Plastic contamination alters soil physical and chemical properties, threatening essential ecosystem functions such as nutrient cycling, water infiltration and soil aggregate stability. The goal of this study is to elucidate how these effects vary with soil texture and test if they could be mitigated by increasing plant species richness.

We conducted a mesocosm experiment combining two soil types (clayey and sandy), four plastic treatments (conventional, biodegradable, a mixture of both, and a control) and a gradient of plant species richness (2–6 species per pot) using seven ruderal species common in urban areas. We measured plant biomass, water infiltration, field capacity, aggregate stability, organic carbon, β-glucosidase activity and combined their individual responses into an overall multifunctionality index.

Conventional plastics significantly disrupted infiltration, aggregate stability and organic carbon to a larger extent than biodegradable plastics. Negative impacts were greater in clayey than in sandy soils; multifunctionality decreased by 15% when conventional plastics were added in clayey soils but slightly increased by 3% in sandy soils.

Plant richness had positive effects on half of the measured functions, either directly or indirectly through plant biomass production. In sandy soils, these positive effects partly compensated for the loss of functioning caused by plastic contamination, as reflected in higher multifunctionality. In clayey soils, however, plastic contamination not only outweighed plant diversity benefits, but also weakened richness–functioning relationships.

The most diverse plant mixtures exhibited transgressive overyielding, indicating functional complementarity among species that enhanced resilience to plastic-induced stress.

Synthesis and applications. Our findings show that the negative effects of plastic contamination on soil functioning strongly depend on soil type and can be partly alleviated by plant diversity. In sandy soils, diverse plant communities buffered functional losses under plastic stress, whereas in clayey soils the benefits of diversity were suppressed. These results highlight that ecological restoration strategies in urban environments should prioritise increasing plant diversity in sandy soils, while managing plastic contamination is crucial in clay-rich soils where biodiversity alone cannot counteract contamination impacts.