measure to counteract the combined Pb 2+ and MPs toxicity. These findings provide novel insights and offer a viable solution to persistent challenges of heavy metals and MPs co-contamination.