Microplastics in agroecosystem: Research status and future challenges

Microplastics (MPs) pollution has attracted global attention in recent years. Despite the remarkable benefits arising from the production of plastic for film mulching, irrigation, and organic fertilizer application, there are increasing concerns associated with the vast amount of plastic entering the agroecosystems and its subsequent potential environmental problems. More specifically, MPs (particles<5 mm in size), typically formed from the disintegration of larger plastic debris by tillage and UV radiation, accumulate in agroecosystems and eventually enter the food chain, threatening human and animal health. On the basis of the current evidence, we summarized the source, abundance, mitigation, and detection methods of MPs in agroecosystems. We evaluated the potential ecological risks of MPs to crop growth, microbial activity, soil nutrient cycling, and greenhouse gas emissions. It is found that MPs could either directly or indirectly impact the plant-soil-microbe interactions once incorporated into soil, through the following mechanisms: First, owing to their chemical inertia and structural characteristics, MPs have been recognized as carriers of hazardous substances (e.g., organic pollutants, heavy metals, and pathogens), in addition to their toxic additives (i.e., plasticizers). After making contact with the soil, the migration of plastic particles likely facilitates the transport of sorbed contaminants and contributes to a great ecological risk for crop growth, enzyme activity, and microbial activity. MPs could also alter soil physicochemical properties, that is, they may change the soil aggregation stability, bulk density, and water holding capacity, resulting in diverse effects on microbial functions and plant growth. MPs could also serve as a novel ecological habitat for microorganisms living at the soil-plastic interface (i.e., microplastic spheres), allowing the formation of unique microbial communities. The second mechanism involves the fact that MPs are particles that contain a high carbon content, typically around 90%, making them relatively unique in relation to other pollutants as they can drive diverse consequences for other element cycles (e.g., nitrogen and phosphorus). Direct effects are likely to be minimal because MPs contain mostly negligible amounts of nitrogen and phosphorus. However, alterations in soil structure and physicochemical properties would be expected to change microbial processes, including the nitrogen and phosphorus related enzymes, since soil properties indirectly control soil oxygen availability, which in turn influences CO₂, N₂O, and CH₄ formation. Due to the high degree of variability in polymer type, size, shape, and concentration, the impacts of MPs on soil biogeochemical processes and their underlying mechanisms remain unclear, and further detailed research is therefore needed. Thus, we propose some research priorities regarding the future challenges of MPs in agroecosystems.