Temperature Dependence of Poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) Biodegradation in Agricultural Soils

Biodegradable polyesters are increasingly used in agricultural applications with soil biodegradation as the targeted postapplication end-of-life. Soil biodegradability is typically established in laboratory soil incubations at constant temperatures exceeding those in many field soils, highlighting the need for robust temperature-biodegradability relationships to transfer laboratory-determined biodegradation rates to field conditions. This work systematically assesses the temperature dependence of the biodegradation of three poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBHHx) variants, containing differing molar amounts of 3-hydroxyhexanoate (where x = 5, 9, and 12%) in three agricultural soils. Scanning electron microscopy images of PHBHH9 films incubated for 18 days showed increasing film surface colonization by fungi and/or filamentous bacteria from 5 to 15, 25, and 35 °C, with dense hyphal networks and film disintegration at the higher tested temperatures. Soil biodegradation of PHBHHx powders over the same temperature range was followed by extracting and quantifying residual PHBHHx mass over time by proton nuclear magnetic resonance spectroscopy. Fitting of these residual masses with a shoulder-log linear kinetic model yielded PHBHHx-variant-independent initial lag phases and biodegradation rate constants. Increasing the temperature shortened the initial lag phase and increased the subsequent rate of PHBHHx biodegradation. Arrhenius rate law analysis revealed soil-specific activation energies that correspond to approximately 2- to 5-fold changes in biodegradation rates per 10 °C. This work establishes temperature as a key abiotic factor controlling polyester biodegradation rates in soils and guides more accurate extrapolation of laboratory-determined biodegradation rates at elevated temperatures to field conditions.