Microbial Communities Associated with Biodegradable Plastic Mulch Films in Two Agroecosystems.

While a promising alternative, there remains considerable uncertainty regarding the breakdown and fate of biodegradable plastic mulch films (BDMs) used in agricultural soils. A better understanding of the microbes involved in decomposing biodegradable mulches (BDMs) is needed. The aim of this project was to characterize microbial communities associated with, and therefore potentially degrading, four different BDMs at both laboratory and field scales. Two of these BDMs were polyester and starch based, one was a polylactic acid (PLA)+ polyhydroxyalkanoate-based experimental film, and the fourth was a PLA+ poly(butylene-adipate-co-terephthalate)-based film. Laboratory enrichment cultures with BDMs were used to select for BDM-degrading microbes, and mulch degradation was assessed by mass loss, microscopy and CO₂ evolution. Physicochemical testing of BDMs in cultures was completed using FTIR, gel permeation chromatography and thermogravimetric analysis. DNA was extracted directly from plastics and bacterial communities characterized by 16S rRNA amplicon sequencing on an Illumina MiSeq. Simultaneously, field weathered BDMs, which had been used for a full season for pumpkin production at two locations (Knoxville, TN and Mount Vernon, WA), were collected at the end of the growing season and the microbiome associated with the four plastic mulch treatments similarly assessed. Bulk soil from the field site was collected in Spring and Fall over two years in TN and WA, and the soil microbiome was assessed using 16SrRNA amplicon sequencing. Results indicated that plastic in laboratory cultures lost mass over time, with increased CO₂ evolution. Physicochemical testing suggested that starch was slightly preferred over the polyesters in microbial conversions. Single colony isolation confirmed the presence of Streptomyces sp, Arthrobacter sp., and Pseudomonas sp. (98%, 95%, 96% identities respectively) in the enriched consortia. In both laboratory and field studies, microbial communities were altered on the plastic surfaces relative to bulk soil, indicating an enrichment of microbes that were likely degraders.