In recent years, environmental pollution has become one of the most pressing global challenges. In addition to the well-known plastic waste, an increasingly urgent threat is emerging: microplastics (MP) and nanoplastics (NP), slight plastic particles smaller than 5 mm (MP) and 1 μm (NP). These ubiquitous contaminants are particularly prevalent in agricultural soils, which are 4–23 times more contaminated with MP than water. Their presence in soil raises concerns about their environmental impact, entry into the food chain and potential impacts on human health. In addition, these tiny particles have a fundamental, yet often overlooked, impact on key biogeochemical cycles, particularly the carbon and nitrogen cycles, which are inextricably linked to greenhouse gas regulation and overall climate stability.

Soil as a haven for plastics and their impact on the climate

An estimated 22,500 tonnes of MPs enter UK soil each year through fertilisers and additives alone. In Europe and North America, this figure is estimated at 110,000–730,000 tonnes per year. The main sources of these plastics in agriculture include:

• Plastic mulch films: Despite their benefits to the crop, they are the main contributor of MP and NP to the soil due to the difficulties of collecting and recycling them. Long-term studies have shown that after 32 years of plastic mulching, MPs constituted 33–56 % of plastic in the soil.
• Sewage sludge (biosolids) and organic fertilizers: Their application annually introduces an estimated 63,000–430,000 tons of MP into soil in North America and 44,000–300,000 tons in Europe.

These plastic particles change the physical properties of the soil, such as water transport and retention, as well as porosity. The key is that alter chemical parameters, including the carbon to nitrogen ratio, and biological factors such as microbial diversity and macrofauna healthIt is these changes that have a direct impact on the carbon and nitrogen cycles, which are essential for climate stabilization.

Microplastics and the carbon cycle: A hidden source of emissions?

Soil is a huge global carbon reservoir, storing approximately 2,500 billion tonnes of organic and inorganic carbon. The presence of MP in soil complicates the assessment of soil carbon dynamics, as MP alone consists of more than 90% carbon. Although this carbon is largely inert and does not readily integrate into natural carbon cycles, the presence of MP may affect carbon release in other ways.

Studies have shown that MPs can significantly increase dissolved organic carbon (DOC) concentrations in soil. Even more alarmingly, MPs can stimulate greenhouse gas emissionsThe meta-analysis found that MPs increased CO₂ emissions by 54.3 % and CH₄ (methane) emissions by 9.7 %, while biodegradable MPs had the most significant impactLaboratory experiments with different types of recalcitrant MPs (PE, PVC, PS, PP and PET) in agricultural soils have also confirmed that the presence of MPs is associated with shifts in microbial communities and increased CH₄ and CO₂ emissions, with PE MP contributing the most. This suggests that the impact on CO₂ emissions likely depends on microbial reactions, rather than just direct plastic degradation.

Microplastics and the nitrogen cycle: Disrupting the climate balance

Nitrogen is crucial for soil, and its balance affects soil quality, yields, and overall biodiversity. Microplastics also disrupt this sensitive cycle. Studies have found that Application of PVC MP to rice soil reduced ammonium and nitrate concentrationsRecalcitrant MPs (e.g., LDPE) have been associated with reduced total soil nitrogen, impaired aggregate stability, increased nitrogen leaching, and increased soil carbon to nitrogen ratios.

The changes also affect emissions of nitrogen gases, which are important greenhouse gases. Although some studies indicate a reduction in nitrous oxide (N2O) emissions in the presence of PE MP, others report that MP addition significantly increased N2O fluxes from soil and repressed functional nitrogen cycle genes in earthworm guts. These different conclusions may be related to MP concentrations, soil types, and soil fauna.

Impacts on soil ecosystems and the overall health of the Earth

Disruption of the carbon and nitrogen cycles has direct consequences for the stability of soil ecosystems and their ability to support life and mitigate climate change. Microplastics alter microbial communities in the soil, creating specialized “niches.” Plasticizers, the main additives in plastics, disrupt microbial activity and affect nitrogen and carbon cycles. Even bioplastics, which are considered a more sustainable alternative, can negatively impact microbial communities and exhibit toxicity similar to conventional plastics. These disruptions affect essential soil functions, including respiration, pH, nutrient availability, and the carbon-to-nitrogen ratio.

Challenges and the need for solutions for the future of climate

It is urgently needed coordinated scientific and regulatory efforts to address the growing problems of agricultural plastic contamination. There is a significant lack of standardized methods for detecting MPs, leading to huge variations in reported concentrations (from 1.3 particles/kg to 236,000 particles/kg of soil). Furthermore, many toxicity studies use exaggerated MP concentrations that do not reflect actual environmental conditions, which may distort the real risks and potential impacts on climate change.

Without decisive action, we will only watch the crisis unfold instead of preventing it. A risk-based strategy that integrates scientific research into regulatory frameworks is essential to set realistic limits on intake and concentrations of MPs, NPs and additives in soil, food and people. Only in this way can we ensure that agriculture, the foundation of our food system, does not contribute to the worsening of climate change, but instead becomes part of the solution. Spring

The study, originally published in the journal  Environmental Sciences Europe

Glossary of Key Terms

• Microplastics (MP): Plastic particles smaller than 5 mm in size.
• Nanoplastics (NP): Plastic particles with a size of less than 1 μm.
• Macroplastics: Plastic objects larger than 25 mm.
• Endocytosis: The process by which cells absorb molecules (or particles such as NPs) by surrounding them with their cell membrane and internalizing them. It includes:
• Phagocytosis: A type of endocytosis in which a cell engulfs large particles, such as microplastics.
• Pinocytosis: A type of endocytosis in which a cell absorbs fluids and small dissolved molecules.
• Apoplastic transport: The movement of water and solutes through the intercellular spaces and cell walls of plants, bypassing cytoplasmic membranes.
• Seed germination: The process by which a seed germinates and begins to grow.
• Oxidative stress: An imbalance between the production of reactive oxygen species (ROS) and the ability of a biological system to detoxify reactive intermediates or repair the resulting damage.
• Reactive oxygen species (ROS): Chemically reactive molecules containing oxygen that can cause cell damage.
• Root hairs: Fine, tubular outgrowths of epidermal cells of roots that greatly increase the surface area of the root for water and nutrient absorption.
• Plastic mulch: The use of plastic films in agriculture to cover soil, which may contribute to soil contamination with microplastics.
• Rubber tire wear: Particles released from vehicle tires due to friction, which are a significant source of microplastics in the environment.
• Soil macrofauna: Larger soil-dwelling organisms, such as earthworms, snails, and centipedes, play a key role in soil health.
• Acetocholinesterase (AChE): An enzyme that breaks down the neurotransmitter acetylcholine, levels of which can be disrupted by toxins, including microplastics.
• Glutathione (GSH) and Superoxide dismutase (SOD): Important antioxidant enzymes whose levels are sensitive to oxidative stress.
• Hyperspectral imaging: A technique that collects and processes information from the entire electromagnetic spectrum, allowing the identification and visualization of the presence of particles in tissues.
• XRF mapping (X-ray fluorescence spectroscopy): An analytical technique used to determine the elemental composition of materials.
• ICP-MS (Inductively Coupled Plasma Mass Spectrometry): A highly sensitive analytical technique used to detect metals and several non-metals at trace concentrations.
• ICP-OES (Inductively Coupled Plasma Optical Emission Spectrometry): An analytical technique used to detect chemical elements.