Size-dependent effects of oxo-degradable plastic contamination on soil quality and Zea mays L. performance

Agricultural plastic mulch films represent 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 eco-friendly alternative to conventional plastic mulch films, however, uncertainty remains around the degradation rate of ODPs in soil and their impacts on soil quality and crop health. Using a controlled mesocosm experiment, we evaluated the behaviour and impact of different concentrations of oxo-biodegradable macro- (>10 mm) or micro-plastic (<5 mm) (0.01, 0.1, 1, and 10% w/w) on the performance of Zea mays L. grown in an agricultural soil over a 6-week period. Contrary to expectation, no major fragmentation or degradation of ODP was observed during the experimental period, however, FTIR revealed the formation of carbonyl groups indicative of oxidation. Overall, our results showed that typical levels of plastic contamination (0.01% w/w) had very little effect on soil physicochemical properties, microbial activity or plant performance. However, higher levels of plastic contamination resulted in significant changes in soil pH, EC, NO3-, bulk density, and soil moisture. At extreme plastic loading rates (10% Can you write w/w), both micro- and macro-sized ODPs caused significant reductions in plant height and foliar chlorophyll content, with microplastic treatments showing consistently greater effects than macroplastics. None of the plastic loading rates had an effect on shoot and root biomass or soil NH4+ and P concentrations or microbial community structure in comparison to the unamended controls. Our findings indicate that at realistic field concentrations, ODPs are likely to have little effect on agroecosystem functioning but that they may persist in soil for long periods of time leading to their progressive accumulation in agricultural soils if used over repeated cropping cycles. Further research is needed to evaluate the longer-term impacts of ODPs on soil quality and crop health under field conditions.