Microplastic Shape, Polymer Type, and Concentration Affect Soil Properties and Plant Biomass

Abstract

Microplastics may enter the soil in a wide range of shapes and polymers. However, little is known about the effects that microplastics of different shapes, polymers, and concentration may have on soil properties and plant performance. To address this, we selected 12 microplastics representing different shapes (fibers, films, foams, and fragments) and polymers, and mixed them each with soil at a concentration of 0.1, 0.2, 0.3, and 0.4%. A phytometer (Daucus carota) grew in each pot during 4 weeks. Shoot, root mass, soil aggregation, and microbial activity were measured. All shapes increased plant biomass. Shoot mass increased by ∼27% with fibers, ∼60% with films, ∼45% with foams, and by ∼54% with fragments, as fibers hold water in the soil for longer, films decrease soil bulk density, and foams and fragments can increase soil aeration and macroporosity, which overall promote plant performance. By contrast, all shapes decreased soil aggregation by ∼25% as microplastics may introduce fracture points into aggregates and due to potential negative effects on soil biota. The latter may also explain the decrease in microbial activity with, for example, polyethylene films. Our findings show that shape, polymer type, and concentration are key properties when studying microplastic effects on terrestrial systems.

Microplastics Increase Soil pH and Decrease Microbial Activities as a Function of Microplastic Shape, Polymer Type, and Exposure Time

“…Our results indicated that microplastic foams and fragments increased soil pH, which can be due to the increase in soil aeration and porosity when these microplastics were added into the soil (de Souza Machado et al, 2019;Lozano et al, 2021a). This along with the leaching of microplastic chemical compounds into the soil (Kim et al, 2020;Waldman and Rillig 2020), may alter soil biota with consequences for soil pH.…”

“…This may could be the reason that PE foams caused higher respiration that PE films. Films and fragments had neutral effects on soil respiration, while positive effects have been observed on this property when a plant species was included in the system (Lozano et al, 2021a). The latter as the presence of roots in the soil matrix contributes to soil aggregation and facilitates water uptake and its redistribution through the soil profile, which in the end promote soil microbial activity (Lozano et al, 2021a).…”

“…As a result of their manufacturing origin and environmental degradation, microplastics may occur in many shapes and a variety of physical and chemical properties (Helmberger et al, 2020;Rillig and Lehmann, 2020). The accumulation of microplastics in soil may impact soil characteristics (Liu et al, 2017;Yi et al, 2020), depending on microplastic properties (Lozano et al, 2021a). Indeed, microplastic shape may determine how microplastics interact with soil particles (de Souza Machado et al, 2018;Rillig et al, 2019a;Lehmann et al, 2020;Rillig and Lehmann 2020).…”

“…In addition, our knowledge of microplastic effects on soil respiration is still rudimentary. Soil respiration, an indicator of the total soil microbial activity (Rousk et al, 2009), is very sensitive to environmental factors, such as soil texture, porosity, moisture, and pH (Luo and Zhou, 2006), soil properties that can be potentially altered by microplastics addition (Rillig et al, 2019b;de Souza Machado et al, 2019;Lozano et al, 2021a). Indeed, recent research has observed that microplastics could alter the soil microbial community (Huang et al, 2019;Fei et al, 2020), suggesting potential effects on soil respiration (Lozano et al, 2021a;Lozano et al, 2021b).…”

Microplastics and phagotrophic soil protists: evidence of ingestion

“…It is possible that the commercial microspheres used in this study are indeed inert and their ingestion may not substantially reduce the protist's food intake. More research using different soils and a wider diversity of MPs types (e.g., varying the parameters shape (Lozano et al, 2021), polymer type , weathering status and additives ) will be required to clarify this.…”