Estimation of microplastics entering agricultural soil through the use of biofertilizers

The use of biofertilizer from anaerobic digestion of organic waste in biogas companies provides a valuable nutrient source for agricultural soils and supports the circular economy. However, their use may also contribute to the input of microplastics (MPs) into soils, and data on this topic remains limited. This study investigated the abundance of MPs (<2 mm) in biofertilizers in biofertilizers and estimated their mass input to the Swedish agricultural soils. Samples from three different biogas industries were collected and pretreated using Fenton and enzymatic treatments to remove organic matter and analyzed by optical microscopy and optical photothermal infrared (O-PTIR) spectroscopy. Morphological characteristics were further examined by scanning electron microscopy (SEM). The highest concentration of MPs detected in the samples was 887,840 particles kg⁻¹ _(dw). The MPs abundance was moderately positively correlated with the proportion of food waste in the feedstock. Mass estimations were up to 6.19 ± 0.56 mg MPs kg⁻¹ _(dw) of biofertilizer. Considering a per-hectare basis, estimated inputs ranged from 0.4 ± 0.06 to 2.0 ± 0.31 g of MPs ha⁻¹ _(dw) and a total annual input of 114 ± 17 to 377 ± 126 kg of plastics yr⁻¹ into the soils through biofertilizer application. The predicted environmental concentration (PEC) of MPs in the soil ranged from 1.0 µg MPS kg⁻¹ after one year to 50 µg MPs kg⁻¹ after 50 years, indicating low ecological risk under realistic agricultural conditions. Fragments within the 5–50 µm particle-size range (75%) were the most common type of MPs (98%, n = 2,325). This result suggests consistent fragmentation of MPs across the biogas facilities and biofertilizer production processes. In addition, chemical composition was determined for 71% of the MPs using O-PTIR spectroscopy, among which paint-derived particles (23%) were the most abundant. Therefore, while the amounts of MPs found in biofertilizer in this study are relatively low, its annual applications can serve as a measurable pathway for MPs input to soils, despite their agronomic and environmental benefits. Thus, in the future, MPs monitoring in biofertilizer should be considered.