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Effects of biodegradable plastics on soil properties and greenhouse gas production
Summary
Biodegradable plastics made from polylactic acid and polybutylene succinate were found to affect soil physicochemical properties and increase CO2 and N2O gas production during decomposition in soil, raising questions about whether they truly avoid the environmental impacts of conventional microplastics.
Microplastics cause environmental problems. Biodegradable plastics have become popular because they aim to avoid such problems. However, their decomposition in the soil may have an impact. This study aims to investigate the effects of biodegradable plastics on the physicochemical properties of soil, specifically the production of CO2 and N2O in the soil and plant growth. Three kinds of biodegradable plastics in the forms of 1) nonwoven fabric sheets made of poly-lactic acid (PLA) and polybutylene-succinate (referred to hereafter as fabric), 2) laminate sheets made of polybutylene adipate terephthalate (PBAT) and pulp (hereafter laminate), and 3) drinking cups made of PLA (hereafter cup), were cut into small pieces (<5 mm), added to soil, then water-holding capacity was determined and incubated aerobically for 4 weeks at 30°C in the dark. Soil and gas samples were collected weekly to measure soil pH, nitrate-nitrogen content, CO2, and N2O productions. These plastics were also tested in a pot experiment with Komatsuna (Brassica napa var. perviridis). We tested for seed germination, plant growth, leaf color, and fresh weight at harvest. Results showed that the water retention capacity was higher in the fabric plastics as compared to the cup plastics and the control. Soil pH with the fabric plastics dropped during the initial 2 weeks of incubation, then recovered to a similar pH to the control (without plastic). Nitrate contents in the soil with laminate plastics were lower than those in the control, while CO2 production in the soil with the laminate plastics was higher than that in the control and the other plastics during the incubation period, and even higher than the one of added plastic-C. N2O was produced rapidly within 1 week of incubation in the soil with the laminate plastics, and cumulative N2O production from incubation was more than that of the control. The seed germination and plant growth tended to be suppressed in the pot experiment with the fabric and laminate plastics. The results indicate that the influence of these biodegradable plastics on soil properties, greenhouse gas production, and plant growth on the kind of plastic and the timing.
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