Microplastics contamination and decomposition in soils

Abstract

A significant amount of conventional plastic waste, especially in the form of microplastics (MPs), has accumulated in soils due to its limited degradation. However, the existence, persistence and impacts of plastics on soil ecosystem services have received little attention compared to aquatic environments. First, this thesis provides a review of plastics in soils including plastic characteristics, methodologies to isolate and characterise plastics from soils, occurrence, sources, impacts of plastics to soil ecosystems and degradation of plastics in soils. Second, the interactions of plastics and soil organisms are complex and inconsistent observations have been made in published studies. Thus, we assessed the effects of plastic on plants, fauna and microbial communities, with a metaanalysis. it was found that overall plastics caused substantial detrimental effects to plants and fauna, but less so to microbial diversity and richness. Third, MP degradation fate and priming effect (PE) on native soil organic matter, are not well known. We examined the impacts of MP composition (i.e, biodegradable bin liner and mulch film, oxo-degradable straw and conventional F&V bag) and size (i.e., <100, 100-250, 250-500, 500-1000, 1000-2000, 2000-4750 μm), soil type (i.e., Ferralsol, Vertisol and Solonetz) and temperature (i.e., 20, 30 and 40°C) on its C mineralisation and PE using a δ13C method. The results demonstrated that MP composition and size, soil type, temperature and their interactions are all key factors on its mineralisation and PE. Furthermore, hitherto no research has been conducted on the abundance of MPs in Australian agricultural soils. We investigated the MP concentration and characteristics in soils treated with sludge, compost and mulch film in New South Wales, Australia. The largest number of MPs were observed in the compost soil with 20776 particles kg-1 soil, followed by soils with sludge (17449) and mulch film (13731) applications.