Plastics Surgin’

The rise of micro- and nanoplastics in agriculture — and what farmers can do about it

The increasing prevalence of microplastics and nanoplastics in agricultural soils represents a significant environmental challenge. Defined as synthetic polymer particles smaller than 5mm and 100nm, respectively, these contaminants impact terrestrial environments like agricultural fields. They originate from multiple sources, contributing to the complexity of their management and mitigation.

The presence of microplastics and nanoplastics in soils poses significant threats to soil health, broader ecological systems and human health. These particles alter soil properties, reduce agricultural productivity and impact soil fauna and flora. Understanding the sources and pathways of these particles into soils is crucial for developing effective strategies to address this pervasive issue.

Sources of Microplastics and Nanoplastics in Agricultural Soils

Agricultural Practices

Plastic-based agricultural products such as mulches, films and synthetic materials are primary pathways for these particles. These materials enhance crop growth, control weeds, and conserve moisture, but they degrade over time due to environmental exposure — UV radiation, mechanical stresses and biological activities — fragmenting into microplastics and nanoplastics. Significant quantities have been identified even on lands without explicit plastic use, indicating accumulation through indirect sources like atmospheric deposition and previous land uses.

Atmospheric Deposition

Microplastics and nanoplastics are also introduced through atmospheric deposition. Aerial transport from urban and industrial areas to farmlands involves wind currents that uplift particles from areas with high plastic usage and waste, depositing them onto soil surfaces. This process disperses particles over large geographic areas, including remote and rural lands, highlighting the role of wind and weather patterns in environmental pollution.

Wastewater and Sludge Applications

Another critical source is treated sewage sludge and biosolids, commonly used in agriculture for their rich nutrient content. Wastewater treatment processes do not completely remove microplastics and nanoplastics, resulting in their presence in biosolids applied to fields. These particles persist, accumulate and potentially affect soil health and crop productivity, representing a direct pathway through which contaminants are added to soil ecosystems. Figure 1 provides a summary of the sources of microplastics related to agricultural activity.

The sources of microplastics and nanoplastics in agricultural soils are diverse and interlinked, contributing to the complex dynamics of soil contamination. Understanding these pathways is crucial for developing targeted strategies to mitigate their introduction and impacts in agricultural settings.

Types of Microplastics and Nanoplastics

Microplastics and nanoplastics in soil manifest in various forms, each with specific properties and implications for soil health and ecosystem dynamics. Their diversity in physical forms and chemical compositions significantly affects their behavior and longevity in soil environments.

Fragments

These are broken-down pieces from larger plastic items and are common in agricultural soils. Their shapes and sizes vary greatly depending on the source material and degradation processes.

Films

Derived from agricultural films and mulches, these thin plastic layers break down into smaller pieces under UV radiation and mechanical action.

Fibers

Originating from sources like atmospheric deposition from textiles or ropes used in agriculture, synthetic fibers are particularly persistent due to their high tensile strength and chemical stability.

Microbeads

Though less common in agricultural settings, microbeads can enter soils through biosolids containing these tiny spherical plastics from cosmetics and cleaning products.

Residence Time of Plastic in the Environment

Polyethylene (PE), polypropylene (PP) and polystyrene (PS) are the most prevalent plastics used today. These polymers are favored for their versatility, durability and cost-effectiveness, making them highly resistant to natural degradation.

As a result, the residence time of microplastics and nanoplastics in soils is critical to their environmental impact. They persist for decades in some cases and pose ongoing risks for soil contamination and uptake by plants and soil organisms. Degradation is slow, influenced by UV exposure, mechanical stress and microbial activity, varying widely with soil type, climate and farming practices. Agricultural films exposed to intense UV light and physical wear can accelerate fragmentation into microplastics that remain long after disintegration.

Due to their long shelf life in the environment, microplastics and nanoplastics significantly influence soil health and crop productivity by altering soil properties like bulk density and water retention, leading to degraded soil structure. These changes adversely affect nutrient and water bioavailability, which are essential for optimal plant growth and productivity.

Microplastics and nanoplastics interfere with root development, reducing plant stability and nutrient uptake. They cause physical blockages and chemical interactions, inhibiting root elongation and branching, which leads to lower crop yields due to reduced water and nutrient absorption.

