Effects of climate, depth, and exposure history on the degradation of biodegradable plastic mulch fragments in soil under field-relevant conditions

Biodegradable plastic mulches are a promising alternative to manage the plastic waste produced by the use of agricultural mulches. Biodegradable plastic mulches are designed to biodegrade in the fields where they are used, turning into carbon dioxide and microbial biomass. This would reduce labor and costs associated with waste management and prevent environmental contamination by plastic waste. Existing biodegradable plastic mulches are certified as biodegradable based on laboratory testing that provides limited information about the behavior of these biodegradable plastic mulches in diverse field conditions. I investigated how one biodegradable plastic mulch fragmented in field conditions in two field sites with distinct climates, the capacity of microplastics and nanoplastics derived from the biodegradable plastic mulch to degrade in field-like conditions and in subsurface soil, and how soil biodegradation might respond to repeated addition of this biodegradable plastic mulch. To do so, I developed a method to quantify microplastics and nanoplastics composed of the biodegradable polymer polybutylene adipate co-terephthalate, the main constituent of the mulch I studied, using nuclear magnetic resonance spectroscopy. In field conditions in Knoxville, Tennessee and San Luis Obispo, California, plastic mulches fragmented into microplastics within 6 months of burial, but total soil plastic did not decrease over 18 months of burial near the soil surface. In laboratory incubations, microplastics and nanoplastics did not significantly biodegrade over 9 months in a Typic Hapludult loam (Shady series, Knoxville, TN) at 20 degrees Celsius, nor in a Pachic Argixeroll (Elkhorn series, Prunedale, CA) over 6 months at the same temperature. The microplastics I tested may degrade 10% more quickly in soil that has been previously exposed to biodegradable plastic compared to similar soil that has not. Because of the slow degradation I observed at two field locations and at field-relevant temperatures in laboratory incubations, I suggest that these products need further development before they are suitable for repeated use in agricultural production systems under the studied conditions. The presence of large plastic fragments and microplastics in the soil, even if temporary, could have negative effects on agricultural productivity and unknown effects on agroecosystems.