Samantha Viljoen

Plastic plays an integral role in modern agriculture, improving crop yields and facilitating food transportation and preservation. However, its extensive use has led to significant environmental challenges, including soil pollution. This research examines the degradation of agricultural plastics, with a focus on the release and ecological impacts of plastic additives on agricultural soil ecosystems. The utilisation of 14C-labelled phthalate acid esters (PAEs) demonstrated that the plastic matrix significantly inhibits additive degradation. The incorporation of dibutyl phthalate (DBP) and di(2-ethylhexyl) phthalate (DEHP) into plastic films reduced soil degradation rates from 79% to 21% for DBP and from 9% to <1% for DEHP over four months. These findings indicate PAEs may persist in soil environments for extended periods, despite evidence that biosolids addition promotes PEA turnover and ultraviolet radiation accelerates PAE mineralisation twofold. Building upon these degradation findings, ecotoxicological assessments revealed a concentration-dependent impact of plastic additives on soil organisms. Experiments with the soil nematode Pratylenchus neglectus demonstrated that additives from both conventional and biodegradable plastics induce significant mortality and oxidative stress. Notably, some biodegradable plastic additives, such as Tartrazine, exhibited more severe ecological effects than their traditional counterparts, challenging assumptions about their environmental safety. To reflect real-world conditions, we conducted a year-long field trial in the agricultural ‘wheat belt’ region of Western Australia (Northam), monitoring the degradation pathways of conventional and biodegradable plastic mulch films. Analytical techniques, including photo image analysis and Fourier Transform Infrared (FTIR) spectroscopy, revealed different degradation patterns across environmental conditions. Significant spectral changes were observed, with above-ground treatments showing more pronounced degradation compared to below-ground environments. Overall, these comprehensive studies highlight the complexity of agricultural plastic degradation as influenced by polymer type, additives, and environmental factors. The findings provide valuable insights for developing more sustainable agricultural plastic products and improved end-of-life management strategies.