Field application of pure polyethylene microplastic has no significant short-term effect on soil biological quality and function

Abstract

Plastics are now widespread in the natural environment. Due to their size, microplastics (MPs; defined as particles <5 mm) in particular, have the potential to cause damage and harm to organisms and may lead to a potential loss of ecosystem services. Research has demonstrated the significant impact of MPs on aquatic systems; however, little is known about their effects on the terrestrial environment, particularly within agroecosystems, the cornerstone of global food production. Soil biology is highly responsive to environmental perturbation and change. Hereby, we investigated the effect of pure low-density polyethylene (LDPE) MP loading (0, 100, 1000, or 10000 kg ha−1) on soil and plant biological health in a field environment over a cropping season. Our results showed that MP loading had no significant effect (p > 0.05) on the soil bacterial community diversity (as measured by amplicon sequencing of bacterial 16S rRNA gene), the size and structure of the PLFA-derived soil microbial community, or the abundance and biomass of earthworms. In addition, metabolomic profiling revealed no dose-dependent effect of MP loading on soil biogenic amine concentrations. The growth and yield of wheat plants (Triticum aestivum L., cv. Mulika) were also unaffected by MP dose, even at extremely high (≥1000 kg ha−1) loading levels. Nitrogen (N) cycling gene abundance before and after N fertiliser application on the MP loaded experimental plots showed relatively little change, although further experimentation is suggested, with similar trends evident for soil nitrous oxide (N2O) flux. Overall, we illustrate that MPs themselves may not pose a significant problem in the short term (days to months), due to their recalcitrant nature. We also emphasise that most MPs in the environment are not pure or uncontaminated, containing additives (e.g. plasticisers, pigments and stabilisers) that are generally not chemically bound to the plastic polymer and may be prone to leaching into the soil matrix. Understanding the effect of additives on soil biology as well as the longer-term (years to decades) impact of MPs on soil biological and ecological health in the field environment is recommended.