Biodiversity and soil type modulate the effect of plastic contamination on soil functioning

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.