Fate, transport, and ecotoxicological impacts of nanoplastics in soil systems: Interactions with earthworms and soil microbial dynamics

Introduction

The global proliferation of plastic materials has led to widespread environmental contamination, with terrestrial ecosystems acting as primary sinks. While microplastics have received considerable attention, nanoplastics represent a more insidious class due to their small size, high surface area, and enhanced reactivity. These properties facilitate their interaction with soil particles, biological membranes, and biochemical pathways, raising concerns regarding their ecological consequences. Agricultural soils are particularly vulnerable due to inputs from sewage sludge, plastic mulching, compost amendments, and atmospheric deposition. Once introduced, nanoplastics can persist, migrate, and interact with biotic and abiotic components. Their small size allows them to penetrate soil aggregates and potentially enter groundwater systems, posing risks beyond the immediate soil environment. Earthworms are keystone organisms in soil ecosystems, influencing structure, aeration, and nutrient cycling through bioturbation. Their interaction with nanoplastics provides a critical pathway for particle redistribution and biological exposure. Concurrently, soil microbial communities-central to organic matter decomposition and nutrient transformations-are sensitive to emerging contaminants, including nanoplastics. This review critically examines current research on nanoplastic transport in soils, their interaction with earthworms, and their influence on microbial activity. It aims to integrate findings across studies, identify inconsistencies, and propose future research directions to advance understanding in this emerging field.

Transport and Mobility of Nanoplastics in Soil

Nanoplastic transport is governed by physicochemical interactions between particles and soil matrices (Table 1). Particle size, surface functionalization, and zeta potential strongly influence mobility. Negatively charged nanoplastics exhibit enhanced movement due to electrostatic repulsion from similarly charged soil colloids. Soil texture plays a decisive role: sandy soils promote rapid percolation, whereas clay-rich soils enhance retention through adsorption and aggregation. Organic matter can act as a binding agent, reducing mobility but increasing localized accumulation. Hydrological processes, including rainfall simulation and irrigation, significantly enhance vertical migration. Column studies demonstrate that nanoplastics can reach subsurface layers, indicating potential groundwater contamination pathways. Aggregation behavior further complicates transport dynamics. High ionic strength environments promote particle clustering, reducing mobility but increasing sedimentation (Figure 1).

Interaction with Earthworms and Soil Fauna

Earthworms serve as both vectors and victims of nanoplastic contamination. Ingestion occurs during feeding on organic matter, leading to internal accumulation. Experimental evidence indicates oxidative stress, reduced biomass, and histological damage in exposed organisms. Bioturbation enhances redistribution of nanoplastics within soil profiles. Burrowing activities facilitate deeper penetration, while casting concentrates particles in nutrient-rich zones. This redistribution alters exposure patterns for other soil organisms. Species-specific responses are evident. Surfacedwelling species exhibit higher exposure rates, whereas deep-burrowing species contribute more to vertical transport. Chronic exposure studies suggest sub-lethal effects that may impair long-term ecosystem functioning (Table 3) (Figure 2).

Effects on Soil Microbial Communities

Microbial communities exhibit both sensitivity and adaptability to nanoplastic exposure. Reductions in microbial biomass and diversity are commonly reported, particularly under high concentrations. Enzymatic activities essential for nutrient cycling-such as dehydrogenase, urease, and phosphatase-are often suppressed, indicating functional impairment. However, some studies report transient increases in respiration, suggesting stress-induced metabolic responses. Nanoplastics also influence microbial community structure by serving as surfaces for biofilm formation. These "plastisphere" habitats can alter microbial interactions and promote the proliferation of specific taxa. Furthermore, nanoplastics can adsorb and transport cocontaminants, enhancing their bioavailability and toxicity. This synergistic effect represents a significant ecological concern (Table 2) (Figure 3).

Integrated Soil System Impacts

The interaction between nanoplastics, earthworms, and microbes creates complex feedback mechanisms. Earthworm-mediated transport exposes deeper microbial communities, while microbial alterations influence soil aggregation and nutrient cycling.These interactions can lead to reduced soil fertility, impaired carbon sequestration, and altered ecosystem functioning. While some resilience is observed at low concentrations, long-term impacts remain uncertain (Picture 1).

Results and Discussion

The collective evidence indicates that nanoplastics are highly dynamic contaminants with significant ecological implications. However, variability in experimental design, particle characteristics, and exposure conditions leads to inconsistent findings. A critical limitation is the use of unrealistically high concentrations in laboratory studies, which may overestimate ecological risks. Additionally, differences in polymer type-such as polystyrene versus polyethylene-result in varying toxicity profiles. Detection and quantification of nanoplastics in soil remain major challenges due to their small size and complex interactions with soil components. This limits the ability to validate laboratory findings under field conditions. Despite these challenges, the overall trend suggests that nanoplastics disrupt soil biological processes, with potential long-term consequences for ecosystem sustainability.

Future Challenges and Directions

To advance the field, future research should focus on:  Standardization of analytical techniques for nanoplastic detection  Long-term field studies under realistic environmental conditions  Investigation of multi-contaminant interactions  Multi-trophic level ecological assessments  Mechanistic studies at molecular and cellular levels  Development of predictive models for transport and risk assessment Integration of advanced imaging techniques and omics approaches will be crucial for understanding nano-scale interactions.

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