The rise of artificial soil carbon inputs: Reviewing microplastic pollution effects in the soil environment

The surge in the use of plastic materials, its poor handling and disposal have led to an increase in microplastic pollution in terrestrial environments. Microplastic pollution in soils is of concern due to potential influences on soil properties which play a critical role in plant growth and soil fertility. Moreover, the soil environment is a key nexus linking the atmosphere, hydrosphere, biosphere and lithosphere, and thus represents a crucial conduit for pollutant migration from the anthroposphere. In this review we evaluate the effects of microplastics in the soil environment with a specific focus on physical properties and biological function in the rhizosphere. Our review reveals that agricultural sources, particularly plastic mulches and waste applications, represent the main source of soil microplastic inputs. Once in the soil environment, microplastic effects on soil properties are highly variable depending mainly on soil type and microplastic characteristics. Soil properties relating to erosion-risk (i.e., bulk density), structural integrity (i.e., aggregate stability, particularly micro-aggregate stability), and water-storage capacity (i.e., evaporation rate, desiccation) are generally adversely impacted by soil microplastic inputs. Soil microplastic effects on rhizosphere function (i.e., plant health and microbial activity) are remarkably varied with some studies revealing positive impacts, such as enhanced plant-symbiotic fungi associations, from soil plastic additions. However, all identified publications reported at least one detrimental MP-induced impact on plant responses. Finally, our review revealed associations between microplastic properties and soil functional parameters - in particular, polymer size and morphology control soil water-holding properties whereas polymer type influences plant response. These associations will be helpful in targeting future research directions on this important topic that intersects all of the Earth's spheres.