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Exploring potential effects of microplastic contamination in Antarctic soils
Summary
Researchers reviewed the potential effects of microplastic contamination in Antarctic soils, synthesizing available evidence on microplastic occurrence, polymer composition, and ecological impacts in one of Earth's most pristine terrestrial environments. The review found that microplastics are present even in remote Antarctic soil ecosystems and may pose risks to soil invertebrates and microbial communities that underpin fragile polar food webs.
Microplastics (MPs; diameter < 5.0 mm) are currently considered ubiquitous contaminants in several environmental matrices around the planet, occurring in a wide range of shapes, sizes, and polymeric compositions. MPs have been recognized as a potential threat to terrestrial ecosystems, in part due to their significant impacts on soil ecosystems. The Antarctic continent is a pristine environment, geographically isolated and dedicated to environmental conservation and scientific studies. Nevertheless, Antarctica has a historical record of anthropogenic pollution, including the presence of MPs in both terrestrial and marine environmental compartments, including soils, which play a crucial role in ecological processes. Despite evidence of MP contamination in Antarctic terrestrial environments, the potential impacts of such contamination on its soils remain largely unknown. This thesis explored the potential impacts of MP presence in Antarctic soils, focusing on evaluating the physical and chemical properties of soils artificially contaminated with MPs and addressing processes driven by freeze-thaw cycles (FTCs). In a first experiment (Chapter I), short-term impacts of contamination by two types of MPs [polyacrylonitrile (PAN) fibers and polyethylene (PE) fragments], added in different concentrations (from 0.001% to 1.0% w w-1), in an Antarctic marine terrace soil were investigated. Rebuilt soil samples in pots were incubated under field conditions for 22 days, with CO2 fluxes monitored every two days. After this period, soil physical and chemical parameters and microbial activity were evaluated. The results showed effects depending on MP type and dose. In general, PAN fibers at high concentrations (0.1%) increased soil porosity and hydraulic conductivity, possibly addressing the observed increase in mean CO2 fluxes. PE fragments did not affect porosity but decreased soil bulk density. The presence of both types of MPs increased soil microbial activity and decreased cation exchange capacity. Potential causes and implications of the observed impacts were discussed. The experimental units from the first experiment were used for a descriptive analysis of thin sections using micromorphological techniques and concepts (Chapter II). Microphotographs evidenced the interaction of MP particles with the soil matrix. PE fragments were observed as part of the soil matrix and clogging pores – corroborating the decrease in hydraulic conductivity – and frequent vesicular and planar pores were observed in samples containing PAN fibers. Features such as clay capping, the presence of ovoidal peds, and planar/vesicular pores suggest the occurrence of processes related to FTCs, even within the short incubation period. Clay capping features on MP particles suggest a possible interaction with active pedogenetic processes in Antarctic periglacial environments. In a second experiment (Chapter III), the potential for MP vertical migration in three different types of Antarctic soils under successive FTC conditions and cumulative impacts on soil physical parameters (porosity and bulk density) were investigated. Soil columns (12.5 cm) with three layers containing or not containing MPs (0.01% v v-1) in a surface layer (2.5 cm) were prepared. Three types of MPs [PE, polyethylene terephthalate (PET), and polylactic acid (PLA)] were used to evaluate potential selective migration. After 44 days of incubation, with or without FTCs, MPs and soil physical parameters were analyzed. The abundance of MPs in the second (2.5 – 7.5 cm) and third (7.5 – 12.5 cm) layers of the soil columns relative to the surface layer, particle diameter, and soil physical parameters were compared across treatments, soil types, and layers. The results confirmed vertical MP migration, depending on soil type, MP type, and the occurrence of FTCs. The soil with higher porosity and organic matter content (S3) showed higher relative MP abundance in the lower layers of soil columns, but FTCs representatively facilitated particle migration in columns of the most weathered and finer-textured soil (S1). PE particles and smaller diameter particles were more abundant in the lower layers of the columns. FTCs caused soil subsidence, increased (macro) porosity in the surface layer, and compaction of the lower layers of the columns, and MPs decreased soil bulk density in S1 columns subjected to FTCs. Potential processes involved on the observed results were discussed. This thesis demonstrated that MP contamination in soils can affect at short-term scale the microbial activity, and soil physical and chemical properties differently according on MP type and soil type, and that active pedogenetic processes in Antarctic periglacial environments interact with MPs, potentially facilitating their vertical migration. However, future studies are needed to better understand the long- term ecological implications of MP contamination in terrestrial ecosystems of Antarctica. Keywords: soil pollution; plastic pollution; maritime antarctica; soil micromorphology; freezing and thawing cycles
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