Impact of Agricultural Weathering on Physicochemical Properties of Biodegradable Plastic Mulch Films: Comparison of Two Diverse Climates Over Four Successive Years

Biodegradable plastic mulch films (BDMs) are essential in the production of vegetable and specialty crops due to their promotion of increased crop yield and quality. Unlike conventional polyethylene (PE) mulches, BDMs can be tilled into the soil after crop harvest to undergo biodegradation, thereby leading to minimal environmental impact. Agricultural weathering impacts both the performance of BDMs during crop production as a barrier to weeds and biodegradability of BDMs in the soil. To better understand the relative importance of climatic factors, the change of physicochemical properties of BDMs during single-season, 3-4 month, field trials for vegetable production at two diverse climates (Knoxville, TN and Mount Vernon, WA) across four successive years (2015-2018) was evaluated. Mulch treatments consisted of four commercially available BDMs composed primarily of polybutylene co-adipate- co-terephthalate (PBAT) that differed in color and polymeric feedstock, a black experimental BDM prepared from polylactic acid/polyhydroxybutyrate (PLA/PHA) blend, and conventional PE mulch. Solar radiation, an important factor to degradation of mulches, was higher in WA than TN in most sampling years. Yet, degradation occurred more greatly for BDMs in TN, which is attributable to higher temperatures in TN. Mulch deterioration did not very extensively between years. Loss of mechanical properties and color was greater than chemical property changes. Differences in the extent of molecular weight decrease between years correlated significantly with solar radiation exposure at the two locations. A black-colored PBAT-based BDM was less susceptible to degradation than equivalent clear and white-on-black films, due to carbon black acting as a photostabilizer. The impact of weathering also differed between three commercially available PBAT-based films. The PLA/PHA mulch was more susceptible to degradation than PBAT-based BDMs, particularly in the warmer location, TN, partially due to a leaching out of PHA and lower-molecular weight polymer molecules. The extent of change for physicochemical properties of BDMs due to agricultural weathering is greatly affected by polymeric composition, and is greater in warmer climates.