Incorporation of microbial strategies for carbon-utilization in interpreting soil priming effects induced by microplastics

In recent years, plastic pollution has gained significant attention from the scientific community and policymakers, with an estimated 250 million metric tons (Mt) of plastic waste projected to enter aquatic systems and 460 million Mt anticipated to enter soil systems from 2016 to 2040. Because of abiotic (e.g., ultraviolet light and dry-wet cycles) and/or biotic factors, plastic residues break into microplastics (MPs; ≥1 μm and <5.0 mm) and nanoplastic debris (<1 μm). These small fragments are highly integrated within terrestrial environments, where plastics interact intensely with soil-dwelling organisms.

Previous studies have shown that microbial activity, or the “plastisphere,” is one of the primary factors controlling the fate of MPs.1 Considering that both soil organic matter (SOM) and MPs can serve as carbon (C) sources for microorganisms, the coupling of SOM and MPs in C-cycling has attracted increasing research attention. However, the impact of different types of MPs on SOM decomposition often varies, leading to conflicting results in the literature. For example, the addition of polystyrene MPs resulted in a reduction of C-utilization efficiency, as well as a decrease of microbial abundance and attenuation of soil respiration, while polybutylene adipate-co-terephthalate MPs triggered a significant increase in soil CO2 emission by selectively enriching specific taxa on their surface.2 A major yet often overlooked reason for these contradictory observations may be attributed to variations in C-utilization strategies among microorganisms, as influenced by the type of MPs or by the emission pathways of MPs in the soil. We assume that the presence of MPs in soil may alter C-utilization strategies of microorganisms by changing SOM bioavailability, providing leachates derived from MPs as alternative C-sources, or promoting the ability of microorganisms to mine C from both SOM or MPs. Our viewpoint will explain these mechanistic pathways in detail (see Figure 1) to allow for proper interpretation of conflicting results within reported studies on the ecological endpoints induced by exposure of soil microbes to MPs.