Effect of Environmental Weathering on Microbial Assimilation of Biodegradable Mulch Films
Cowan-Banker, C. D., M. B. Anunciado, L. C. Wadsworth, and D. G. Hayes.  2018.  15th International Symposium on Bioplastics, Biocomposites, and Biorefining, July 24 27, 2018, Guelph, Ontario, Canada.

Abstract:
Plastic mulch films are an integral component in the production of vegetables and other specialty crops. Mulches prevent weed formation, evaporative loss of soil moisture, and reduce soil erosion, among other benefits. However, conventional polyethylene (PE) -based mulches are infrequently recycled and cannot be disposed of sustainably. Residual PE fragments remain in the soil or become dispersed in the environment, and as a result reduce soil fertility and contribute to the formation of micro- and nano-plastics in watersheds. Biodegradable plastic mulches (BDMs) have been developed to address the problems associated with PE mulches end-of-life problems. BDMs are designed to be plowed into the soil after crop harvest, where they will undergo biodegradation; or, they can be retrieved and then composted. However, the long-term impacts of BDMs on soil ecosystems has not been investigated. In this study, we are investigating the effect of environmental weathering on the biodegradability of BDMs under both ambient soil and industrial composting conditions. Environmental weathering during field trials for growing pie pumpkins in Knoxville, TN, has led to depolymerization and loss of mechanical strength, both of which may accelerate biodegradation. But, for some mulches, we have evidence that photodegradative reactions may have produced cross-linking, which would hinder biodegradation. We compared the time course of CO2 evolution for weathered vs. unweathered mulches by using standardized laboratory tests, ASTM D5988 and D5338 for soil and industrial compositing conditions, respectively. Biodegradation rates in soils were measured for an experimental polylactic acid and polyhydroxyalkanote film (PLA/PHA), and three polybutyrate (PBAT) -enriched BDMs that are commercially available in North America. The rate and extent of biodegradation in soil for weathered mulches was greater than for un-weathered PLA/PHA BDM, while the effect of weathering on biodegradability differed in trend between the three PBAT-enriched BDMs. Soil biodegradation resulted in a decrease of molecular weight for both PLA/PHA and PBAT-enriched BDMs. Thermogravimetric analysis demonstrated all major polymeric components of the four BDMs underwent a decrease of thermostability, likely due to depolymerization, and the PHA content decreased while that of PLA increased for the experimental BDM, indicating preferential utilization of PHA by microbes. Microscopic particles from the four BDMs (< 250 microns) were detected in the soil after biodegradation, as observed via microscopy. Under composting conditions an un-weathered PBAT-enriched mulch film produced more CO2 than its weathered counterpart. Therefore, the impact of weathering on biodegradation of BDMs differs between the environment, polymetric composition, minor components of the BDMs, and in trend between phases of the time course CO2 evolution.