Determining the spatial and temporal distribution of macro- and microplastics in soil and their impact on soil function.

Ramage, Stuart John Frederick Forde

Abstract

Microplastics are long-term anthropogenic terrestrial ecosystem stressors that are ubiquitous in the natural environment, found in all ecological niches including aquatic and terrestrial environments. Consequently, microplastics have been found within organisms and plants, where they have the potential to induce adverse effects. Little is known of microplastic distribution in Scotland (or worldwide), their fate over time, and how they interact with organic pollutants or the impact of these interactions on soil function. It was the overarching aim of this project to determine the extent of macro- and microplastics in Scottish surface soils, both spatially and temporally, and evaluate the impact of microplastics on soil function and ecosystem services. The novel high-gradient magnetic separation (HGMS) methodology was developed to overcome the challenges faced using the commonly used density separation method, and to improve recovery rates of all microplastic types, compositions and sizes regardless of soil type. HGMS was able to recover microplastics consistently across different soil types and their recovery rates ranged between 91- 96%, which was statistically higher than that achieved by density separation (0-89%; p < 0.05). The HGMS method not only allowed for the recovery of high-density microplastics, but it also showed capability for recovering fibres held in fibre-soil aggregate complexes. The HGMS method was then applied to investigate the fate of microplastics in a long-term sewage sludge study and to determine spatial distribution of microplastics on a national scale. Sewage sludges applied to agricultural soils are sources of microplastic pollution; however, little is known about the accumulation, persistence or degradation of these microplastics over time. In a longterm study (25 years), the abundance and degradation of microplastics was assessed in soils sampled biennially from an experimental field managed under an improved grassland regime following the application of five different sewage sludges. Microplastic abundance, dominated by fibres, significantly increased by 1417-3700% following sewage sludge applications (p < 0.05) with abundance remaining relatively constant over time (22 years) (p > 0.05). Fibres degraded over time, showing the greatest reduction in size, highlighting the possibility that fibres release secondary micro(nano)plastics. Films were also highly susceptible to degradation, while other microplastic morphologies were more resistant. Coloured fibres showed dye loss over time, which possibly leached into soil. The sludges contained different microplastic compositions (e.g., polyester, polyurethane), reflecting the possible different sources of the sludges. This evidence is useful in informing regulation on sewage sludge use and management, and in assessing the fate and impact of microplastics in soil. The first national-scale study on microplastic prevalence in soils was conducted using the National Soils Inventory of Scotland. Results highlighted that microplastic "hotspots" could be identified through distribution mapping, which demonstrated that geography played a role in microplastic distribution e.g., spatial variation in land use or rural locations impacted by tourism. Fibres were the most abundant microplastic morphology (≥ 88.91%). Microplastic data was correlated to anthropogenic, topographical, soil physicochemical and environmental factors that influenced microplastic distribution in Scotland. Largely, land use was a significant factor in the distribution of microplastics, such that high-intensity land management resulted in higher microplastic pollution (average 3756 microplastics L-1 soil) compared to unmanaged soils (average 562 microplastics L-1 soil) (p < 0.05). Microplastics were also correlated with persistent organic pollutants (rS = 0.18-0.46), including compounds linked to plastic manufacturing and degradation, suggesting that microplastics may contribute to pollution of these contaminants through leaching or that they share a common anthropogenic source. Further, sorption and desorption kinetics of 16 priority polycyclic aromatic hydrocarbons (PAHs) were investigated using two different sizes of polyethylene microfilms derived from mulch film mixed into an agricultural soil. Sorption data fitted a first order equilibrium model while desorption data fitted an exponential decay model. Sorption and desorption kinetics were influenced by physico-chemical properties of PAHs, and no significant differences between the rate of sorption or desorption were observed. Decreases in soil organic matter indicated that smaller-sized microfilms sequestered a higher amount of PAHs (3.36% decrease compared to 2.93% for the larger microfilm) and sorption to microfilms prevented soil acidification likely caused through microbial degradation (2.67% decrease in soil pH opposed to 4.10%). The effect of microfilms, chrysene and chrysene-bound microfilms on ecosystem function (soil respiration) and microbial activity (substrate-induced respiration) was then investigated. The treatment groups showed differences in function and activity, with chrysene inducing reduced soil respiration and microbial activity (p < 0.05), microfilms inducing increased soil respiration and microbial activity, and chrysene-bound microfilms not significantly changing ecosystem function compared to a soil negative control. These were attributed to changes in the microbial biomass. Evaluation of the microbial community composition through sequencing of the 16S rRNA gene showed differences in the microbial communities was more pronounced at a lower taxonomic level (family) and that there were significant changes in beta diversity of the microbial communities between treatments (p < 0.05). Sorption of chrysene to microfilms appear to limit the availability of chrysene to soil micro-organisms and may alleviate the immediate effects of chrysene. However, this may pose a greater threat, as chrysene could be slowly released into the soil environment and persist over a prolonged period. Key findings are discussed throughout this thesis, which highlight the importance of understanding the extent and impact of microplastics in Scotland to enable future work to be conducted, and inform future mitigation and remediation strategies for the protection of one of Scotland's most valuable natural assets.

Citation

RAMAGE, S.J.F.F. 2024. Determining the spatial and temporal distribution of macro- and microplastics in soil and their impact on soil function. Robert Gordon University, PhD thesis. Hosted on OpenAIR [online]. Available from: https://doi.org/10.48526/rgu-wt-2801146

• Thesis Type: Thesis
• Deposit Date: Apr 22, 2025
• Publicly Available Date: Apr 22, 2025
• DOI: https://doi.org/10.48526/rgu-wt-2801146
• Keywords: Microplastics; Soil pollution; Persistent organic pollutants; Sorption kinetics; Ecosystems; Microbial activity; Scotland
• Public URL: https://rgu-repository.worktribe.com/output/2801146
• Award Date: Oct 31, 2024