Impacts of Microplastics on the Soil Biophysical Environment

Abstract

Soils are essential components of terrestrial ecosystems that experience strong pollution pressure. Microplastic contamination of soils is being increasingly documented, with potential consequences for soil biodiversity and function. Notwithstanding, data on effects of such contaminants on fundamental properties potentially impacting soil biota are lacking. The present study explores the potential of microplastics to disturb vital relationships between soil and water, as well as its consequences for soil structure and microbial function. During a 5-weeks garden experiment we exposed a loamy sand soil to environmentally relevant nominal concentrations (up to 2%) of four common microplastic types (polyacrylic fibers, polyamide beads, polyester fibers, and polyethylene fragments). Then, we measured bulk density, water holding capacity, hydraulic conductivity, soil aggregation, and microbial activity. Microplastics affected the bulk density, water holding capacity, and the functional relationship between the microbial activity and water stable aggregates. The effects are underestimated if idiosyncrasies of particle type and concentrations are neglected, suggesting that purely qualitative environmental microplastic data might be of limited value for the assessment of effects in soil. If extended to other soils and plastic types, the processes unravelled here suggest that microplastics are relevant long-term anthropogenic stressors and drivers of global change in terrestrial ecosystems.

Figure 1

Four microplastic types considered in the current study. Polyacrylic fibers (A), polyamide beads (B), polyester fibers (C), and polyethylene high-density fragments (D). The white bar in each panel represents 1 mm size.

Figure 2

Integration of microplastic particles to the soil biophysical environment. Structure of control soil (A) was not visually distinct under the stereomicroscope from soil contaminated with polyamide beads (SI S1D). Polyethylene fragments (B), and polyester (C) or polyacrylic fibers (D) resulted in visually apparent soil features. The white bar in each panel represents 1 mm size.

Figure 3

Effects of microplastic particles on soil bulk density. (A) Visualization of the impact of microplastics on bulk density over the range of treatment concentrations. (B) Focusing on effects of microplastic concentration irrespective of microplastic type. For A and B, data were represented by mean and standard error (N = 5 per microplastic treatments, 10 for controls). (C) Summary of effect range of microplastic types for bulk density combining treatment concentration. Data distribution was depicted by violin plots with median, interquartile range and 95% confidence interval overlaid. Dots represent outlying data. Microplastic types are color-coded: controls (dark gray), polyacrylic (yellow), polyamide (green), polyester (red) and polyethylene (blue) treatments (i.e., linear microplastics are in warm colors- yellow and red, nonlinear microplastics are in cold colors- blue and green).

Figure 4

Effects of microplastic particles on water holding capacity. (A) Visualization of the impact of microplastics on water holding capacity over the range of treatment concentrations. (B) Focusing on effects of microplastic concentration irrespective of microplastic type. For A and B, data were represented by mean and standard error (N = 5 per microplastic treatments, 10 for controls). (C) Summary of effect range of microplastic types for bulk density combining treatment concentration. Data distribution was depicted by violin plots with median, interquartile range and 95% confidence interval overlaid. Dots represent outlying data. Microplastic types are color-coded: controls (dark gray), polyacrylic (yellow), polyamide (green), polyester (red), and polyethylene (blue) treatments (i.e., linear microplastics are in warm colors: yellow and red, nonlinear microplastics are in cold colors: blue and green).

Figure 5

Effects of microplastic particles on the soil structure. (A) Visualization of the impact of microplastics on bulk density over the range of treatment concentrations. (B) Focusing on effects of microplastic concentration irrespective of microplastic type. For A and B, data were represented by mean and standard error (N = 5 per microplastic treatments, 10 for controls). (C) Summary of effect range of microplastic types for bulk density combining treatment concentration. Data distribution was depicted by violin plots with median, interquartile range and 95% confidence interval overlaid. Dots represent outlying data. Microplastic types are color-coded: controls (dark gray), polyacrylic (yellow), polyamide (green), polyester (red), and polyethylene (blue) treatments (i.e., linear microplastics are in warm colors: yellow and red, nonlinear microplastics are in cold color: blue and green).

Figure 6

Effects of microplastic particles on soil microbial activity. (A) Visualization of the impact of microplastics on bulk density over the range of treatment concentrations. (B) Focusing on effects of microplastic concentration irrespective of microplastic type. For A and B, data were represented by mean and standard error (N = 5 per microplastic treatments, 10 for controls). (C) Summary of effect range of microplastic types for bulk density combining treatment concentration. Data distribution was depicted by violin plots with median, interquartile range and 95% confidence interval overlaid. Dots represent outlying data. Microplastic types are color-coded: controls (dark gray), polyacrylic (yellow), polyamide (green), polyester (red), and polyethylene (blue) treatments (i.e., linear microplastics are in warm colors: yellow and red, nonlinear microplastics are in cold colors: blue and green).

Figure 7

Soil functional relationship between microbial activity and soil aggregation in control (A), or experimentally contaminated soils containing polyacrylic (B), polyamide (C), polyester (D), and polyethylene (E). The general association between these two proxies of soil health is unraveled on panel F (F = 13.01, r² = 0.13, p < 0.001). In all panels dark gray, yellow, green, red, and blue colors represent control, polyacrylic, polyamide, polyester, and polyethylene treatments, respectively (i.e., linear microplastics are in warm colors: yellow and red, nonlinear microplastics are in cold colors: blue and green).