Vertical transport of microplastics from agricultural mulching films and associated chemical additives in soil ecosystems

Agricultural soils receive substantial inputs of microplastic pollution from a range of different sources. The degradation and fragmentation of mulching films during and after use is expected to represent an important source of microplastic particles to soils, backed up by emerging evidence from monitoring activities. Yet, the transport and fate of these particles after they enter soil environments remains a persistent knowledge gap. This is important as soils may effectively retain these particles – resulting in increasing pollution with successive inputs – or particles may be mobilised from soils, contaminating other environments such as groundwater or surface waters. The processes that control retention versus export of particles have been poorly constrained and are expected to be complex. Chemical additives are routinely added to plastic mulching films to bestow a range of specific material properties. The extent to which these chemicals may leach out of particles in soil environments, and their potential to be transformed or mobilised in soils following release is not well known.This study tracked the vertical transport of microplastic fragments derived from two relevant mulching films and associated chemical additives within soils: one biodegradable mulching film and one conventional plastic mulching film. The study specifically investigated the influence of factors expected to exert a control on particle/chemical fate: bioturbation and soil water inputs. The experiment was conducted in the CLIMECS (CLImatic Manipulation of ECosystem Samples) facility at Vrije Universiteit Amsterdam, which comprises 40 soil columns that simulate ecosystems – with soil, vegetation, and fauna – and are individually controlled for different environmental conditions. The two types of microplastic fragments were added to the upper layer (10 cm) of soil columns (total length: 40 cm) to represent a contaminated plough layer. Different treatments consisted of high and low microplastic concentrations, high and low watering regimes, and the presence and absence of earthworms. The columns were maintained for a period of twelve weeks. Microplastic and chemical additives content was measured in six different depths within each core at the end of the three months, to assess the extent of vertical transport. The results from this study reveal important insights on the mobility of mulching film fragments within soil systems and elucidate some of the important controls on particle movement. This provides crucial context related to the exposure of soil environments to soil microplastic and chemical additive pollution derived from mulching film use.