Microplastic incorporation into soil aggregates of arable land

Abstract

Agricultural mulch films are widely applied to increase crop productivity but are a significant source of microplastics (MPs) in the environment. We investigated the incorporation of conventional and biodegradable MPs into soil aggregates from agricultural fields in Finland, Germany and Spain, hypothesising that (i) MPs become increasingly occluded within soil aggregates over time, (ii) the extent of occlusion differs between conventional and biodegradable MPs, and (iii) occluded MPs differ in morphology and surface characteristics from free MPs. Arable fields were spiked with MPs derived from either a biodegradable polybutylene adipate terephthalate-starch blend (PBAT/starch) or conventional linear low-density polyethylene (LLDPE) mulch films. Soil samples were collected after two barley-growing seasons, separated into free and occluded fractions, and analysed using digital microscopy and deep learning to quantify MP abundance, size and shape. Surface properties were assessed using scanning electron microscopy. Total MP abundance declined between cropping seasons for both polymer types, while the proportion of aggregate-occluded MPs increased, reaching 76% for PBAT/starch and 65% for LLDPE. Soil exposure altered the size, shape and surface properties of MPs in a polymer-specific manner. Occluded PBAT/starch MPs were larger and more solid than their free counterparts, consistent with protection against weathering and fragmentation within aggregates. Occluded LLDPE MPs were smaller than free particles but also exhibited higher solidity, likely reflecting preferential incorporation of particles with smoother, more uniform edges. We conclude that soil aggregate occlusion is a key mechanism governing MP fate in agricultural soils, promoting particle stability while simultaneously reducing mobility and bioavailability.