Microplastics in agricultural soils : effects on physical, chemical, and microbiological processes

Microplastics (MPs) affect key soil properties relevant to agriculture: physical structure, chemical properties, and microbial processes, with their specific functions. This thesis examines how pristine and degraded conventional MPs (CMPs: polyethylene, PE, and polyethylene terephthalate, PET) and biodegradable MPs (BMPs: polybutylene adipate terephthalate, PBAT) affect different soil types. It integrates five studies that combined greenhouse and laboratory experiments to assess MPs impacts on soil physical aggregation and water holding capacity (WHC), carbon storage, respiration, nutrient cycling, and microbial community shifts. As the basis for studying combined physical (aggregation and WHC), chemical (pH, C, N, nutrients), and microbial (abundance and diversity) properties among differing soil types (silty loam and sandy loam), a greenhouse experiment was conducted (Study 1, Greenhouse Experiment) with maize and MPs amendments (types: PBAT, PE, and PET; concentrations: 0.1 and 1% w/w; size ranges: 75–400, 200–400, 75–200, and <75 µm) over 18 weeks. A 15N-labeled ammonium-nitrate fertilizer traced nutrient fate. Further complimentary studies were conducted: a respiration experiment assessed CO2 emissions, microbial biomass and community shifts (Study 2, Soil Respiration); a UV-weathering experiment evaluated accelerated photodegradation of PE and PET for size fragmentation and surface reactivity (Study 3, Plastic Reactivity); a method development for quantification of MPs in soil with thermal desorption-gas chromatography-tandem mass spectrometry (TD-GC-MS/MS) (Study 4, Method); and a conceptual viewpoint reconsidering the size definition of MPs in soil (Study 5, Viewpoint). Soil physical functions, aggregation and water retention, were minimally affected by CMPs. In contrast, BMPs enhanced microaggregate stability and WHC, but only under plant growth. This suggests soil structural improvement was mediated by biological functions, such as microbial activity and root exudations, which are more active in arable soils such as the silty loam. Sandy loam, with poor inherent structure, remained unaffected by MPs. As for chemical functions, MPs contributed to soil total carbon in proportion to their polymer carbon content. However, BMPs triggered microbial priming effects, as evidenced by increased CO₂ emissions and nitrogen immobilization. These effects were amplified in the nutrient-poor, unstructured sandy soil, where microbial communities likely responded more rapidly to the BMP-carbon inputs. CMPs, however, showed limited chemical influence unless degraded, as plant growth appeared to mask their effect on nutrient cycling. Microbial activity and community composition varied between soil and polymer types. BMPs stimulated microbial biomass and significantly altered prokaryotic community composition, particularly in sandy loam, which showed enrichment of microbial genera associated with plastic degradation and nitrogen cycling. This suggested that lower quality soils may be more microbially responsive to MPs inputs due to resource limitations. To evaluate potential long-term risks associated to increase in MPs surface reactivity, CMPs were artificially weathered to simulate environmental degradation. UV-weathered PE increased surface oxidation, hydrophilicity, negative surface charge, and cation exchange capacity (CEC), indicating increased environmental reactivity. In contrast, PET remained chemically stable under the same conditions. These findings demonstrate that degradation state critically alters CMP functions in soil, with PE potentially causing long-term risks to soil CEC and contaminant mobility. Methodological advances included the development of a mass-based method for polymer quantification in soils without cleanup. Here again, the role of soil type differentiation in MPs detection and interpretation became clear with the developed method, as it discovered plastics quantification requires correction for humic substance interference in organic-rich soils. As plastic size was critical to previous findings, a viewpoint emerged that challenges the established <5 mm definition of MPs as overly broad for soil systems. As most soil processes operate at the micro- to nanoscale, the thesis proposes a revised classification aligned with the SI: microplastics as 1–1000 µm and nanoplastics as 1–1000 nm. This refined framework would better align MP research with ecologically relevant soil process scales. In conclusion, BMPs demonstrated a dual role: enhancing physical structure in structured soils (silty loam) but disrupting chemical and microbial processes in vulnerable soils (sandy loam) due to its rapid biodegradability and microbial stimulation. CMPs, in contrast, showed longer-term risks primarily after degradation, with PE exhibiting high environmental reactivity after weathering. Collectively, these findings highlight that plastic effects in the environment are not universal but depend on polymer properties and soil-specific conditions. For agroecosystem risk assessments, it is essential to consider soil type, degradation state, and particle size when evaluating the sustainability of conventional or biodegradable plastic use in agricultural soils of varying quality.