Abstract
Microplastic as an anthropogenic pollutant accumulates in terrestrial ecosystems over time, threatening soil quality and health, for example by decreasing aggregate stability. Organic matter addition is an efficient approach to promote aggregate stability, yet little is known about whether microplastic can reduce the beneficial effect of organic matter on aggregate stability. We investigated the impacts of microplastic fibers in the presence or absence of different organic materials by carrying out a soil incubation experiment. This experiment was set up as a fully factorial design containing all combinations of microplastic fibers (no microplastic fiber addition, two different types of polyester fibers, and polyacrylic) and organic matter (no organic matter addition, Medicago lupulina leaves, Plantago lanceolata leaves, wheat straw, and hemp stems). We evaluated the percentage of water-stable aggregates (WSA) and activities of four soil enzymes (β-glucosidase, β-D-celluliosidase, N-acetyl-b-glucosaminidase, phosphatase). Organic matter addition increased WSA and enzyme activities, as expected. In particular, Plantago or wheat straw addition increased WSA and enzyme activities by 224.77 or 281.65% and 298.51 or 55.45%, respectively. Microplastic fibers had no effect on WSA and enzyme activities in the soil without organic matter addition, but decreased WSA and enzyme activities by 26.20 or 37.57% and 23.85 or 26.11%, respectively, in the presence of Plantago or wheat straw. Our study shows that the effects of microplastic fibers on soil aggregation and enzyme activities are organic matter dependent. A possible reason is that Plantago and wheat straw addition stimulated soil aggregation to a greater degree, resulting in more newly formed aggregates containing microplastic, the incorporated microplastic fibers led to less stable aggregates, and decrease in enzyme activities This highlights an important aspect of the context dependency of microplastic effects in soil and on soil health. Our results also suggest risks for soil stability associated with organic matter additions, such as is common in agroecosystems, when microplastics are present.
Highlights
Microplastics as a group of anthropogenic contaminants are pervasive and persistent
We found a similar relationship between effects of microplastic fibers in the presence of different types of organic matter and effects of organic matter on β-Glucosidase activity as we found in water-stable aggregates (WSA); meaning, the more positive an effect a specific organic material, the more detrimental was the impact of added microplastic fibers (Figure 4)
All types of organic matter increased WSA substantially compared to the control, with wheat straw having the most positive effect on WSA (30.7% [95%-confidence interval (CI): 26–35.1]), followed by Plantago (24.4% [95%-CI: 20.6–29.1]), hemp stems (24.2% [95%-CI: 19–28.4]), and Medicago (16.4% [95%-CI: 11– 22.8])
Summary
Microplastics as a group of anthropogenic contaminants are pervasive and persistent. Microplastic is widely studied in marine ecosystems (Eriksen et al, 2014; Bergmann et al, 2015; Jambeck et al, 2015), and only in recent years has attention shifted to terrestrial ecosystems (Rillig, 2012; Bläsing and Amelung, 2018; de Souza Machado et al, 2018a; Lozano and Rillig, 2020; Zhou et al, 2020). Atmospheric deposition of microplastic fibers is an important source of soil contamination, as hundreds of particles are deposited from the atmosphere per square meter per day (Cai et al, 2017; Dris et al, 2017; Brahney et al, 2020). Further anthropogenic activities, such as tillage, and movement by soil animals can accelerate the incorporation of microplastic fibers into the soil (Huerta Lwanga et al, 2017; Rillig et al, 2017)
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