Microplastics and hydrocarbons in soils: Quantification as an anthropic carbon source

The literature on the presence of microplastics (MPs) and their potential impact on terrestrial ecosystems is still scarce. Interestingly, soil MPs are detected as organic carbon (SOC) using traditional quantification methods (e.g., loss on ignition [LOI]), although its dynamics in the environment will be different. The objective of this study was to quantify the carbon (C) contribution of MPs to the SOC in superficial soil samples from a coastal urban wetland (Avellaneda, Buenos Aires, Argentina) with the features of a humid subtropical forest and compare with hydrocarbon contribution. Soil samples were split for analysis of moisture content; texture (sieve and pipet method); organic matter as a LOI (8 h at 450 °C); total hydrocarbons (THCs; gravimetry of solvent extractable matter); n‐alkanes (solvent extraction and gas chromatography‐flame ionization detection analysis); and extraction of MPs (floatation in NaClaq, filtration, H2O2 digestion, and visual sorting under a stereomicroscope). The superficial soil was a sandy clay loam with a large organic matter content (19%–30%). The THC averaged 2.5 ± 1.9 g kg and the marked predominance of odd‐numbered carbon n‐alkanes maximizing at C29 and C31 show the contribution of the terrestrial plant waxes. The average number of MPs was 587 ± 277 items kg of dry soil, predominantly fibers. Taking account of the C content, THCs and MPs add to the soil 1.23 ± 1.10 ton C ha and 0.10–0.97 ton C ha, respectively. Therefore, in this system with humid forest characteristics, the MPs represent between 0.12% and 1.25% of soil estimated carbon, in a magnitude similar to the C contribution of THCs (0.6%–4.2%). This preliminary study shows the relevance of discriminating MPs from other carbon sources and presents a description of their impact on soils to advance future research or tools for decision‐makers. Integr Environ Assess Manag 2023;19:698–705. © 2022 SETAC Key Points This work quantifies the contribution of carbon by microplastics to soil and compares it with the contribution of total hydrocarbons. This study shows that for the moderately productive area Reserve Municipal Coast of Avellaneda, with a slow decomposition of leaf litter, microplastics account for between 0.12% and 1.25% of the total estimated carbon of the topsoil. Because of the resistance of microplastic to decomposition, it would be expected to accumulate in the soil and should be taken into account to assess the soil carbon storage function.