Bioaccumulation of microplastics in soil-crop systems and potential risks to food safety

Introduction

The first report linking plastic fragments to agricultural soil appeared in the early 2010s, but the problem was decades old by then. Plastic mulch films, irrigation with treated or untreated wastewater, organic fertilisers derived from municipal waste, and atmospheric deposition have been delivering synthetic polymers to farmland since the widespread adoption of plastics in the 1960s [1,2] . Global estimates now suggest that agricultural soils receive 4-23 times more microplastic mass annually than the oceans [3] . Once in the soil matrix, MPs alter porosity, water-holding capacity, and microbial community composition [4] . More concerning from a public health standpoint is the growing evidence that plant roots can internalise particles smaller than 1 μm and translocate them to above-ground tissues [5] . Whether these particles reach edible organs at concentrations that pose a dietary risk is still debated, partly because standardised extraction and quantification methods for plant matrices were only agreed upon in 2020 [6] . Sub-Saharan Africa, where wastewater reuse is common and plastic mulch adoption is rising fast, has been almost entirely absent from this literature [7] . This research measured MP distribution across soil depth profiles and crop tissues under three contrasting input scenarios in central Uganda over two cropping years.

Research area description

The trial was conducted at Makerere University Agricultural Research Institute, Kabanyolo (MUARIK), located 21 km north of Kampala (0°28′N, 32°37′E, 1,190 m a.s.l.). The site has a bimodal rainfall pattern with peaks in March-May and September-November, totalling about 1,290 mm yr⁻¹. Soils are Ferralsols with 38% clay, pH 5.4, and organic carbon 2.1%. The area has received municipal wastewater for irrigation since the 1990s through an informal canal network from the Lubigi wetland treatment system.

Material and methods

Material

Three treatment blocks were established: (i) plots irrigated with partially treated wastewater from the Lubigi channel since at least 2008, (ii) rainfed plots using polyethylene mulch film (25 μm thickness, replaced each season since 2015), and (iii) rainfed control plots with no known plastic input history. Each treatment had four replicate plots (10 × 12 m). Test crops were maize (cv. Longe 10H), common bean (cv. NABE 15), and tomato (cv. Rio Grande). Density separation used ZnCl₂ solution (1.8 g cm⁻³); identification employed a Bruker Lumos II micro-FTIR spectrometer with ATR mode.

Methods

Soil cores were collected at six depth intervals (0-5, 5-10, 10-15, 15-20, 20-25, 25-30 cm) using stainless-steel corers. Five cores per plot were composited by depth. MPs were extracted by density separation (30 g soil in 200 ml ZnCl₂, shaken 2 h, settled 24 h), filtered onto glass-fibre filters, and counted under a stereomicroscope [8] . Particles > 50 μm were identified by micro-FTIR (spectral match ≥ 70%). Crop tissues (root, shoot, and edible organ) were digested in 30% H₂O₂ at 65 °C for 48 h, filtered, and analysed identically [6] . Sampling occurred at four time points across two cropping cycles (March 2020-February 2022). Treatment effects were tested by one-way ANOVA with Tukey's HSD (α = 0.05). Results Polyethylene fragments accounted for 48.6% of all identified particles, followed by polypropylene (22.7%) and polyester fibres (14.3%).

Comprehensive interpretation

A positive linear correlation existed between topsoil MP concentration and root MP content across all treatments (r = 0.91, p<0.001), confirming that soil contamination level drives plant uptake. The root-to-edible-organ transfer factor averaged 0.07-0.17, indicating that the plant's vascular barrier retains most internalised particles.

Discussion

The topsoil MP concentrations recorded under wastewater irrigation (347 particles kg⁻¹) fall within the range reported for peri-urban farms in China (260-570 particles kg⁻¹ [9] ) but exceed values from European sewage-sludge-amended fields (78-210 particles kg⁻¹ [10] ). The difference likely reflects the lower treatment efficiency of Uganda's informal canal system compared to European tertiary treatment [11] . The steep depth gradient in all treatments suggests that vertical migration of MPs is limited in these clay-rich Ferralsols, consistent with pore-size exclusion of particles above 50 μm [4] . The detection of MPs in edible organs, even in control plots, underlines the pervasive nature of atmospheric and background contamination [12] . That said, the concentrations in grain and seed (0.2-2.3 particles kg⁻¹) are roughly two orders of magnitude below the provisional tolerable intake recently proposed by WHO working groups [13] . Tomato fruit showed higher uptake than cereal grain, a finding that warrants attention because tomatoes are consumed fresh and in large quantities in Ugandan diets [14] .

Limitations

This research has several constraints worth noting. First, micro-FTIR identification was limited to particles > 50 μm; nanoplastics (< 1 μm), which may be more biologically active, were not quantified. Second, the two-year duration may not capture long-term accumulation trends in soils receiving wastewater for over a decade. Third, the ZnCl₂ density separation method recovers approximately 85-92% of common polymer types, potentially underestimating total MP load. And the control plots, while free of intentional plastic input, still received atmospheric deposition, setting a non-zero baseline.

Conclusion

Wastewater irrigation delivered roughly eight times more microplastics to topsoil than background deposition, and plastic mulch tripled the load. Root tissues acted as the main accumulation site across maize, bean, and tomato, while edible organs contained far fewer particles. Current concentrations in grain and fruit remain below proposed safety thresholds, but continued wastewater use without improved filtration will raise soil MP stocks over time. Periodic monitoring of MP in periurban food systems and investment in low-cost wastewater filtration are both warranted.

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