Behavior of microplastics and plastic film residues in the soil environment: A critical review

It is now widely acknowledged that microplastic pollution represents one of the greatest anthropogenically mediated threats to Earth-system functioning. In freshwater and marine ecosystems the presence of large amounts of microplastic appears almost ubiquitous, with frequent reports of negative impacts on aquatic health. In contrast, however, the impact of plastic in terrestrial environments remains poorly understood. In agroecosystems, microplastics (particles < 5 mm) can enter the soil environment either directly (e.g. from biosolids application, irrigation water, atmospheric deposition), or indirectly through the in situ degradation of large pieces of plastic (e.g. from plastic mulch films). Although we have encouraged the use of plastics over the last 50 years in agriculture to promote greater resource use efficiency and food security, the legacy of this is that many soils are now contaminated with large amounts of plastic residue (ca. 50-250 kg ha^-1). Due to difficulties in separating and quantifying plastic particles from soil, our knowledge of their behavior, fate and potential to transfer to other receptors (e.g. surface and groundwater, air) and enter the human food chain remains poor. This information, however, is critical for evaluating the risk of soil-borne microplastic pollution. In this critical review, we systematically summarize (i) the distribution and migration of microplastics in soils, (ii) highlight the separation, extraction, and identification methods for monitoring microplastics in soils, (iii) discuss the ecological effects and pollution mechanisms of soil microplastics, (iv) propose mitigation strategies to help prevent and reduce microplastic pollution, and (v) identify the most important future challenges in soil microplastics research.