Minimal vertical transport of microplastics in soil over two years with little impact of plastics on soil macropore networks.

Plastics used in agriculture improve productivity and resource efficiency. As they fragment over time, microplastics are unintentionally released into soil, raising concerns regarding long-term implications for soil structure and fertility. Here we investigated microplastics transport and their impact on soil structure through a two-year field experiment. 45 re-packed soil columns were installed with three treatments: indium-doped polyethylene terephthalate fragments or fibers in the top 2 cm and a control with no microplastics. Soil pore structure was monitored with X-ray tomography, and microplastics vertical transport was assessed via the indium tracer. With time macropore volume, biopore fraction and critical pore diameter increased independent of microplastic addition. Microplastic transport was minimal, with only ~1% reaching below 8 cm soil depth in two years. This experimental design, simulating natural soil conditions, suggests that microplastics have a negligible influence on soil macropore architecture and its transport rate is limited in the short term.