Microplastics in soil ecosystems: impacts on microbial communities, nutrient cycling, and agronomic sustainability

Microplastics (MPs), defined as synthetic polymer particles smaller than 5 mm, have become pervasive contaminants in terrestrial ecosystems. Soils, particularly agricultural soils, are now recognized as major sinks for MPs due to inputs from plastic mulching, sewage sludge, compost, irrigation water, and atmospheric deposition. Their persistence, diverse morphology, and chemical complexity enable MPs to interact directly with soil microorganisms, which are central to nutrient cycling, organic matter decomposition, and plant health. This review synthesizes current knowledge on the ecological risks and microbial responses associated with MPs in soil. MPs alter soil physical properties such as porosity, bulk density, and water retention, thereby reshaping microbial habitats. They also act as carriers of persistent organic contaminants, heavy metals, and antibiotics, amplifying combined toxicity. At the microbial level, MPs induce shifts in community composition, favoring taxa capable of colonizing plastic surfaces or tolerating oxidative stress, while reducing the abundance of nitrifiers and ammonia oxidizers. Functional consequences include disrupted carbon, nitrogen, and phosphorus cycling, altered enzyme activities, and increased greenhouse gas emissions. Plant–microbe symbioses, including nitrogen-fixing bacteria and arbuscular mycorrhizal fungi, are impaired, leading to reduced nutrient uptake and crop productivity. Although certain microorganisms demonstrate limited biodegradation potential, ecological benefits are outweighed by concerns regarding long-term accumulation, persistence of additives, and potential horizontal gene transfer of resistance traits. Significant knowledge gaps remain, particularly regarding chronic field-scale impacts under diverse agro-climatic conditions. Future research should integrate molecular, ecological, and biogeochemical approaches to assess cumulative effects and inform sustainable strategies for mitigating MP pollution in soils.