Growth Performance and Whole-Plant Bacterial Assemblages of Spirodela polyrhiza Under Swine Biogas Slurry Dilution Gradients

Swine biogas slurry is a nutrient-rich livestock effluent whose treatment could be coupled with plant biomass production. We evaluated the short-term bioremediation potential of Spirodela polyrhiza by cultivating plants for 7 days in 5–20% (v/v) slurry or reference media. We measured growth and aqueous-phase chemistry and characterized bacterial assemblages recovered from rinsed whole plants using 16 S rRNA sequencing. In the primary experiment, the 5% slurry treatment showed the highest mean fresh biomass and fresh-weight relative growth rate among slurry treatments, although differences in growth were modest. Apparent aqueous-phase NH3-N concentrations decreased by more than 90% across all four slurry dilutions. In a supplementary experiment using a separate slurry batch, dry-weight relative growth rates did not differ significantly among the four slurry treatments (p = 0.168). After volume correction, NH3-N depletion was 94.6–96.8% in plant-containing systems versus 20.8–27.9% in plant-free controls; corresponding total nitrogen depletion was 91.5–93.9% versus 19.7–29.0%. Slurry amendment was associated with generally higher bacterial richness, while Shannon diversity was highest at 5%; whole-plant community composition differed among treatments. Together, these short-term findings show how the growth of S. polyrhiza, aqueous-phase nutrient depletion, and whole-plant bacterial assemblages varied with slurry concentration. They provide a basis for further evaluation of duckweed-assisted biogas slurry treatment in relation to biomass production and nutrient recycling.