Microplastics and Lead Team Up to Worsen Soil Damage and Stunt Plant Growth

In a world drowning in plastic, the quiet casualties may be the soils beneath our feet. A new study from Nigerian researchers, published in Discover Toxicology, has revealed that when polypropylene microplastics and lead contamination occur together in soil, they deliver a punishing one-two punch that degrades soil chemistry and stunts the growth of a widely cultivated ornamental plant. The findings offer some of the clearest evidence yet that the coexistence of plastic particles and heavy metals in terrestrial environments creates a toxic synergy far worse than either pollutant alone.

The research team, led by Promise C. Odoh of the University of Ilesa together with colleagues from Obafemi Awolowo University and Elizade University, set out to answer a question that has puzzled soil scientists for years: what happens when two of the planet’s most stubborn pollutants share the same patch of ground? Global plastic production exceeds 300 million tons annually, and roughly 80 percent of that plastic eventually finds its way into the environment. As these materials fragment through mechanical and environmental weathering, they break down into microplastics, particles ranging from 0.1 to 5 millimeters, which persist in soils, oceans, and freshwater systems alike. Because of their hydrophobic surfaces and enormous surface-area-to-volume ratios, these particles act as ideal carriers for other contaminants, including toxic metals like lead, the second most hazardous heavy metal after arsenic.

To probe this interaction, the researchers collected soil from a relatively undisturbed site at the Obafemi Awolowo University Research Farm in Ile-Ife, Nigeria. The soil was air-dried, crushed, and sieved through a 2 millimeter steel mesh to ensure uniformity before being packed into 5 kilogram pots. The team then artificially contaminated the soil with polypropylene microplastics of three different sizes, 1, 2, and 4 millimeters, prepared by pulverizing household plastic containers, and with lead nitrate salts at concentrations of 250, 500, and 750 milligrams per kilogram. Thirty grams of microplastics were mixed into the treated soils, which were left for one week to equilibrate before seedlings of Ficus benjamina, a popular ornamental fig with known phytoremediation potential, were transplanted into the pots. The experiment followed a factorial design arranged in a completely randomized layout with three replications, and the plants were monitored for four months.

The results were striking. Polypropylene microplastics alone drove soil pH down from an initial 6.57 to as low as 5.57, pushing the soil toward acidity. The researchers attribute this acidification to acidic substances released as the plastic degrades, though they note that some earlier studies have reported the opposite effect, likely because different polymer types and environmental conditions behave differently. Lead contamination intensified the acidification, with the most pronounced drop occurring at the highest concentration of 750 milligrams per kilogram. Intriguingly, the smallest 1 millimeter particles appeared to exert a slight buffering effect, possibly through interactions between the plastic surfaces and soil minerals.

The damage extended deep into the soil’s nutritional architecture. Soil organic carbon declined consistently as lead concentrations rose, falling to as low as 24.30 grams per kilogram compared with initial values of 50.70. Soils without microplastics retained more organic carbon, while the addition of the larger 4 millimeter particles accelerated carbon loss. The team suggests that organic carbon can become sequestered on microplastic surfaces, effectively locking it away from the soil microbes that would normally decompose it and recycle its nutrients. Total nitrogen followed a similar downward trajectory, with the lowest values recorded at the highest lead levels in combination with 4 millimeter microplastics, a pattern consistent with heavy metal toxicity disrupting microbial nitrogen fixation and other nitrogen-transforming processes.

Phosphorus availability and cation exchange capacity also suffered. Available phosphorus decreased significantly with rising lead concentrations, with the steepest decline observed in soils treated with 4 millimeter polypropylene particles, likely because heavy metals interfere with phosphate solubility and microbial phosphorus cycling. Exchangeable calcium, magnesium, potassium, and sodium all declined as contamination increased, indicating that the pollutants impair the soil’s ability to retain nutrients. Meanwhile, exchangeable acidity, driven by hydrogen and aluminum ions, climbed with lead concentration regardless of microplastic size, further compounding the chemical stress on plant roots. Notably, the soil’s physical particle size distribution remained essentially unchanged, suggesting the pollution operates primarily through chemistry rather than texture.

To confirm the identity of the microplastics, the researchers extracted particles from the soil using saturated zinc chloride solution and analyzed them with Fourier-transform infrared spectroscopy. The post-contamination spectra revealed the unmistakable chemical fingerprint of polypropylene: stretching vibrations of methylene groups in the polymer backbone, along with carbonyl and aromatic peaks that signal oxidative degradation of the plastic. Before contamination, the soil showed only a single aromatic band, underscoring how thoroughly the experiment introduced the polymer into the system.

The plant responses told the biological side of the story. Ficus benjamina seedlings exposed to combined microplastic and lead treatments suffered significant reductions in leaf area, root length, root number, and total dry biomass, with the effects growing worse as lead concentrations increased. The most severe leaf area declines occurred at 500 milligrams per kilogram of lead combined with 1 millimeter particles, indicating dose-dependent toxicity. Because leaf area directly governs photosynthetic capacity and carbon assimilation, its reduction compromises the plant’s entire energy budget, hindering growth and its ability to tolerate or sequester pollutants. Root systems were particularly vulnerable to the smaller 1 and 2 millimeter particles, which likely physically hinder root penetration, while the larger 4 millimeter particles disproportionately suppressed biomass, perhaps through mechanical stress that impedes nutrient and water uptake.

Perhaps the study’s most important finding lies in the statistics. Analysis of variance revealed significant interactions between lead contamination and polypropylene microplastics across all measured plant traits, meaning the combined effect exceeded what either pollutant could achieve alone. At low lead concentrations, the plants mounted an adaptive response by producing more roots, but at higher contamination levels this resilience was overwhelmed and root growth collapsed. Heavy metals and microplastics may also interfere with chlorophyll synthesis and electron transport in the photosystems, further throttling biomass production.

The authors caution that their controlled screenhouse conditions cannot fully replicate the complexity of natural field environments, and that results from a single species may not generalize across ecosystems. Still, the message is urgent and clear: the global accumulation of plastic waste, projected to reach roughly 11 billion tons by 2025, is not merely a problem of visible litter. As microplastics mingle with industrial heavy metals in the world’s soils, they reshape nutrient cycles, acidify the ground, and quietly undermine the plants that anchor terrestrial food webs. Understanding and mitigating these combined effects, the researchers argue, will be essential to protecting soil health and agricultural productivity in the decades ahead.

Subject of Research: Combined effects of polypropylene microplastics and lead contamination on soil properties and plant growth

Article Title: Effects of polypropylene microplastics and lead (Pb) contamination on soil properties and the growth response of Ficus Benjamina

Article References: Odoh, P. C., Awotoye, O. O., Ekpa, D. E., Dada, O. E., & Akpan, N. J. (2025). Effects of polypropylene microplastics and lead (Pb) contamination on soil properties and the growth response of Ficus Benjamina. Discover Toxicology, 2(1), Article 27. https://doi.org/10.1007/s44339-025-00038-6

Image Credits: AI Generated

DOI: 10.1007/s44339-025-00038-6

Keywords: microplastics, polypropylene, lead contamination, soil health, Ficus benjamina, heavy metals, soil pH, phytoremediation, soil organic carbon, plant growth, cation exchange capacity, toxicology