The Impact of Microplastic Material, Size, and Concentration on Greenhouse Gas Emissions in Soil Environments

Abstract

The large wasteful legacy left behind by plastic waste has been detrimental to ecosystem functions. Especially Microplastics (MP) with a diameter size of 1mm to 5mm, a byproduct of larger plastic breakdown. Although soil environments are estimated to have 4 to 23 times the amount of MP found in Marine environments, hitherto, not much attention has been given to the impact of MPs on these environments (Horton et al., 2017). The research project aims to understand emissions produced as a direct cause of MP’s characteristics such as MP’s material, size, and concentration. A laboratory model using 64 different GC glass vials filled up with one-third soil mixture (75% topsoil, 25% compost soil, and the rest of the vial was left empty for gas accumulations) was conducted. The experimental condition included a comparison of conventional Microplastics such as Polypropylene (PP) to biodegradable Microplastic such as Cellophane (CP). Microplastic sizes included a small MP particle size of 1mm and a medium size of 3mm. MP concentration was investigated by using 0.0%, 0.5%, 1%, 1.5%, and 2% (w/w) concentrations. Each combination of MP material, size, and quantity was replicated 5 times, with 4 control samples of soil vials without any MPs. Samples were incubated under anaerobic conditions for thirty days (± 48 hours) in the dark at 29 °C (± 1 °C). The results showed CP had the most morphological changes and surface roughness compared to PP. In addition, the difference between the material of MP was dependent on the size of the MP. For the larger-sized MP (3mm), CP decreased CH4 emissions by an average of 44.6%. Whereas PP increased emissions by 24.2%. The smaller-sized (1mm) MP showed the opposite effect as PP decreased CH4 emissions by 68.8% at 1% concentration and no impact was observed from CP on emissions. Secondly, with disregard to the type of MP used, size comparison showed that 1mm MP reduced CH4 emissions by 38.85%. Both the material and size of MP resulted in no impact on CO2 emissions. Lastly, the concentration of the MP showed that smaller-sized MP (1mm) had a negative correlation with CH4 emissions. Whereas CO2 emissions were negatively correlated with increased (3mm) MP concentrations. Smaller concentrations of 0.5% (w/w) showed no impact on emissions across all different MP conditions. Moreover, PP showed a threshold for CH4 emissions at 1% concentrations with a sharp decrease in overall emissions at larger concentrations. In conclusion, the material of MP plays a role in the decomposition and emissions produced as a result of their molecular structures and carbon material. The size of the MP is an important feature in soil interactions as smaller MP allow for easier microbial contact and growth due to having a smaller edge-to-surface ratio as well as their ability to fill up soil pores and reduces the oxygen level in deeper soils. Lastly, the impact of concentration was codependent on MP size. Overall, the impact of MP on terrestrial environments is still in its infancy of understanding. Hence, the recommendation for future studies should include a model demonstrating the impact on soil emissions and microbial interaction carried out in different soil environments. In addition, the impact of material, size, and concentrations should be repeated in aerobic environments to reflect emissions impact on shallow surfaces rather than deeper surfaces. In addition, further understanding of the impact of MP on pore space availability is needed within soil environments, specifically at which size between 1mm and 3mm do size impact begins to fade. Lastly, longer incubation periods should be used to measure the impact on CO2 emissions in aerobic environments.