Microplastic pollution in neotropical rainforest, savanna, pine plantations, and pasture soils in lowland areas of Oaxaca, Mexico: Preliminary results

Microplastic (<5 mm plastic pieces) pollution is a worldwide serious problem, particularly in terrestrial ecosystems. The presence and abundance of such contaminants in tropical areas are unknown, despite their potentially harmful effects on human health and biodiversity. Tropical lowland areas often are densely populated by humans. However, the level of microplastic contamination in human-modified tropical ecosystems remains unknown. In the tropical lowland coastal plain of the Gulf of Mexico, we compared the concentration and particle size distribution of soil microplastics in five contiguous ecosystems: a neotropical rainforest, a native savanna, two pine plantations, one with the understory removed and the other with the understory left intact, and a human-induced grassland used for cattle grazing. Microplastic extraction was performed using the density separation and filtration method. Microplastics (only fibers and fragments) were present in the soils of all the studied ecosystems. We do not detect the presence of films and pellets. We found between 1.53 and 1.49 (g⁻¹ dry soil) particles of fibers and fragments. Microplastics from 150 to 500 µm were the most common, whereas those from 40 to 60 µm were the rarest. The concentration of fibers and fragments was significantly different among the studied ecosystems, being higher in the pasture than in the tropical rainforest, the pine plantations, and the savanna, which displayed similar concentrations. Microplastics (fibers and fragments) appear to increase as the levels of clay and sand increased. The observed high abundance of microplastic in tropical lowland areas warrants further studies on their abundance and ecosystem impact.