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Effects of polypropylene microplastics and lead (Pb) contamination on soil properties and the growth response of Ficus Benjamina
Summary
Researchers found that polypropylene microplastics and lead contamination together cause greater harm to soil chemistry and plant growth than either contaminant alone, with Ficus plants showing significantly reduced leaf area, root length, and total biomass when exposed to both. Microplastics also lowered soil pH and depleted essential nutrients, compounding the toxic effects of the heavy metal.
Soil contamination is becoming a major source of concern because of its consequences on the overall well-being of the soil ecosystem. The presence of microplastics in the environment which may co-exist with other contaminants could serve as double jeopardy to the life forms in the soils. This study investigates the effects of polypropylene (PP) microplastics and lead (Pb) contamination on soil physicochemical properties and the growth performance of Ficus benjamina, a widely cultivated ornamental plant. The soil samples used in this study were obtained from a Research Farm, Obafemi Awolowo University (OAU), Ile-Ife, Nigeria. The soil was air-dried at room temperature, crushed, and sieved using a 2 mm steel sieve to ensure homogeneity before being packed into 5 kg pots. Three sizes of polypropylene (PP) microplastics (1 mm, 2 mm, and 4 mm) and Pb applied as nitrate salts (250, 500, and 750 mg/kg) were used for artificial contamination of the soil. Thereafter, two weeks old seedlings of. Ficus benjamina were planted into experimental pots. The experiment was in a factorial combination (2 × 2 × 2) arranged in a completely randomized design with three replications. Standard methods were adopted in determining all the soil physiochemical parameters (pH, organic carbon, total nitrogen, available phosphorus, particle size distribution), Pb, Cd, exchangeable acidity and cation exchange capacity (CEC: Na⁺, K⁺, Ca²⁺, Mg²⁺). Fourier-transform Infrared Spectroscopy (FT-IR) was used to identify the presence of Microplastics (MPs) in the soil. Plant responses were evaluated based on leaf area, root length, number of roots, and biomass accumulation. Descriptive statistics and analysis of variance (ANOVA) using SPSS software with Duncan’s Multiple Range Test (DMRT) were employed in this study. The findings showed that MPs reduced the soil pH from 6.57 to 5.57. Exchangeable cations (Ca²⁺, Mg²⁺, K⁺, and Na⁺) decreased progressively with decrease in PP MP sizes while the exchangeable acidity (H⁺ and Al³⁺) increased with higher Pb concentrations regardless of the sizes of PP MP. Increased Pb concentrations significantly (p < 0.05) impaired plant growth (leaf area, root length, and total biomass). However, the combination Pb and PP MPs had more pronounced negative effects on the growth of Ficus compared to individual contaminants. Significant (p < 0.05) interactions existed between Pb contaminations and PP MPs irrespective of the Pb concentrations and microplastics sizes. Further studies on the effects of interactions between various sizes of microplastics and heavy metals on terrestrial environment should be explored.