Microplastics make their way into the soil and rhizosphere: A review of the ecological consequences

Microplastics (MP) are ubiquitous contaminants of great concern due to their high persistence and potential hazardous impact on the environment. Depending on their size and shape, as well as the chemical additives they can have in their polymeric structure, MP can be taken up by organisms, ultimately leading to direct and indirect toxic effects. In this review, we discuss the primary sources, fate, and impact of MP on the rhizosphere ecology, focusing in particular on how soil physical-chemical properties, plant physiology, and soil biodiversity are modulated by the interaction of MP in the plant-soil system. Current knowledge on soil ecotoxicology shows that MP directly affects soil quality and fertility via alteration of soil nutrient cycling and microbial communities and indirectly by changing soil bulk density, pH, porosity, electric conductivity, and nutrient bioavailability. MP is also known to affect soil animals by altering their feeding, mobility, and reproductive behavior. Toxic effects of MP on the multiple and co-occurring interactions among the soil, soil organisms and plants, particularly in the long-term, remain unstudied. Indeed, a better understanding of rhizosphere-MP interactions at a functional level (e.g., nutrient availability, pollutant immobilization, root exudates, etc.) is urgently needed to develop risk assessment frameworks of soil pollution by MP.