Impact of plastic mulch and their associated leachates on seed germination

The benefits of plastic mulch in agriculture, including higher crop yields, early seedlings development, and earlier harvests, are countered by challenges associated with their complete removal from the soil and proper disposal, ultimately contributing to plastic pollution. Plastic mulching is identified as a major source of microplastic pollution in agricultural soils, with an increasing number of reports indicating its impact on both soil and plant health. However, the causes of the adverse effects are still not clear. The risk posed by microplastics can arise from the plastic particles or as a consequence of the toxicity of chemical additives and plastic-derived compounds, which can be leached over time. In this study we assessed the impact of macro- and microplastic particles, along with their associated leachates, from both conventional (LDPE-based) and biodegradable (PBAT-based) mulch films on the germination of three plant species: arabidopsis (Arabidopsis thaliana), cotton (Gossypium hirsutum L.), and tomato (Solanum lycopersicum). We developed a comprehensive methodology for leachate extraction from macro and microplastics at concentrations of 0.2% and 2% w/w. This process involved monitoring various parameters over time including electrical conductivity (E.C), pH, total dissolved solids (TDS), and UV-visible absorbance indices. Additionally, we employed a semi-automated germination scoring pipeline, obtaining cumulative germination data that allowed the estimation of multiple germination parameters, including the maximum germination capacity (Gmax), the time required for 50% of the seeds to germinate (t50), and the area under the germination curve (AUC). Leachates from PBAT-based mulch showed a significant increase in the solution electrical conductivity (E.C), total dissolved soils (TDS), and absorbance indices for both macro and microplastics over time. Furthermore, a gravimetric weight loss of the biodegradable mulch samples occurred, indicating the release of certain compounds to the solution. Although there were no significant changes in the assessed proxies of leachates derived from LDPE-based mulch after one week, the leachates started to be released more pre-eminently after 35 days.  The seed germination assay revealed a negative effect of the leaching solution from the higher concentration of PBAT-based  leachates on the germination of arabidopsis seeds, as indicated by reduced Gmax(%), increased t50, and decreased AUC in comparison with the control. Although a slight reduction in germination was observed for the other plant species, the values were not statistically significant. When testing the impact of leached or new microplastic particles on the seeds, they did not significantly impact the seed germination parameters. This suggests a potential toxic effect of plastic leachates rather than the microplastics themselves. Further detailed characterization of leachate solutions will be conducted to gain a better understanding of the leaching process and the compounds released to determine the potentially hazardous substances affecting seed germination. These findings can offer valuable insights to the plastic industry, indicating the need to reduce or replace certain plastic building blocks and additives while exploring environmentally friendly alternatives. Moreover, the study underscores the need for increased attention to the environmental repercussions of plastic pollution in agriculture and its potential effects on plant health.