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.

Additives in agricultural plastics can leach into the surrounding soil during use or improper disposal. Their subsequent degradation rates directly regulate whether they persist and accumulate to levels with ecotoxicological effects or are rendered benign. However, which soil properties primarily regulate the degradation of additives remains unclear (e.g. soil carbon, pH, available nutrients, microbial biomass and community structure). We assessed the degradation of the common plastic additives with different functionalities (DEHP (di(2-ethylhexyl) phthalate; plasticiser), 2-hydroxy-4-n-octyloxybenzophenone (benzophenone-12; BP12; UV stabiliser) and AO168 (tris(2,4-di-tert-butylphenyl) phosphite; antioxidant)) in soils under controlled moisture and temperature conditions over 21 days across contrasting agricultural soils from six countries across a global transect (Australia, Brazil, Egypt, India, Vietnam and the UK). DEHP followed zero-order degradation kinetics, with negligible degradation in soils with low microbial biomass. BP12 degraded fastest via first-order degradation kinetics via ether cleavage and hydroxyl loss. The degradation of DEHP and BP12 was correlated with soil microbial biomass and nitrate concentration. BP12 degradation products detected included benzophenone and benzoic acid. DEHP is degraded via β-oxidation of alkyl groups to dibutyl phthalate and diethyl phthalate and through ester hydrolysis to phthalic acid. AO168 degraded via abiotic oxidation and phosphate ester hydrolysis to 2,4-di-tert-butyl-phenol, and degradation was not well correlated with any measured soil variable. Overall, these results show that the components of additive mixtures leached into soils will degrade at different rates due to varying mechanisms and controls exerted by the soil microbial biomass. Plastic additives have differing potentials to persist in agricultural soils globally, with some likely to accumulate to levels that may impact soil function and pose an ecotoxicological threat to soil biota.