UV-aged microplastic fibres and drought affect productivity, soil properties, and biodiversity effects in selected cover-crop mixtures

Microplastic contamination and drought increasingly co-occur in agricultural soils, yet their combined effects on plant productivity, soil functioning, and biodiversity-effect components in selected crop mixtures remain poorly resolved. Glasshouse experiment examined whether an applied UV-ageing treatment of polypropylene microplastic fibres modified community productivity, Loreau-Hector biodiversity-effect components, and measured soil responses under contrasting water regimes. Four cover-crop species were assembled as monocultures, four predefined two-species mixtures, and one fixed four-species mixture. Shoot biomass, root biomass index from standardised soil cores, shoot-to-root-core biomass ratio, water-stable aggregates (WSA), nitrate (NO₃⁻), and ammonium (NH₄⁺) were quantified at harvest. Mixture overyielding was partitioned into net diversity effect (NE), complementarity effect (CE), and selection effect (SE) using treatment-matched monoculture references. Drought reduced shoot biomass and lowered the shoot-to-root-core biomass ratio, while the UV-aged fibre treatment was associated with lower shoot biomass, root biomass index, WSA, and NO₃⁻ availability, especially under drought. Biodiversity-effect partitioning showed that overyielding within the selected mixture set was mainly associated with the statistical CE component, whereas SE was weaker and more variable. The fixed four-species mixture showed stronger NE and CE than selected two-species mixtures, but this represents a planned composition-confounded contrast, not a general richness effect. Direct treatment effects and interactions for NE, CE, and SE were not significant. Soil responses were consistent with possible mechanisms affecting mixture performance, but did not demonstrate causal pathways linking fibre ageing to CE.