Impacts of conventional and alternative plastics on soil ecosystems

Plastic is an essential component of agriculture globally, but its widespread use has resulted in significant environmental pollution. As biodegradable and bio-based alternatives gain attention as replacements for fossil-based, non-degradable plastics, the ecological impacts of these alternatives remain poorly understood, particularly as they degrade and release leachates. This research aimed to address critical questions: How do conventional and alternative plastics and their leachates affect soil health, plant performance, and soil organisms? Do biodegradable and bio-based plastics offer ecological benefits over conventional plastics? What mechanisms potentially drive these impacts in soil-plant systems? To answer these questions, the research hypothesised that both conventional and alternative plastics and their associated leachates negatively impact soil and plant systems by disrupting soil processes, altering planting stress responses, and shifting soil fauna behaviours, with biodegradable plastics causing similar disruptions to conventional plastics. These hypotheses were tested through a combination of laboratory and field experiments. The findings demonstrate that both conventional and alternative plastics, whether as microplastics or mesoplastics, and whether in pristine or aged from, or as leachates, adversely affect soil and plant systems. Exposure to plastics consistently reduced plant biomass and altered chlorophyll content, while increasing oxidative stress in two agronomically important plant species, Lolium perenne and Hordeum vulgare. In soils, exposure to plastics altered water content, organic matter, and pH, while increasing CO2 respiration rates. These changes indicate disrupted soil processes, including water flow and carbon cycling, which are critical for ecosystem health. Biodegradable plastics, including bio-based polymers (e.g., polylactic acid and polyhydroxybutyrate) and fossil-based alternatives (e.g., oxo-biodegradable polyethylene), caused effects broadly similar to those of conventional plastics (polyethylene and polypropylene), challenging the assumption that these materials are inherently safer for the environment. Notably, leachates from biodegradable fossil-based polymers induced significant stress in plants and altered soil properties, raising concerns about their long-term ecological viability. Field experiments further showed reduced plant yields and declines in the diversity and abundance of soil faunal groups such as Collembola, with potential implications for soil ecosystem functioning. The research also explored the underlying mechanisms of these impacts. Soil fauna exhibited behavioural responses, such as avoidance or attraction to soil contaminated with mesoplastics and their leachates, indicating possible disruptions in soil ecosystem processes. Physical analyses demonstrated that mesoplastics impaired soil hydrological properties, decreasing water flow and increasing soil bulk density, with potential consequences for plant growth and water distribution. This comprehensive investigation highlights the ecological risks of both conventional and alternative plastics in agricultural systems. The findings call for a rigorous evaluation of alternative plastics, stricter regulatory frameworks, and the development of truly sustainable agricultural practices to mitigate the environmental impacts of plastic use.