Piece by piece: plastics in soil ecosystems

Plastics have significantly advanced human life and research, but their overuse and poor waste management have led to the widespread presence of plastics, microplastics, and nanoplastics across all ecosystems. This thesis focus on soil ecosystems, where plastic contamination—particularly from conventional and biodegradable plastic mulches — is increasingly concerning and yet poorly understood. The nine chapters collectively address plastic and microplastic environmental impacts, methodological challenges, and biodegradation dynamics, aiming to inform all stakeholders involved. Chapter 1 introduces the scope, key definitions, and contextualize the thesis identifying critical research questions. The first two chapters reviewed existing literature to provide a status quo of microplastic research. Chapter 2 provides a timeline of global policymaking tackling (micro)plastic contamination, covers and discuss preventive and mitigative strategies to cope with the issue. Chapter 3 digs into soil ecosystems and examines plastic impacts on soil biota, highlighting risks to invertebrates, microbial communities, and terrestrial plants. Chapter 4 dives into methodological approaches and assess seven analytical methods used across Europe to detect eight types of microplastics in soils, revealing that there is no silver bullet to analyze microplastics universally, and advocating a hypothesis-driven approach to address this contamination. Chapters 5, 6, and 7 encompass our efforts to accelerate and understand the degradability of plastic mulches. Chapters 5 shows Streptomyces fulvissimus potential to degrade PLA below its glass transition temperature, while Chapter 6 explored the hypotheses of carbon limitation/unavailability and niche reopening and highlight several microbial treatments (mostly composed of Aspergillus sp.) to accelerate the degradation of LDPE and PBAT/PLA in carbon-free media, autoclaved compost, and compost. Puzzled by the delayed degradation of PBAT/PLA plastic mulches, Chapter 7 stand out as a two-year experiment to demonstrate how temperature and soil moisture content affect plastic mulch biodegradation dynamics, showing its soil-texture specific and time-dependent characteristics. Chapter 8 shifts focus to chemical leachates from plastics, revealing that compounds from PBAT/PLA and LDPE plastic mulches can inhibit seed germination and continue leaching for extended periods. The findings stress the need for long-term toxicity assessments and regulatory updates. Finally, Chapter 9 synthesizes the thesis, urging for realistic expectations of biodegradability, straightforward ecotoxicological evaluations, thorough hypothesis-driven methodological selection, and the careful implementation of biodegradable mulches. Overall, this thesis underscores the complex, often underestimated risks of plastic pollution in soils and calls for interdisciplinary, long-term strategies to address this global issue.