Microplastics in liquid biofertilizers: an overlooked threat to agricultural soil?

This study investigated the presence of microplastics (MPs, >5 μm to <2 mm) in biofertilizers and estimated their mass input to Swedish agricultural soils. Collective samples from representative biogas facilities were analyzed using optical microscopy and optical photothermal infrared (O-PTIR) spectroscopy, with selected MPs examined by scanning electron microscopy (SEM). The highest MPs concentration detected was 888 items g⁻¹. Preliminary observation indicates that MPs abundance was moderately correlated with the proportion of food waste used as biogas feedstock. Mass concentrations reached up to 6.19 ± 0.56 mg MPs kg⁻¹(dw) biofertilizer. Estimated inputs to agricultural soils ranged from 0.4 ± 0.06 to 2.0 ± 0.31 g MPs ha⁻¹ yr⁻¹, corresponding to a total annual input of 114 ± 17 to 377 ± 126 kg of plastics to Swedish soils via biofertilizer application. The total predicted environmental concentration (PECtotal) after 50 years was estimated at 0.11 mg kg⁻¹ soil for mineral-amended soils and 1.52 mg kg⁻¹ for sewage sludge-amended soils, indicating that MPs from biofertilizer represent only a minor fraction of total soil MPs contamination. Fragments dominated the MPs composition (98%), with most particles in the 5-50 μm size range. Polymer identification was achieved for 71% of particles, with paint-derived materials being most abundant (23%). Compared with other biosolids such as sewage sludge, biofertilizers represent a minor but measurable pathway for MPs inputs to agricultural soils. Nevertheless, the dominance of small-sized particles and long-term accumulation highlights the need to consider possible adverse effects of repeated biofertilizer application on soil ecosystems.