Microplastics in the Agroecosystem: An Emerging Threat to the Plant and Soil Health

Thin plastic mulch films (4–8 μm thick) are widely used in agroecosystems to improve crop growth and water use efficiency. Although extremely effective, after crop harvest they are notoriously difficult to recover from soil and are rarely recycled and fragmented consequently by tillage and UV radiation. Of these, microplastics (MPs; particles <5 mm in diameter), typically formed from the disintegration of larger plastic debris, are thought to be the most environmentally damaging. Due to our poor understanding of MPs behavior in soils, it is currently not possible to make informed decisions on future policies relating to the use and disposal of agricultural plastics. Based on current evidence, we aim to evaluate whether MPs represent an emerging threat to plant-soil health in agroecosystems. We assess the ecological risks to plant-microbe-soil interactions associated with MPs and discuss the consequences of MPs on soil carbon (C), nutrient cycling, as well as greenhouse gas emissions in agroecosystems. We also identify knowledge gaps and give suggestions for future research. We conclude that MPs can alter a range of key soil biogeochemical processes by changing its properties, forming specific microbial hotspots, resulting in multiple effects on microbial activities and functions. Because of the diverse nature of MPs found in soils, in terms of polymer type, shape and size, we also see differing effects on soil organic matter (SOM) decomposition, nutrient cycling, and greenhouse gases production. Importantly, increased bioavailable C from the decomposition of biodegradable MPs, which enhances microbial and enzymatic activities, potentially accelerates SOM mineralization, and increases nutrient competition between plant and microbes. Thus, biodegradable MPs appear to pose a greater risk to plant growth compared to petroleum-based MPs. Although MPs may confer some benefits in agroecosystems (e.g. enhanced soil structure, aeration), it is thought that these will be far outweighed by the potential disbenefits.