Plant Uptake and Ecological Impacts

Recent research shows that plants can absorb microplastics and nanoplastics through their root systems, raising concerns about bioaccumulation and biomagnification in the food chain. These particles adversely affect plant health and pose risks to animals and humans consuming these plants. Microplastics can adsorb and carry pollutants, including heavy metals and organic contaminants, further degrading plant and soil health. Figure 2 provides a general overview of where plants take up plastics (black dots) in various parts of the plant.

Root Absorption

Microplastics and nanoplastics are absorbed by plant roots and translocated to aerial parts through the xylem. For instance, polystyrene nanoparticles accumulate in roots and move to shoots and leaves via the xylem. Studies have shown that PS nanoparticles can enter the root tissues and move through the vascular system to other parts of the plant, including leaves and stems.

Stomatal Uptake

Nanoplastics can enter leaves through stomata and move to other plant parts, including roots, via the vascular system. PS nanoparticles can attach to and penetrate stomata, moving through the phloem to reach roots and other plant areas.

Crack-Entry Mode

Microplastics and nanoplastics can enter plant tissues through cracks in the root epidermis, allowing direct access to vascular tissues. This has been observed in crops like wheat and lettuce, where PS nanoparticles entered the root xylem through cracks.

Translocation and Accumulation

Microplastics and nanoplastics can be translocated to various plant parts, including stems, leaves and fruiting bodies. Studies using confocal microscopy and scanning electron microscopy have shown these particles in the vascular tissues of different plant parts, highlighting the complex internal pathways for their distribution.

Plant Parts and Accumulation

Roots: Microplastics and nanoplastics are primarily absorbed through the root system and can be found in high concentrations within root tissues.

Stems: Particles can move from roots to stems via the xylem, with significant accumulation in vascular tissues.

Leaves: Nanoplastics can enter leaves through stomata and accumulate within leaf tissues, often transported through the phloem.

Fruiting Bodies: Some studies indicate that microplastics and nanoplastics can accumulate in the flowers and fruits of plants, although this is less common compared to roots and leaves.

Species Concentration of Microplastics Effects on Plants Accumulation Pathway Zea mays (Maize) 28 days exposure to PS Plastic accumulates in leaves and roots Root absorption, translocation Lactuca sativa (Lettuce) 35 days exposure to PS nanoparticles (0.1 and 1 mg/L) Nanoparticle uptake by leaves via stomata, translocation downwards Foliar uptake, root absorption Cucumis sativus (Cucumber) 65 days exposure to PS (100, 300, 500, 700 nm) PS uptake by root, transported to leaves, flowers, fruits Root absorption, translocation Allium cepa (Onion) 72 hours exposure to PS (0.01, 0.1, 1 g/L) PS enters different cellular compartments Root absorption Arabidopsis thaliana (Thale Cress) 35 days exposure to PS Positive-charge nanoparticles have significant effects on roots Root absorption Triticum aestivum (Wheat) and Lactuca sativa nanoparticle (0.2 and 2 µm) exposure PS enters roots via crack-entry mode, translocation Root absorption, translocation Lepidium sativum (Garden Cress) Micro- and nanoparticles (103–107 mL/) exposure Microplastic delays seed germination Seed pore accumulation Table 1 provides a summary of published studies that looked at plastic uptake in specific crops. These mechanisms highlight the potential for microplastics and nanoplastics to accumulate in edible plant parts, posing risks to food safety and human health. Understanding these pathways is crucial for developing strategies to mitigate crop contamination.

Impacts on Soil Micro- and Macrofauna

Microplastics disrupt soil ecosystems, affecting soil organisms like earthworms, which play a crucial role in soil aeration and nutrient cycling. Ingested microplastics accumulate in their guts, causing harm or mortality. This disruption affects their digestive processes, reducing mobility and effectiveness in soil mixing, leading to diminished soil fertility and structural stability.

Microplastics also alter microbial community structures, affecting bacteria and fungi essential for decomposing organic matter and cycling nutrients. These disturbances lead to long-term changes in soil biodiversity and functionality, reducing soil ecosystem resilience to environmental stresses.

Quantifying Micro- and Nanoplastics in the Environment

Identifying and quantifying microplastics and nanoplastics in soil requires robust analytical techniques to understand contamination and its impacts on agriculture. Key methods used in research include:

Spectroscopic Analysis

Fourier Transform Infrared Spectroscopy (FTIR) and Raman Spectroscopy are essential for identifying the chemical composition of microplastics and nanoplastics. FTIR detects various polymers based on their unique absorbance spectra. Piehl et al. (2018) used FTIR to identify polyethylene and polypropylene in agricultural soils. Raman Spectroscopy complements FTIR by providing detailed molecular structure information, crucial for identifying chemically complex plastics.

