Biodegradation of commercial mulch films in Swiss agricultural soils: Results from combined laboratory, mesocosm and field incubations

Non-biodegradable polyethylene mulch films are widely used in agriculture to allow for an extended growing season and to increase crop yields. These mulch films are, however, difficult to completely recollect from the field after use, particularly when they are thin (< 25 µm). Residual mulch film pieces can accumulate in soils over time, thereby negatively impacting soil productivity and possibly turning agricultural soils into sources of plastics to surrounding environments. Mulch films certified as biodegradable in soils promise to be a solution to these problems. While such mulch films are already commercially available, a thorough assessment of the biodegradation dynamics of biodegradable mulch film products in soils in the field is lacking. So far, certification relies exclusively on laboratory soil incubations coupled to respirometric analysis of CO₂ formed from the mulch films during biodegradation. Respirometric analyses are, however, very challenging to implement in field incubation studies. Past studies determining concentrations of biodegradable mulch films in field soils and attempts to follow their biodegradation dynamics in the field have relied on approximate quantification approaches, such as determining the decrease in surface area or gravimetric mass of film pieces recollected by hand. To advance a more robust and quantitative analytical approach for residual mulch film quantification in soils, we present a methodology to solvent extract and quantify the main synthetic polymeric components of commercial biodegradable mulch films, poly(butylene adipate-co-terephthalate) (PBAT) and polylactic acid (PLA), from soil. The methodology is based on exhaustive Soxhlet extraction using chloroform/methanol coupled to quantitative ¹H-NMR of the extracted residual PBAT and PLA. We show full recovery of these polymers added to soils in spike-recovery experiments. Here, we use this approach to assess the biodegradation of two commercial biodegradable mulch films in three Swiss agricultural soils in a multiyear incubation study. These incubations are conducted at three experimental incubation scales: flasks in the laboratory, mesocosms in a greenhouse and the actual field. We statistically compare biodegradation rates and extents between three soils, two tested films across the three incubation scales, as well as differences in the relative rates of biodegradation between PBAT and PLA. Thereby, we assess the transferability of biodegradation results from laboratory incubations to field incubations. Our results highlight variations in biodegradation between soils and polyesters and indicate that laboratory soil incubations show faster biodegradation than measured in the same soil in the field.