Effect of Polyethylene Microplastic and Electrical Conductivity of Composts on Seed Germination

Commercial-scale conversion of food and organic wastes into compost emerges as a promising solution to enhance waste management and promote organic farming. Compost and fertiliser might pose little concern for microplastic contaminated soil and agricultural products. The microplastics might be either stimulants or inhibitors in germination. Mung bean seeds were employed in this research due to their widespread use in various authentic dishes as bean sprouts. Polyethylene (PE) microplastic with an average particle size of 125 µm, with ultra-high molecular weight and surface modified powder, was used in this study. Food wastes from the dining and kitchen were composted to produce the organic compost. In this research, four composts from different sources were examined: T1, T2 and T3 referred to composts from dining food waste, kitchen waste (non-edible vegetables and fruits), and the conversion of kitchen waste to vermicompost by earthworms, respectively, while T4 was the chemical organic fertiliser derived from the chemical extraction of human hair. The food waste from dining had high salt and seasoning content, resulting in high specific electrical conductivity (EC) values in the T1 treatment, whereas food waste from the kitchen consisted of non-edible vegetables and fruits, leading to slightly lower EC values in the T2 treatment. The T3 treatment exhibited higher EC than T2, while T4 had the highest EC values among all treatments. The high EC values in compost or fertiliser refer to the high ionic strength. The composts (T1, T2, and T3) and chemical organic fertiliser (T4) were diluted with DI water at different mass ratios (1:1, 1:10, and 1:20) to examine the phytotoxicity of each compost to mung bean seeds. Control treatments included deionised (DI) water (C1) and PE microplastic-contained solutions at different concentrations (0.2-1.0% w/v) (C2). One hundred mung bean seeds underwent a seed germination bioassay. After soaking in solutions for 8 h at room temperature, they were placed on tissue paper inside closed containers, and wrapped to prevent light exposure. Every 2 days, 5 mL of solution was sprayed to maintain moisture. The number of germinated seeds was counted, and root lengths were measured after 5 days of incubation using thread for accuracy due to their non-linear form. The relative seed germination (RSG), relative radicle growth (RRG) and germination index (GI) were then analysed. Results indicated that C2 was less phytotoxic, even at 1.0% w/v of PE, and the threshold limit concentration was observed at 0.8% w/v, while a PE concentration of 0.4% w/v was a stimulant for seed germination due to the highest GI. The germination test inferred that not only the microplastics but also the EC of fertiliser can be phytotoxic. The composts T1 and T2 had threshold limit concentrations of 1:10 and 1:20 (% w/v), respectively. Both T1 and T2 underwent the same composting procedure, but the effective microbes (EM) were unable to reduce the salt content in the waste. As a result, T1 exhibited stronger phytotoxicity than T2, attributed to its higher ionic strength. In contrast, the earthworms slightly increased the ionic strength of vermicompost (T3), with a threshold limit concentration of 1:20 (% w/v). However, severe phytotoxicity was observed at a concentration of 1:1 (% w/v). Conversely, T4 acted as an inhibitor, even at the lowest concentration of 1:100 (% w/v). The presence of microplastics, combined with high EC, raises concerns when the organic composts contaminated with microplastics are applied to agricultural fields.