Biodegradable mulch films exhibit slower-than-expected degradation with negligible effects on soil microbial communities

The adoption of biodegradable plastic mulch films (PMFs) has increased as a sustainable alternative to conventional PMFs. However, evidence suggests that current biodegradability standards may not reflect in-field conditions, leading to potential accumulation of biodegradable PMFs in soil with yet undetermined effects on soil microbial communities. We therefore conducted a field experiment to investigate the above and below ground degradation of one conventional and seven biodegradable PMF blends in a temperate climate (UK) over 12 months. We found that PMFs had no effect on soil properties and a negligible effect on microbial community structure. The main factors driving change in relative abundance of bacteria and fungi were exposure time, followed by the potential of meso- and macrofauna interactions. PMFs buried in soil showed no sign of fragmentation, regardless of presence of meso- and macrofauna, which was likely the cause for no overall difference in microplastic concentrations. The slow degradation of all PMFs combined with the lack of physical fragmentation under natural conditions, indicates that degradation is slower than expected according to current biodegradability standards. This highlights the importance of reframing standards by treating degradation as a system property, influenced by a variety of environmental and intrinsic factors