Density Separation

This method separates microplastics and nanoplastics from soil components based on density. Using a saturated salt solution, microplastics and nanoplastics float while denser materials sink. Researchers like Huang et al. (2020) used this technique to isolate microplastics from urban soil samples for further spectroscopic analysis.

Visual Identification

Microscopy enhances visual identification, allowing direct observation and counting of microplastics and nanoplastics in soil samples. This method, often combined with other techniques, helps confirm the presence and assess the size and shape distribution of particles. Piehl et al. (2018) used visual identification with microscopic analysis to quantify microplastics in soil samples.

Each method has strengths and limitations. Spectroscopic techniques provide detailed chemical information but require expensive equipment and expertise. Density separation is cost-effective but may not work for all plastic types due to overlapping densities with organic matter. Visual identification offers direct evidence but can be labor-intensive and may miss very small particles or those resembling natural soil components.

Correcting the Problem

Addressing microplastic and nanoplastic pollution in agricultural soils requires a multifaceted approach. Key strategies include bioremediation, phytoremediation, policy and regulatory measures, mechanical removal, and education and awareness programs.

Bioremediation

Bioremediation uses microorganisms to degrade microplastics. Certain bacteria and fungi can break down polyethylene and polystyrene. However, their efficiency in field conditions is still under research.

Phytoremediation

Phytoremediation involves plants absorbing, accumulating and potentially degrading microplastics. Certain plants can isolate microplastics within their root systems, reducing their spread and contributing to soil stabilization. The effectiveness depends on the plants and soil characteristics.

Policy and Regulatory Approaches:

Regulations are crucial for managing microplastic pollution. Implementing guidelines to reduce plastic use, improve waste management, and promote biodegradable materials can significantly decrease microplastics in soils. Policies encouraging recycling of agricultural plastics and treating wastewater before land application are essential. The EU, for instance, is developing policies to reduce plastic waste and promote circular economic practices in agriculture.

Mechanical Removal

Mechanical removal involves physically extracting microplastics from soil using techniques like sieving or flotation. Effective for larger particles, this method is labor-intensive and not always feasible on a large scale, depending on soil type and microplastic concentration.

Education and Awareness Programs

Raising public awareness about the impact of microplastics can lead to more sustainable practices in plastic use and disposal. While not directly removing existing contamination, these programs are crucial for preventing future pollution. Success is typically measured by changes in public behavior and reductions in plastic use over time.

Method Description Potential Benefits Success/Removal Rates Implementation Complexity Bioremediation Use of microorganisms to degrade plastics Reduces microplastic levels, potentially restoring soil health Moderate (varies by study and conditions) High (field application challenges) Phytoremediation Use of plants to absorb or degrade plastics Isolates or removes microplastics, stabilizes soil structure Low to moderate (dependent on species and conditions) Moderate (requires specific plants and monitoring) Policy & Regulation Development and enforcement of guidelines Prevents future contamination, promotes sustainable practices High (effective with strict enforcement) High (requires legal and regulatory frameworks) Mechanical Removal Physical extraction of microplastics from soil Immediate reduction of microplastic concentration Moderate to high (effective for larger particles) High (labor-intensive, limited to accessible areas) Education and Awareness Promoting sustainable practices through education Long-term reduction in microplastic pollution Indirect (challenging to quantify) Low (wide-reaching impact)

These strategies collectively address both the symptoms and sources of microplastic pollution in agricultural soils. Effective implementation requires a combination of these methods, tailored to specific environmental, economic and social contexts.

Microplastics and nanoplastics in agricultural soils present significant challenges, impacting productivity, food safety and ecosystem health. These pollutants originate from various human activities, contaminating soil and carrying other pollutants, which amplify their impacts.

These particles disrupt soil structure and fertility, hinder crop growth and lead to bioaccumulation in the food chain, compromising food quality and safety. They also threaten biodiversity and water quality, contributing to broader environmental degradation. The challenges posed by microplastics and nanoplastics are significant but solvable. Combined efforts in research, technology, and policy can effectively address this pollution, allowing us to safeguard agricultural landscapes, protect food resources and maintain ecological balance.

Patrick Freeze is a soil health scientist, research and development manager, and technical specialist at Ward Labs. He earned his Ph.D. in soil chemistry from Washington State University where he studied soil health and heavy metal chemistry as a USDA NIFA Needs Fellow and in Thailand as a U.S. Fulbright Scholar.