Degradation of microplastic seed film-coating fragments in soil

Authors: ACCINELLI, CESARE - University Of Bologna; Abbas, Hamed; SHIER, W.T. - University Of Minnesota; VICARI, ALNERTO - University Of Bologna; Little, Nathan

Submitted to: Chemosphere
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/25/2019
Publication Date: 3/29/2019
Citation: Accinelli, C., Abbas, H.K., Shier, W., Vicari, A., Little, N. 2019. Degradation of microplastic seed film-coating fragments in soil. Chemosphere. 226:645-650.

Interpretive Summary: Seeds can be coated with petroleum-derived plastic coatings or with biodegradable plastic coatings, which can carry biological control agents or commercial chemicals such as insecticides. This research showed that for corn seeds, bioplastic coatings had fewer small pieces that got dislodged during processing like handling or planting than petroleum-derived plastic coatings, and the small pieces of seed coatings that got dislodged had different rates of degradation in soil. The biodegradable bioplastic seed coating fragments completely degraded in soil within 32 days and degradation was faster when anti-fungal bacteria were incorporated into the coating (24 days). In contrast, petroleum-derived plastic seed coating fragments required 48 days to degrade in soil. This study shows that the use of bioplastic seed coatings reduces both the amount of seed coating fragments entering soil and how long they persist there, both of which should decrease the environmental impact of any pesticides incorporated in them.

Technical Abstract: Encapsulating fungicides and/or insecticides in film-coatings applied to agronomic seeds has become a widely accepted method for enhancing seed germination and overall seedling health by protecting against many diseases and early-season insect pests. Despite advancements in seed film-coating technologies, abrasion of the seed coating can occur during handling and mechanical planting operations, resulting in variable amounts of detached fragments entering the soil. The present study investigated the degradation in soil of these plastic-like, small-sized fragments, referred to here as microplastic coating fragments. Degradation of microplastic coating fragments in soil was found to be highly variable. The lowest degradation rate (=48 days) was observed in fragments detached from seeds coated with a commercial polymer mixture, while fragments from a biodegradable plastic formulation degraded completely within 32 days. When spores of the plant growth-promoting bacterium, Bacillus subtilis, were incorporated into the bioplastic, degradation was even more rapid (=24 days). The fragment degradation rate was unaffected by incorporating two commonly used neonicotinoid insecticides, imidacloprid or thiacloprid, into either coating formulations, but insecticide dissipation rates in soil were more rapid when added associated with seed coating fragments than when spiked in directly. Half-lives of these two insecticides were reduced by up to 27% in fragments from bioplastic-coated seeds. These results are consistent with variable and not easily predicted soil degradation rates for seed coating fragments, with enhanced dissipation of coating-entrapped pesticides and with a higher degradation rate for biodegradable seed coating incorporating selected microbial strains.