Enchytraeids drive horizontal redistribution of small microplastics in soils

Technical Summary: Enchytraeids Drive Horizontal Redistribution of Small Microplastics in Soils

Problem Statement
Microplastics represent a global environmental concern, with soils acting as a major sink. While the detrimental effects of microplastics depend heavily on their bioavailability and redistribution from point sources, existing research on bioturbation (organism-driven transport) is heavily biased toward earthworm-mediated vertical transport. Consequently, the role of smaller, abundant soil mesofauna, specifically enchytraeids, in the horizontal redistribution of microplastics remains largely overlooked. Furthermore, quantitative data on the transport of very small microplastics (<10 µm) is scarce due to limitations in conventional separation and identification methods, and the potential for horizontal redistribution from topsoil hotspots—a critical zone for soil biota and plant roots—has not been sufficiently studied.

Methodology
The study employed a stepwise experimental design to quantify enchytraeid-driven horizontal transport of fluorescent polystyrene (PS) spheres (2–50 µm) under both in vitro and soil microcosm conditions.

• Experiment 1 (In Vitro): Agar plates (15 cm diameter) were inoculated with 0 or 50 enchytraeids. After settlement, 10 ng of PS spheres (2.21, 10.06, or 50 µm) were spiked into the center. Following a 3-day incubation, microplastic redistribution from the center to surrounding sections (I–IV) was quantified via microscopic counting.
• Experiment 2 (Soil Microcosms): Loamy sand soil microcosms (15 cm diameter, 1 cm depth) were sterilized and inoculated with 135 enchytraeids (approx. 7,850 individuals/m²). After a 1-week acclimatization, 1.28 × 10⁶ ng of 1.99 µm fluorescent PS spheres were added to the center. The system was incubated for 108 days. Microplastic concentrations in surrounding sections were extracted and quantified using fluorescence microscopy calibrated against a mass-concentration curve.
• Analysis: A "weighted mass–distance" metric was calculated by multiplying the mass of relocated microplastics by the transport distance. Statistical significance was evaluated using one-sided one-sample t-tests and Welch's t-tests to compare treatments against controls and different particle sizes.

Key Results

• In Vitro Transport: Enchytraeids significantly redistributed all three microplastic sizes from the center to surrounding sections within 3 days, whereas no transport occurred in controls.
  • Size Dependence: Smaller particles were transported more effectively. For 2 µm spheres, 39.2% reached Section I, with significant amounts also found in Sections II and III. For 10 µm spheres, significant transport was observed in Sections I and II. For 50 µm spheres, significant transport was limited to Section I (5.2%).
  • Weighted Mass–Distance: The transport efficiency (weighted mass–distance) was significantly higher for 2 µm (9.8 ng·cm) and 10 µm (7.5 ng·cm) spheres compared to 50 µm spheres (1.0 ng·cm).
• Soil Microcosm Transport: Over 108 days, enchytraeids markedly enhanced horizontal transport compared to controls.
  • In Section I, microplastic amounts were approximately tenfold higher in the presence of enchytraeids (0.68% vs. 0.07% in controls).
  • Significant increases were also observed in Sections II–IV, though to a lesser extent.
  • The weighted mass–distance was significantly greater in soils with enchytraeids, with the organisms relocating an average of 1×10³ ng of microplastics over an average distance of 33.0 cm.
• Mechanism: The study observed that smaller microplastics (2 µm) were readily ingested and egested, suggesting gut passage is a primary driver. However, the sharp drop in transport efficiency at 50 µm suggests that larger particles are likely redistributed via surface adhesion and physical disturbance rather than ingestion.

Significance and Claims
The paper claims to provide the first quantitative evidence that soil enchytraeids drive the horizontal redistribution of microplastics from hotspots to surrounding areas, particularly for particles ≤10 µm. The authors assert that this mesofauna pathway complements the well-documented vertical transport by earthworms.

Key contributions include:

1. Filling a Knowledge Gap: Addressing the oversight of mesofauna in microplastic transport models and providing data on the transport of sub-10 µm particles, which are difficult to quantify via conventional methods.
2. Ecological Relevance: Highlighting that horizontal redistribution from topsoil hotspots can broaden the spatial overlap of microplastics with biologically active microhabitats and plant roots, potentially increasing food-web exposure risks beyond what physical transport models predict.
3. Methodological Framework: Validating in vitro agar-based assays as rapid screening tools for organism-driven transport while emphasizing the necessity of quantifying redistribution under realistic soil conditions, where complex matrices constrain mobility.

The authors conclude that while enchytraeids have a lower individual transport capacity compared to earthworms, their continuous, biota-driven dispersion of localized microplastic point sources constitutes an ecologically relevant pathway controlling microplastic fate in terrestrial ecosystems.