Size-dependent effects of oxo-degradable plastic contamination on soil quality and the growth of $lt;I$gt;Zea mays$lt;/I$gt;

Agricultural plastic mulch film represents a significant source of microplastic contamination in soils, raising concerns about soil health and food security. Oxo-degradable plastics (ODPs) have emerged as a potentially more sustainable alternative to current plastic mulch films, however, uncertainty remains around the degradation rate of ODPs in soil and their impacts on soil quality and crop health. The study evaluated the dynamics and impact of different concentrations of micro- and macroplastics derived from a commercial d2w ODP (0.01%, 0.1%, 1% and 10% w/w) on the growth of Zea mays in an agricultural soil over a 6-week period. Chemical analysis revealed the ODP contained about 0.29% additives by weight, primarily comprising antioxidants (tris(2,4-di-tert-butylphenyl) phosphite and its oxidized form) and lubricants, with minimal heavy metal content. ODP degradation was mainly limited to chain scission and had only partial formation of oxygenated groups, without an increase in carbonyl groups. The rate of ODP degradation was found to be inversely related to the ODP concentration in soil. Overall, typical field levels of plastic contamination (0.01% w/w) had negligible effect on soil quality or plant performance. However, higher levels of ODP contamination resulted in significant changes in soil pH, EC, NO3− and bulk density. At extreme plastic loading rates (10% w/w), both micro- and macro-sized ODPs caused significant reductions in plant growth, with microplastic treatments having consistently greater effects than the macroplastic treatments. Changes in bacterial 16S rRNA community composition were driven by Acidobacteriota and Gemmatimonadota. Macroplastics significantly altered these bacterial communities, while microplastics had minimal effect. These findings indicate that at realistic field concentrations, ODPs are likely to have little effect on agroecosystem functioning in the short-term but might persist in soil for long periods of time, leading to their progressive accumulation in agricultural soils if used over repeated cropping cycles.