Shape matters: Microplastic fibers disrupt soil carbon cycling via shape-driven physical disturbance

Soil microplastic (MP) pollution is an emerging concern with uncertain carbon (C) cycling impacts. Among MPs, fiber-shaped particles may exert distinct effects due to their high surface area and dissimilarity from natural soil structures. This study aimed to assess the physical and chemical mechanisms by which MP addition alters soil C dynamics, especially through their impacts on priming effect. We assessed two common MP polymers and shapes —low-density polyethylene beads and acrylic fibers—at two addition rates (0.1% and 1% w/w), paired with inert glass particle controls mimicking the same MP shapes and a negative control with soil only. Natural abundance δ¹³C isotopic analysis distinguished MP-derived and soil-derived CO₂ emissions over a 63-day incubation period. Fiber-shaped additions of either glass or MPs reduced total CO₂ emissions by 25%, whereas bead-shaped additions had no measurable effect. Plastic-derived CO₂ constituted a minor but detectable portion of total emissions, with similar decomposition rates across polymer types. Surprisingly, higher MP concentrations decomposed up to 10 times slower than lower concentrations, suggesting that MPs may clump together, reducing the surface area available for microbial colonization, thereby slowing down breakdown. Our results show that fiber-shaped MPs significantly disrupt soil C cycling, mainly through physical disturbance, while chemical effects (i.e., presence/absence of plastics) are secondary. Finally, soil priming effect increased with the amount of MPs added, raising additional concerns about the consequences of rising soil plastic contaminations worldwide.