Water hyacinth (Eichhornia crassipes) biomass characterization for a potential exploration as an agriculture soil enhancer: Linking multi-location biogeochemical profiles to ecotoxicological safety

Water hyacinth (WH) is an invasive aquatic species for which no universal biomass management strategy exists, although many developing countries use it in agriculture with limited understanding of its potential environmental impacts. As WH is an effective bioaccumulator it is essential to assess its composition and quantify potentially harmful elements before this surplus green biomass can be effectively valorised. Determining the thresholds for their effects is crucial to define safe and sustainable uses. In this context, this study characterized WH biomass from six Portuguese locations (four northern and two southern), focusing on nutrient and potentially toxic element (PTE) profiles, sugar, protein, and structural composition. Furthermore, the ecotoxicological profile of aqueous extracts from each WH biomass was evaluated using several freshwater species (Raphidocelis subcapitata, Brachionus calyciflorus, Daphnia magna, and Danio rerio) and multiple endpoints, to benchmark safe agricultural application rates. Structural analysis revealed tissue type (leaves, floaters, roots) had greater influence than sampling location, with roots showing highest absorbance linked to lignin, proteins, and cellulose. These wall components provide metal-binding sites, explaining root PTE levels being higher than other tissues. Elemental composition showed high primary nutrients (e.g., potassium, phosphorus), meeting EU requirements for organic soil improvers (EU Regulation 2019/1009). Whole-plant WH water extracts had high conductivity (≥ 6.98 mS/cm), nutrient and PTE concentrations, and caused adverse effects on all aquatic species. No clear toxicity ranking emerged, though Bico and Pateira extracts were least toxic, and Sorraia extract most severe (algal inhibition, zooplankton mortality, zebrafish effects at 0.78 % dilution). The findings indicate that WH biomass incorporation into soils should be considered on a site-specific basis, owing to variations in PTEs accumulation across locations, requiring contaminant screening and regulatory guidance before large-scale use. The results evidenced multispecies, multi-endpoint ecotoxicity that might justify the need for dilution strategies and controlled application rates of WH biomass on soils to minimize putative downstream impacts.