Temperature impacts polymer biodegradation rates in soils in predictable ways but with dependencies that differ between soils

Biodegradable polymers can play an important role in helping to overcome the plastic pollution problem by replacing conventional, persistent polymers in specific applications. These include applications in which plastics are used directly in the environment and cannot be completely recollected (e.g., mulch films and seed coatings) and for bags used to collect biowaste for industrial composting. Temperature is an important system factor that determines the rate at which biodegradable polymers biodegrade in both natural and engineered environments. Yet, there is a limited quantitative understanding of how temperature impacts polymer biodegradation rates in the open environment, specifically in soils. Temperature not only affects the activity of soil microorganisms but also their extracellular enzymes that hydrolyze backbone bonds in biodegradable polymers. Here, we assessed the impact of temperature on the biodegradation of poly-3-hydroxybutyratehydroxyhexanoate (PHBH) in agricultural soils. We incubated PHBH in three different standards soils and at four temperatures (5, 15, 25 and 35 ºC). We determined the amount of residual PHBH in soil over time by extracting PHBH from the soil using a chloroform-methanol mixture and then quantifying the extracted polymer using proton nuclear magnetic resonance spectroscopy (¹H NMR). We find that the rate of PHBH biodegradation increased with increasing temperatures in all three soils, but that the rates and temperature dependence of the rates variedbetween soils. The fastest biodegradation occurred in LUFA 6S (clay) followed by LUFA 2.4 (loam), and the slowest biodegradation was in LUFA 2.2 (sandy loam). The soil-dependence likely reflects differences in the abundance and activity of microbial degraders in these soils. At lower incubation temperatures, there was a noticeable lag-phase prior to the onset of biodegradation, which was most pronounced in soil LUFA 2.2 . When the lag-phase is included in the kinetic modeling, the temperature-dependence of the PHBH biodegradation rate can be described reasonably well by the Arrhenius rate law but differs between soils. We further investigated the microbial colonization dynamics of PHBH film surfaces during the lag-phase using optical and scanning electron microscopy. After incubation of solvent-cast PHBH films in the soil at the aforementioned temperatures, microscopic analyses revealed that fungal hyphae were involved in both colonization and initial breakdown of the PHBH films, and that fungal activity increased with increasing temperature. Taken together, these results suggest that a careful determination of the temperature dependence of polymer biodegradation in different soils is needed to predict, from standard tests run at elevated and constant temperature, how quickly biodegradable polymers will biodegrade in the open environment where temperatures are lower and variable.