● Film mulching decreased soil organic C content in soil aggregates with 0.053–0.25 mm diameter.

Microplastic distribution is non-homogeneous in agricultural soil following plastic film degradation. However, the distribution of microplastics by shape and particle size in different soil aggregates remains unknown. To elucidate the distribution of microplastic shapes and particle sizes in soil aggregates with increasing years of film mulching, four paired fields with film mulching (FM) and no mulching (NM) were examined at 1, 5, 10, and 20 years after continuous mulching. An increase in soil aggregates of 0.053–0.25 mm diameter was observed; however, soil organic carbon content decreased after long-term FM. Microplastics primarily combined with 0.053–2 mm soil aggregates. Specifically, long-term FM was associated with dominance of film- and fiber-shaped microplastics in soil aggregates of 0.25–2 mm and 0.053–0.25 mm diameter, respectively. Fiber- and granule-shaped microplastics of 0.25–1 mm diameter primarily combined with 0.053–0.25 and 0.25–2 mm soil aggregates, respectively. Film-shaped microplastics of diameter > 1 mm and diameter 0.05–0.25 mm primarily combined with 0.25–2 mm soil aggregates. Therefore, distribution of microplastics in soil aggregates can be used to monitor soil health and quality, greatly enhancing our understanding of the risk posed by microplastics to the environment.

● Microplastics (MPs) increased activities of N and P hydrolases in paddy soil.

Microplastics provide a new ecological niche for microorganisms, and the accumulation levels of microplastics (MPs) in terrestrial ecosystems are higher than those in marine ecosystems. Here, we applied the zymography to investigate how MPs – polyethylene [PE], and polyvinyl chloride [PVC]) at two levels (0.01% and 1% soil weight) impacted the spatial distribution of soil hydrolases, nutrient availability, and rice growth in paddy soil. MPs increased the above-ground biomass by 13.0%–15.5% and decreased the below-ground biomass by 8.0%–15.1%. Addition of 0.01% and 1% MPs reduced soil NH₄⁺ content by 18.3%–63.2% and 52.2%–80.2%, respectively. The average activities of N- and P-hydrolases increased by 0.8%–4.8% and 1.9%–6.3% with addition of MPs, respectively. The nutrient uptake by rice plants and the enzyme activities in hotspots increased with MP content in soil. The accumulation of MPs in paddy soil could provide an ecological niche that facilitates microbial survival, alters the spatial distribution of soil hydrolases, and decreases nutrient availability.

● Occurrence of microplastics was widespread in long-term mulched farmland soils.

Soil contamination from film debris following the prolonged application of mulching film has emerged as a worldwide concern. However, the extent that mulching films contribute to soil microplastics, and the spatial distribution of soil contamination from film debris remain unclear. In this study, the cotton field in Xinjiang (China), which underwent film mulching for a prolonged period of 5−30 years, was selected as the research location. A total of 360 soil samples were collected, aiming to study the spatial distribution characteristics of mulching film debris pollution. The samples were extracted using the density flotation method combined with stereomicroscopic; the source, composition, abundance, and distribution characteristics of soil MPs were identified by the scanning electron microscopic, and Fourier transform infrared spectroscopic analyses. In soil mulched for a 30 year period, the abundance of microplastics across the studied soil depth (0−60 cm) was 78.51±2.57 n/(100 g). The μ-FTIR analyses revealed that the composition of the microplastics matched that of polyethylene materials. Therefore, plastic mulching could be inferred as a major contributor to microplastic pollution in agricultural lands. Overall, it is necessary to study the distribution characteristics of plastic film remaining for further study of plastic pollution in farmland soils.

• We collated and synthesized previous studies reporting on impacts of microplastics in soils.

We collated and synthesized previous studies that reported the impacts of microplastics on soil parameters. The data were classified and integrated to screen for the proportion of significant effects, then we suggest several directions to alleviate the current data limitation in future experiments. We compiled 106 datasets capturing significant effects, which were analyzed in detail. We found that polyethylene and pellets (or powders) were the most frequently used microplastic composition and shape for soil experiments. The significant effects mainly occurred in broad size ranges (0.1–1 mm) at test concentrations of 0.1–10% based on soil dry weight. Polyvinyl chloride and film induced significant effects at lower concentrations compared to other compositions and shapes, respectively. We adopted a species sensitivity distribution (SSD) and soil property effect distribution (SPED) method using available data from soil biota, and for soil properties and enzymes deemed relevant for microplastic management. The predicted-no-effect-concentration (PNEC)-like values needed to protect 95% of soil biota and soil properties was estimated to be between 520 and 655 mg kg^-1. This study was the first to screen microplastic levels with a view toward protecting the soil system. Our results should be regularly updated (e.g., quarterly) with additional data as they become available.