Combined effects of microplastics and salinization on soil health

Microplastic (MP) accumulation and salinization frequently co-occur in terrestrial ecosystems, yet their combined impacts on soil health remain elusive. This thesis first optimized MP characterization and then investigated MP–salinity interactions on earthworm health and soil biogeochemical cycling.
First, we optimized wet laser diffraction for accurate determination of MP particle size distributions (PSDs). Across three polyethylene (PE) size classes (<35, <125, and <500 μm), ethanol outperformed water and other dispersants, producing stable, unimodal PSDs with low uncertainty; volume-weighted mean diameters closely matched those determined by microscopy. Thus, ethanol is recommended for PSD determination of hydrophobic MPs, especially fine particles.
Second, we examined joint effects of NaCl and PE MPs on earthworm health. MP effects on avoidance behaviour were strongly size-dependent and synergistically enhanced by salinity for small MPs. While high MP loadings alone did not increase mortality, MP co-exposure reduced NaCl-induced mortality, indicating antagonism. Under moderate salinity, MP exposure reduced growth but markedly increased cocoon production, suggesting an energy trade-off under stress. Both salinity and MPs altered the earthworm gut microbiome, although MP effects were weaker under saline conditions. Overall. MP–salinity interactions were generally additive or antagonistic.
Finally, we assessed the combined effects of NaCl and PE or polyhydroxyalkanoates (PHA) MPs on soil GHG fluxes, carbon and nitrogen pools, enzyme activities, and functional pathways using metagenomics and metatranscriptomics. PE MPs had minimal effects on CO₂ and CH₄ emissions, whereas PHA MPs stimulated carbon emissions via enriching Azotobacter and Methanosarcina and promoting their acetate-mediated cross-feeding; this effect was suppressed by salinity. Under saline conditions, environmentally relevant PE level increased N₂O emissions, whereas high PE level and PHA MPs mitigated N₂O release via enhanced competitive dissimilatory nitrate reduction to ammonium (DNRA) and/or N₂O reduction. Overall, salinity acted as a double-edged regulator, underscoring MP risk assessments should integrate polymer type, size, dose, and salinity gradients.