Temperature Sensitivity of Biodegradable Plastic Mulches to Microbial Decomposition.

Plastic agricultural mulches provide weed control, moisture retention, soil temperature optimization, and decreased soil erosion in the production of fruits and vegetables. Because conventional mulches are composed of polyethylene, a poorly biodegradable plastic, few sustainable disposal methods are available. Many landfills will not accept them due to the presence of adsorbed pesticides, and few recycling options exist. A more sustainable alternative is the use of biodegradable plastic mulches (BDMs) which, after being tilled into the soil subsequent to service life, biodegrade into carbon dioxide, water, and microbial biomass as part of the soil carbon cycle. However, the role of soil temperature in biodegradation of these carbon-based materials is not well known. A manipulative incubation experiment was conducted to look at in situ biodegradability of three types of mulch films: a poly lactic acid/ polyhydroxyalkanoate experimental product (PLA/PHA), a starch/polyester blend, and a cellulosic paper mulch (the latter serving as a control) at different soil temperatures (10, 20, and 30°C). The evolution of CO₂ was measured biweekly over 16 weeks to monitor the time course of biodegradation. In addition, total organic carbon, dissolved organic carbon, and microbial biomass carbon measurements provide insight into the transport and accumulation of plastic mulch material, to help understand the underlying mechanisms. There were significant differences in the cumulative CO₂ produced during the 30°C incubation in the starch/polyester and PLA/PHA trials when compared to the no mulch control (0.624±0.012 mg C g^-1 dry soil, 0.596±0.035 mg C g^-1 dry soil and 0.440±0.003 mg C g^-1 dry soil, respectively) indicating microbial assimilation of the mulch. Cumulative CO₂ produced by the starch/polyester and PLA/PHA mulches increased with temperature with the starch/polyester blend having the highest cumulative CO₂ at each temperature. The significant influence of air temperature on the rate of degradation for biodegradable mulches could help determine the impact of climate and environment on microbial assimilation.