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Contrasting responses of soil-microbial-plant systems to co-application of biochar and microplastics with/without additives in acidic versus slightly alkaline heavy metal-affected soils
Summary
Farmland contaminated with both microplastics and heavy metals (like cadmium and lead) is a growing concern, since these toxins can build up in crops we eat. This study found that adding biochar—a charcoal-like soil additive—helped plants grow better and absorb fewer heavy metals in acidic soil, but didn't work as well (and sometimes backfired) in slightly alkaline soil. This matters because it shows that soil cleanup strategies need to be tailored to soil type, rather than assuming one fix works everywhere, which could affect how safe and nutritious our food crops are.
Microplastics (MPs) and heavy metals threaten agricultural soil health through progressive accumulation. Despite well-known remediation effects, biochar application on these contaminated environments, especially with contrasting pH, remains in question. We investigated the effects of biochar co-applied with three types of MPs (polystyrene, BP; acrylonitrile-butadiene-styrene, BB; polyvinyl chloride, BV) and their additive-containing counterparts (MPAs: polystyrene with additives, BPA; acrylonitrile-butadiene-styrene with additives, BBA; polyvinyl chloride with additives, BVA) on soil-microbial-plant systems in acidic and alkaline soils. Biochar alone or with MPs/MPAs increased electrical conductivity (EC), nutrients bioavailability (P, S, K, Na) and Fe by 11-127%, while reducing metal ions contents (Al, Cd, Cu and Zn) by 10-92% in both soils. Soil pH and Mg had divergent responses to the treatments, with both increasing in acidic soils whereas Mg decreased in alkaline soils. For soil microorganisms, bacterial alpha-diversity consistently increased after incubation, but an absent response was in fungal alpha-diversity in acidic soils. Specifically, in acidic soils, Proteobacteria, Planctomycetes, and Myxococcota increased but Acidobacteria decreased. Conversely, the relative abundance of Proteobacteria and Actinobacteria increased in alkaline soils. Regarding plant growth, the co-application of biochar and MPs/MPAs alleviated plant oxidative damage, improved plant growth, and reduced metal accumulation by 11-80% (except for Cu) in acidic soils. These treatments, however, had limited positive effects on plant growth in alkaline soils and even increased plant uptake of Ni, Pb, and Zn. Overall, our study demonstrates that biochar is more effective at remediating co-contamination of MPs/MPAs and heavy metals in acidic soils than in alkaline soils.