Organic fertilizers from biowaste fermentation act as a vehicle for microplastic particles to enter the terrestrial environment, with the amount of microplastic particles differing based on pre-treatment methods and plant type, a new study shows. Recycling of organic waste through composting or fermentation, followed by application of the resulting fertilizer products to agricultural land, is considered environmentally friendly, but the results of this study challenge that common perception and highlight an overlooked source of pollution. Due to their small size, microplastic particles (MPPs) -- mainly fragments, fibers and spheres of plastics, all smaller than five millimeters -- presumably enter the food web, and may even end up in human food. Considerable attention has thus been given to MPPs, but largely to those discovered in the ocean. Only more recently have researchers expanded their study to include microplastic pollution in terrestrial environments. To better understand MPP production and initial entry into terrestrial ecosystems, Nicolas Weithmann and colleagues screened different organic fertilizers produced at recycled biowaste plants for MPPs greater than one millimeter in size. Their screens focused on outputs of biowaste composting plants (which processed biowaste from households) and on outputs of biowaste digester plants (which create biogas from manure). As a reference, they also looked at MPP loads from agricultural energy crop digestion plants, which process only energy crops like corn, and no biowaste. All fertilizer samples converting biowaste contained MPPs, they report - usually between two and five mm in size -- while none of the samples from the agricultural energy crop digester plants contained significant amounts of the tiny plastics. The amount of MPPs from biowaste treatment differed based on pretreatment and plant type; the MPP load produced from biowaste digester plants, for example, was greatest. In particular, the biowaste digester plant directly supplied by waste from commerce contained the highest numbers of MPPs (895 MPPs per kilogram of dry weight). Based on their findings, further study into the possible consequences and impacts of MPP contamination from these types of biowaste fertilizers is necessary before any risk can be assumed, Weithmann et al. say.